I'lIII-IIIIIIIIIEII I I THESlS l llllllllllll Q g?- -.. .2 This is to certify that the dissertation entitled SEISMOLOGICAL STUDIES IN NORTHEASTERN RUSSIA presented by Kevin G. Mackey has been accepted towards fulfillment of the requirements for Phd. Geological Sciences degree in flw 5r Jj Major fifessor Date )mM’a. /p7. 19 9:; MS U is an Affirmative Action/Equal Opportunity Institution 0' 12771 LEBRARY Michigan State University PLACE IN RETURN BOX to remove this checkout from your record. To AVOID FINE return on or before date due. MAY BE RECALLED with earlier due date if requested. DATE DUE DATE DUE DATE DUE 11m WM“ SEISMOLOGICAL STUDIES IN NORTHEAST RUSSIA By Kevin G. Mackey A DISSERTATION Submitted to Michigan State University in partial fulfilhnent of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Geological Sciences 1999 ABSTRACT SEISMOLOGICAL STUDIES IN NORTHEASTERN RUSSIA By Kevin G. Mackey A seismicity catalog and associated list of seismic phases for larger events has been compiled for northeast Russia using published and unpublished data from the regional networks operating in eastern Russia (primarily Magadan, Yakutsk, and Amur), the western Alaska network, and international data files. The catalog contains over 40,000 events and over 110,000 arrival times. The resultant catalog is contaminated by industrial explosions, particularly in the Amur and central Magadan districts. The level of contamination is analyzed using the temporal distribution of events as anthropogenic events occur primarily during local day. Dramatic differences are observed between daytime and nighttime seismicity for the Amur district. Removal of anthropogenic sources allows easier identification of active faults. A seismicity trend was found to extend westward from the Seward peninsula to northern Kamchatka, which is interpreted to define the northern boundary of a proposed Bering plate. Clockwise rotation of this plate about an Euler pole in northeast Chukotka is suggested to be driven by terrane accretion is southern Alaska and coupling with the Pacific plate. A preliminary crustal velocity model is developed by obtaining best fit travel time curves over 3 x 5 degree regions. The velocities obtained are generally in agreement with inferred tectonic regimes with high velocities in Precambrian platforms, low velocities in active rifts, and average velocities in Mesozoic terrane assembledges. The velocity model is then used to relocate larger regional events. Relocated events are used to develop a preliminary regional upper mantle tomographic model of northeast Russia. ACKNOWLEDGMENTS First, I would like to thank my advisor, Kaz Fujita, for all the guidance, support, and help over the past 6 years while I worked on this project. I would also like to thank the members of my guidance committee: Bill Cambray, Dave Hyndman, Larry Ruff, and Tom Vogel. Many ideas resulted from helpful conversations with other scientists both in Russia and the US, including David Stone, Paul Layer, Dan McNamara, Guy Tytgat, Marina Odinyets, Boris Sedoff, Art Grantz, Doug Christensen, Simon Klemperer, Thomas Hearn, and Jeff Amato. In the process of assembling the database of northeastern Russia, there have been many people who have assisted in the acquisition or entry of data For data acquisition, I would like to thank Boris Koz'min, Larissa Gounbina, Valery Irmev, Valentin Kovalev, Andrey Savchenko, Sasha Larionov, Natasha Koz'mina, Evgeni Gordeev, and the station operators in Stekolnyi and Seimchan for Russian regional network data, Charlotte Rowe, Guy Tytgat, and Roger Hansen for Western Alaska network data, and Bob Engdahl for ISC data. For data entry, I would like to thank Boris Koz'min, Larissa Gounbina, Nastia Antropova, Trent Faust, Rob McCaleb, Alexandra Dejong, Alan McNamara, Paula Figura, Melissa McLean, and Steve Riegel. This project would not have been possible without the help of many individuals. Many thanks go the all the peOple who provided logistical support while in the field. In Magadan, this includes the Gounbin family (Larissa, Dima, Tanya, and Keesa), the Kovalev family (Valentin and Zena), Andrey Savchenko, and all the people of the Magadan EMSD iv (Thanks for all the agates l), and Lerun Izmailov and Pavell Minyouk of NEISRI. In Yakutsk, I thank the Koz’min family (Boris, Gala, Natasha, Zhenya (Irkutsk), and Baba Tanya), the Imaev family (Valery, Luda, and Timor), Len Parfenov, Zena Komilova, Pavell Izbekov, and Vilodia Oxman. I would also like to thank Svetlana and Olga in Ust’Nera, Anna and Albina in Talaya, the Matrosova Gold Mine, and Gala in Batagai. In Alaska, I thank David Stone, Paul Layer, Roger Hansen, Ann Trent, Diane and Cliff Gray, and Dena and David Vought. In California many thanks to Sharron Morrison. Last but not least, I would like to thank my wife, Michelle, for all the years of love, encouragement, support, and understanding while I have worked on this dissertation, especially with all the nights at the lab until 5 am and summers when I ran off to Siberia. I would also like to thank my parents for their love, encouragement, and support since I started graduate school. Support for this project was provided by, the National Science Foundation Office of Polar Programs grants 92-24193, 94-24139, and 98-06130, Incorporated Research Institutions for Seismology Joint Seismic Pro gran and Global Seismic Network Pro gram, the Defense Threat Reduction Agency contract DTRA01-98-C-0168, and Michigan State University. Their support is gratefully acknowledged TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES INTRODUCTION Overview of study A brief overview of the tectonic regime and geologic history References CHAPTER 1 The Northeastern Russia Seismicity Database Introduction Data sources Zemletiyseniya v SSSR Materialy po Seismichnosti Sibiri Seismologicheskii Bulletin - Dalnie Vostok Unpublished Magadan Network Bulletin Unpublished Yakutsk Network Draft Material Unpublished Kamchatka Seismicity Catalog Western Alaska Network Data Tape Seismograms Other Seismic networks and database assembly Yakutsk Regional Network Temporary stations in the Lena River Delta, Laptev Sea, and New Siberian Islands Magadan Regional Network Data of Station Iul’tin (ILT) and Magadan’s Chukotka Network Northeast Russia Test Network Western Alaska Network Kamchatka Peninsula Network Amur Regional Network Sakhalin Island Network Irkutsk Regional Network Conclusion References CHAPTER 2 Explosion Contamination in the Northeast Russia Seismicity Catalog Introduction Data Sources Previous Work vi 98 98 99 99 Discussion Amur district Polyarnyi—Leningradsky-Plamennyi Kolyma gold belt Ust’Nera Lazo Deputatsky Kular Stolb Yugorenok South Yakutia Red Dog Interpretative results Conclusion References CHAPTER 3 Relocations of Northeast Russia Earthquakes Introduction Previous work Discussion Results Future work Conclusions References CHAPTER 4 Tomography of Northeast Russia Introduction Previous work Methodology Tomography code Data selection Discussion Initial models Models using relocations Regional model Local Magadan model Conclusions References CHAPTER 5 Seismicity of the Bering Strait Region: Evidence for an Independent Bering Sea Plate Introduction Seismicity map Regional seismotectonics and geology Seward Peninsula vii 102 106 125 128 138 138 143 143 146 146 146 155 155 162 165 167 167 167 172 187 198 202 203 204 204 204 205 205 208 209 209 215 215 224 224 226 227 227 231 231 231 Chukchi Peninsula Koryak Highlands Aleutian Arc Discussion Conclusions References CONCLUSIONS APPENDIX A Alphabetized list of northeastern Russia seismic stations and station parameters. APPENDIX B Yearly plots of seismicity in northeast Russia. APPENDIX C 1999 Digital deployments and station observations. APPENDIX D Output of event relocations for comparison with Iul’tin and western Alaska network data APPENDIX E Event relocations for northeastern Russia. APPENDIX F An alternate method for hypocentral depth determination using Pn residuals. 235 238 240 241 246 247 252 256 265 307 321 325 335 Table 1-1 Table 1-2 Table 1-3 Table 1-4 Table 1-5 Table 1-6 Table 1-7 Table 1-8 Table 1-9 Table 1-10 Table 1-11 Table 1- 12 LIST OF TABLES Seismic stations operated by the Yakutsk network. 26 Temporary seismic stations from the Yakutsk region. The 1989 South Yakutia and the 1971 Artyk deployments were aftershock studies. 29 Temporary stations in the New Siberian Islands, west of the Lena River Delta (Avetisov, 1983; Avetisov, 1996) and the Laptev Sea (Kovachev et al., 1996). 38 Seismic stations operated in the Magadan region. All stations were operated by the Magadan EMSD. 44 Travel time curve used for locating earthquakes in the Magadan network. This table depicts travel times for a hypocentral depth of 5 km. 48 Permanent seismic stations to Operate in Chukotka All but station Iul’tin were operated by the Magadan EMSD. Station Uelen was abandoned shortly after opening because noise prevented return of any useful data. 55 Comparison of origin times and epicenters for earthquakes located by ILT, the Western Alaska network (W AK), and those relocated in this study. 57 Seismic stations and station parameters from the temporary network established in northeast Russia in the mid 1960's. Parameters from Mishin (1967). 65 Seismic stations of the Western Alaska network. Parameters from Biswas et al. (1980), and Biswas et al. (1983). 69 Seismic stations and station parameters of the Kamchatka network. 73 Seismic stations and station parameters of the Amur network. 79 Seismic stations and station parameters for the Sakhalin Island network. 83 Table 1-13 Table 2-1 Table 2-2 Table 3-1 Table 5-1 Table F-l Table F-2 Seismic stations and station parameters for selected stations of the Irkutsk network. 88 Town and mine locations from which explosions are reported in the unpublished Magadan bulletins. 101 Focal mechanisms of the Amur region. Planes are given as Strike - Dip - Rake. 161 Best fit velocities and number of events per region. 185 Focal mechanisms for western Alaska and Chukotka. Mechanisms are listed from west to east. Those followed by an asterisk are plotted of Figure 5-3. 233 Depth determination table based on Pn residuals. 341 Depth determinations from the Magadan test area. All depths (h) are in kilometers. 344 Figure I-l Figure 1-2 Figure I-3 Figure I-4 Figure 1-1 Figure 1-2 Figure 1-3 Figure 1-4 Figure 1-5 Figure 1-6 Figure 1-7 LIST OF FIGURES Location of the northeastern Russia study area. Index map of the study area in northeast Russia. Plate tectonic map of northeast Russia with teleseisrnic earthquakes, representative focal mechanisms, and relative plate mOtions. Plates are North American (NA), Eurasian (EU), Amur (AM), Okhotsk (OK), Pacific (PA), and Bering (BE). Geologic index map of northeast Russia. Seismic network boundaries in northeastern Russia. Plot of seismicity catalog compiled for northeastern Russia Seismicity related to subduction of the Pacific plate under Kamchatka, the Kurile Islands, and the Aleutian Islands are omitted. Events in the seismicin catalog for which phase data has been acquired. Nomogram for determining K-class values of earthquakes in the in the Magadan region using a SKM seismometer. Horizontal axis is distance in km, while vertical axis is the amplitude of ground motion (maximum P wave amplitude plus maximum S wave amplitude) in microns. The K-class value is read off the diagonal lines. Seismicity and seismic stations of the northern Yakutsk network. Temporary seismic stations deployed after the 1971 Artyk earthquake are shown as squares. Seismicity and seismic stations of the southern Yakutsk network. Temporary seismic stations deployed after the 1989 south Yakutia earthquake are depicted as squares. Relationship between K-class and ISC reported magnitude in the Yakutsk network. U.) ON 14 16 17 19 30 31 35 Figure 1—8 Figure 1-9 Figure 1-10 Figure 1-11 Figure 1-12 Figure 1-13 Figure 1—14 Figure 1—15 Figure l-16 Figure 1-17 Figure 1-18 Summer seismic station deployments in the New Siberian Islands region from Avetisov (1996). Stations shown as triangles and located events as circles. Summer seismic station deployments in the Lena River Delta region from Avetisov (1996). Stations shown as triangles and located events as circles. Stations deployed for refraction profiles shown as lines. Numbers indicate years of operation The 1989 summer deployments of ocean bottom seismometers shown as hexagons and located seismicity shown as circles. Events and stations from Kovachev et al., (1995). Yakutsk network stations in gray for reference. Seismicity and seismic stations of the Magadan network. Crustal velocity profile used for locating earthquakes in the Magadan region. Assumed raypaths for a 3 km deep event (asterisk) are shown. Travel time curves used for earthquake locations in the Magadan network. These curves are calibrated for a hypocentral depth of 5 Ian Relationship between K-class and ISC reported magnitude in the Magadan network. The heavy gray line depicts the K-M relationship cited by Andreev et al. (1967) for the Magadan region. Seismicity recorded by station Iul’tin (ILT) from 1966-1982. The large cluster of seismicin to the northwest of Iul’tin is most likely explosion contamination (See chapter 2). Major mine locations are also shown. Seismicity and seismic stations of the Chukotka network. Relationship between K-class and ISC reported magnitude in the Chukotka network. Relationship between local magnitude and K-class as reported by Kondorskaya and Shebalin (1982). 37 40 41 43 47 49 53 54 58 60 61 Figure 1-19 Figure 1-20 Figure 1-21 Figure 1-22 Figure 1-23 Figure 1-24 Figure 1-25 Figure 1-26 Figure 1-27 Figure 1-28 Figure 1-29 Figure 1-30 Figure 2-1 Relationship between K-class and magnitude in the Chukotka network. Magnitudes greater than 3.5 are from the ISC catalog, while those less than 3.5 are from the Western Alaska network. The January 16, 1982 event is depicted as a square. ISC magnitudes are Mb, while those from the Western Alaska network are M1. Relationship between K-class and magnitude in the Chukotka network. Magnitudes greater than 3.5 are from the ISC catalog, while the remaining magnitude 3.1 event is from the Western Alaska network. Seismicity and seismic stations of the northeast Russia temporary network (1963- 1967). Seismicity and seismic stations in the western Alaska network Seismicity and seismic stations of the Kamchatka network. Only seismicity north of 56° North are included. A few stations associated with volcano monitoring are omitted for clarity. Relationship between K-class and ISC reported magnitude in the Kamchatka network. Seismicity and seismic stations of the Amur network. Relationship between K-class and ISC reported magnitude in the Amur network. Seismicity and seismic stations of the Sakhalin network. Relationship between K-class and ISC reported magnitude in the Sakhalin network. Seismicity and seismic stations of the Irkutsk network. Seismicity shown is primarily from 1970 and 1971. Plot of seismicity catalog compiled for northeastern Russia. Plate boundaries are depicted as gray lines, network boundaries as dashed lines, and seismic stations as triangles. Explosion sources in northeast Russia listed in Russian bulletins. Small dots are individual located explosions, and large dots are towns or mines with multiple explosions. 63 66 68 72 77 80 81 85 87 91 92 100 Figure 2-2 Figure 2-3 Figure 2-4 Figure 2-5 Figure 2-6 Figure 2-7 Figure 2-8 Figure 2-9 Figure 2- 10 Figure 2-1 1 Figure 2-12 Figure 2-13 Figure 2-14 Figure 2-15 Figure 2-16 Percentage of seismicity occurring during local “daytime”. Numbered regions are discussed individually in the text. Histogram of event origin times from aftershocks of the 1989 South Yakutia earthquake. Note a slight bias towards “nighttime” events. Daytime seismicity of the Amur region. Temporal statistics of clusters of epicenters in gray boxes are shown in figures of corresponding numbers. The Baikal-Amur railway is indicated with the heavy gray line. Teleseismically recorded events of magnitude greater than 4.0 are depicted with large open circles. Nighttime seismicity of the Amur region. Temporal variation of probable tectonic earthquakes. Temporal variation of reported seismicity in the Raychikhinsk mining region. Temporal variation of reported seismicity in the Khingansk mining region. Temporal variation of reported seismicity in the Komsomolsk’ na Amur mining region. Temporal variation of reported seismicity in the Chegdomyn mining region. Temporal variation of reported seismicity in the Svobodniy region. Temporal variation of reported seismicity in the Shimanovsk mining region. Temporal variation of reported seismicity in the Oktyabrskiy placer mining region. Temporal variation of reported seismicity in the Taldan region. Temporal variation of reported seismicity in the Ekirnchan mining region. Temporal variation of reported seismicity along the central segment of the Baikal-Amur railway. xiv 104 105 107 108 110 111 112 113 114 115 116 117 119 120 122 Figure 2-17 Figure 2-18 Figure 2- 19 Figure 2-20 Figure 2-21 Figure 2-22 Figure 2-23 Figure 2-24 Figure 2-25 Figure 2-26 Figure 2-27 Figure 2-28 Figure 2-29 Figure 2-30 Figure 2-31 Figure 2-32 Temporal variation of reported seismicity along the northern segment of the Baikal-Amur railway. Temporal variation of seismicity in the Tynda region. Iul’tin seismicity with mine locations. Stippled area indicates region used in temporal analysis of event origin times. Temporal variation of reported seismicity in the Polyarnyi, Leningradsky, and Plamenny mining region. Temporal variation of explosions from the Polyarnyi, Leningradsky, and Plamenny mining region. “Daytime” seismicity of the Kolyma gold belt. Shaded areas indicate regions used in temporal analysis of event origin times and other regions of explosion contamination. Note ring of seismicity around Susuman. “Nighttime” seismicity of the Kolyma gold belt. Shaded areas indicate regions used in temporal analysis of event origin times. Temporal variation of reported seismicity in the region northwest of Susuman. Temporal variation of explosions in the Susuman region. Temporal variation of seismicity from the region of the Kolyma dam Temporal variation of reported seismicity in the Kulu region. Temporal variation of explosions in the Kulu region. “Daytime” seismicity of northern Yakutia Shaded areas indicate regions used in temporal analysis of event origin times. “N ighttime” seismicity of northern Yakutia. Shaded areas indicate regions used in temporal analysis of event origin times. Temporal variation of reported seismicity in the Lazo mining region. Temporal variation of reported seismicity in the cluster of epicenters south of the Lazo mining region. XV 123 124 126 127 129 130 131 132 133 135 136 137 139 140 141 142 Figure 2-33 Figure 2-34 Figure 2-35 Figure 2-36 Figure 2-37 Figure 2-38 Figure 2-39 Figure 2-40 Figure 2-41 Figure 2-42 Figure 2-43 Figure 2-44 Figure 2-45 Figure 2-46 Temporal variation of reported seismicity in the Deputatsky mining region. Temporal variation of reported seismicity in the Kular mining region. Temporal variation of reported seismicity in the Stolb mining region. Temporal variation of reported seismicity in the Yugorenok mining region. “Daytime” seismicity of southern Yakutia. Shaded areas indicate regions used in temporal analysis of event origin times. “Nighttime” seismicity of southern Yakutia Shaded areas indicate regions used in temporal analysis of event origin times. Temporal variation of reported seismicity in the Aldan mining region Temporal variation of reported seismicity in the Chulman mining region. Temporal variation of reported seismicity in the Spokoynii mining region. Temporal variation of reported seismicity in the Red Dog mining region. Daytime and teleseismic seismicity of the Amur region with mapped faults. Black lines indicate strike-slip faults and dark gray lines, thrust faults. Gray indicates regions of explosion contamination. Nighttime and teleseismic seismicity of the Amur region with mapped faults. Black lines indicate strike- slip faults and dark gray lines, thrust faults. Focal mechanisms of the Amur region with nighttime seismicity and reinterpreted faults based on seismicity trends. “Nighttime” seismicity of northeastern Russia. xvi 144 145 147 148 149 150 152 153 154 156 157 158 160 163 Figure 3-1 Figure 3-2 Figure 3-3 Figure 3-4 Figure 3-5 Figure 3-6 Figure 3-7 Figure 3-8 Pn phase arrival travel time curve for northeast Russia. Russian reported hypocenters and origin times were used. The plot contains 9,342 arrivals. Data shown is only from northeastern Russia stations. Pg phase arrival travel time curve for northeast Russia. Russian reported hypocenters and origin times were used. The plot contains 35,257 arrivals. Sn phase arrival travel time cm've for northeast Russia Russian reported hypocenters and origin times were used. The plot contains 4,873 arrivals. Data shown is only from northeastern Russia stations. Sg phase arrival travel time curve for northeast Russia. Russian reported hypocenters and origin times were used. The plot contains 68,811 arrivals. Composite reduced travel time curve for northeastern Russia. Only data from northeastern Russia stations are shown. Pg reduced travel time curves for 75 events used in Mackey (1996) and Mackey et al. (1998). Upper curve uses original Russian determined hypocenter parameters and reflects a velocity of 6.1 km/s. Lower curve plots the same data after all events were relocated and inverted for velocity and origin time. Reduction velocity is 6.0 km/s. Figure from Mackey (1996). Pn reduced travel time curves for 75 events used in Mackey (1996) and Mackey et al. (1998). Upper curve uses original Russian determined hypocenter parameters. Lower curve plots the same data after all events were relocated and inverted for velocity. Reduction velocity is 8.0 kin/s. Figure from Mackey ( 1996). Reduced compo site travel time curve from 75 events used in Mackey (1996) and Mackey et al. (1998). All data plotted use the Pg phase relocated epicenters and origin times determined from inversion of the data after high residual arrivals were removed. The Pg-Pn crossover point is consistent with a regional crustal thickness of 37 km. Pg velocity plotted is 5.99 km/s and Pn velocity is 7.96 km/s. Reduction velocity is 8.0 kin/s. Figure from Mackey (1996). xvii 168 169 170 171 173 174 175 176 Figure 3-9 Grid of individual regions where calibrated crustal velocities were determined. 178 Figure 3-10 Pg-Sg velocity residual graph for the region 60-63°N x 145-150°E. 181 Figure 3-11 Pg-Sg velocity residual graph for the region 54-57°N x 125-130°E. 182 Figure 3- 12 Grid of calibrated Pg velocities. Original epicenters shown for reference. 1 83 Figure 3-13 Grid of calibrated S g velocities. Original epicenters shown for reference. 1 84 Figure 3- 14 Pg velocities for northern Yakutia determined by using a moving window. 1 8 8 Figure 3- 15 S g velocities for northern Yakutia determined by using a moving window. 1 89 Figure 3-16 Original vs. relocated epicenters for the Amur region. Arrows indicate locations of improved definition of some seismicity clusters and trends. 191 Figure 3- 17 Original vs. relocated epicenters for the Magadan region. Ulakhan fault shown by gray line. Note improvement in relative locations of clusters indicated with arrows, as well as many other clusters. 192 Figure 3-18 Original vs. relocated epicenters for Chukotka. Arrows indicate improved lineations. Network boundaries shown in gray. 193 Figure 3- 19 Original vs. relocated epicenters for northern Yakutia. 195 Figure 3-20 Histogram of relocated event depths. Most event depths are around 10 km 196 Figure 3-21 Composite regional travel time curve for northeast Russia using hypocenter parameters from relocations. Sn data are omitted. 197 Figure 3-22 Travel time curve for the region 60-63° N x 145-150° E comparing original (open circles) with relocated event parameters (closed circles). Note the significant reduction in scatter of data points. Sn data are omitted. 199 xviii Figure 3—23 Figure 3-24 Figure 4-1 Figure 4-2 Figure 4-3 Figure 4-4 Figure 4-5 Figure 4-6 Figure 4-7 Travel time curve for the region 54-57° N x 125-130° E comparing original (Open circles) with relocated event parameters (closed circles). Note the significant reduction in scatter of data points. Sn data are omitted. 200 Travel time curve comparing the regions 60-63° N x 125-150° E (solid; Magadan) and 54-57° N x 125-130° E (open; south Yakutia). Note increased velocities for relocated events in south Yakutia. Sn data are omitted. 201 Pn is assumed to be a head wave propagating along the Moho. Static corrections are calculated for the event (down going leg) and the receiver (up going leg). The big moat around the station holds fish and lizards that are disturbed by the earthquakes. 206 Pn arrivals based on original Russian hypocenters. Gray points fall outside the 7.4 km/s and 8.4 krn/s velocity criteria and are not used. Vertical lines denote the accepted distance range of values used. 210 Raypath coverage for the preliminary tomography model using Russian hypocenters. 21 1 Pn tomography of northeastern Russia using Russian determined hypocenters. Contours of velocity perturbations are in percent deviation from 8.0 an5. Points represent cell locations where perturbations were calculated. Rhythmic appearance of contours in northern Yakutia are an artifact of the automated contouring. 213 RMS residual vs. Iteration based on data from original Russian hypocenters. The minimum residual IS on iteration 3. 214 Pn tomography of the boundary between the Magadan and Yakutsk networks. A shift from higher velocities in the Magadan network to lower velocities in the Yakutsk network correlates with the network boundary. Different location procedures between networks may result in the tomography mapping the network boundaries and not real Moho velocity. Contours of velocity perturbations are in percent deviation from 8.0 km/s. Points represent cell locations where perturbations were calculated. 216 Static station corrections for tomography using original Russian hypocenters. Static corrections range from minus 15 to plus 12 seconds. 2 17 Figure 4-8 Pn arrivals selected for use in tomography based on relocated hypocenters. All points shown fall between the 7.4 km/s and 8.4 km/s velocity criteria. Vertical lines denote the accepted distance range of values used. 219 Figure 4-9 Raypath coverage for the tomography model using relocated hypocenters. 220 Figure 4- 10 Pn tomography of northeastern Russia using relocated hypocenters. Contours of velocity perturbations are in percent deviation from 8.0 km/s. Points represent cell locations where perturbations were calculated. 221 Figure 4-11 RMS residual vs. Iteration for tomography based on data from relocated hypocenters. 222 Figure 4- 12 RMS residual vs. Iteration for tomography of the western Magadan region based on data from relocated hypocenters. 225 Figure 5-1 Seismicity map of the Bering Strait region. 228 Figure 5-2 Neotectonic and index map of the Bering Strait region. Labeled faults are Kaltag (KT) and Kugruk (ICU). 229 Figure 5-3 Regional tectonics of the Bering Plate, with representative focal mechanisms. Star denotes Euler pole. 230 Figure 5-4 Focal mechanism and synthetic seismo grams from the October 10, 1971 Chukchi earthquake (R. McCaleb, pers. comm). T0p traces show actual digitized records, while bottom traces show synthetics. All digitized records are short-period vertical components from the station indicated. 236 Figure 5 -5 Crustal seismicity of interior Alaska showing activity on the Denali fault. Data compliments of the Alaska Earthquake Information Center (AEIC), Geophysical Institute, University of Alaska, Fairbanks. Triangle denotes Fairbanks. 244 Figure 5-6 Extrusion tectonics of southeast Asia (Peltzer and Tapponnier, 1988). Note similarity to Alaska and the Bering Plate in relative locations of indenter, rift zones, and faults. 245 Figure C-l. ’ Nighttime seismicity and active faults of northeastern Russia. Major faults are labeled and other faults are located based on interpretation of seismicity lineations. 254 Figure B-l Figure B-2 Figure B-3 Figure B-4 Figure B-5 Figure B-6 Figure B-7 Figure B-8 Figure B-9 Figure B-lO Figure B-ll Figure B-12 Figure B-13 Figure B- 14 Figure B- 15 Figure B-16 Figure B-17 Figure B-18 Figure B-19 Figure B-20 Figure B-21 Figure B-22 Figure B-23 Historic seismicity of northeastern Russia (pre 1950). Seismicity of northeastern Russia in 1950-1959. Seismicity of northeastern Russia in 1960. Seismicity of northeastern Russia in 1961. Seismicity of northeastern Russia in 1962. Seismicity of northeastern Russia in 1963. Seismicity of northeastern Russia in 1964. Seismicity of northeastern Russia in 1965. Seismicity of northeastern Russia in 1966. Seismicity of northeastern Russia in 1967. Seismicity of northeastern Russia in 1968. Seismicity of northeastern Russia in 1969. Seismicity of northeastern Russia in 1970. Seismicity of northeastern Russia in 1971. Seismicity of northeastern Russia in 1972. Seismicity of northeastern Russia in 1973. Seismicity of northeastern Russia in 1974. Seismicity of northeastern Russia in 1975. Seismicity of northeastern Russia in 1976. Seismicity of northeastern Russia in 1977. Seismicity of northeastern Russia in 1978. Seismicity of northeastern Russia in 1979. Seismicity of northeastern Russia in 1980. 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 Figure B-24 Figure B-25 Figure B-26 Figure B-27 Figure B-28 Figure B-29 Figure B-30 Figure B-31 Figure B-32 Figure B-33 Figure B-34 Figure B-35 Figure B-36 Figure B-37 Figure B-38 Figure B-39 Figure B-4O Figure B-4l Figure C-1 Figure C-2 Seismicity of northeastern Russia in 1981. Seismicity of northeastern Russia in 1982. Seismicity of northeastern Russia in 1983. Seismicity of northeastern Russia in 1984. Seismicity of northeastern Russia in 1985. Seismicity of northeastern Russia in 1986. Seismicity of northeastern Russia in 1987. Seismicity of northeastern Russia in 1988. Seismicity of northeastern Russia in 1989. Seismicity of northeastern Russia in 1990. Seismicity of northeastern Russia in 1991. Seismicity of northeastern Russia in 1992. Seismicity of northeastern Russia in 1993. Seismicity of northeastern Russia in 1994. Seismicity of northeastern Russia in 1995. Seismicity of northeastern Russia in 1996. Seismicity of northeastern Russia in 1997. Seismicity of northeastern Russia in 1998. Map of the Magadan region showing locations of seismic stations deployed in 1999. Permanent stations (closed triangles), temporary stations (open triangles), and near future station (circle). Recording of 1,500 kg blast from the Matrosova gold mine. Trace 1 - time, traces 2,3,4 - from Matrosova mine (250 In from blast), traces 5,6,7 - station at Stokolviya (25 km north of Matrosova). 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 309 313 Figure C-3 Figure 04 Figure C-5 Figure F-l Figure F-2 Figure F-3 Figure F-4 Local event recorded at Susuman on June 24, 1999. Epicenter unknown. Regional distance recording from Nelkoba of Mb 6.2 Ktuile Island event of September 18, 1999. Teleseismic vertical component recordings of the MI, 5.8 California-Nevada border region event of August 1, 1999. Stations are, from top to bottom, Ust’Nera, Nelkoba, and Susuman. Illustration showing that Pn residuals are low when depth is 0, and high when depth is 15 km. Events here were located by Pg and Sg arrivals with the best fitting velocities. The Pg and S g arrivals are generally unable to constrain the depth which was allowed to vary from a minimum of 0.0 km to a maximum of 15 km Events used here are from the Magadan test region. Simplified crustal model and diagram used to calculate differences in Pn travel time from hypocenters of varying depth to point A. Path length from point A to the seismic station is the same for all depths. Empirical relationship between change in depth from 10 km and associated shift of origin time. Data are fit with a second order regression (solid line) with 95 % confidence intervals (dashed lines). Comparison of depths determined in the normal location routine with depths determined by the Pn residual method described here. Note the near 1:1 correlation. Open circles without residual offset correction. Closed circles with residual offset correction. 315 316 317 337 339 342 346 INTRODUCTION OVERVIEW OF STUDY From a tectonic standpoint, Northeastern Russia (Figure I- 1; Figure I-2) is one of the least studied large continental regions in the world. This was due to its geographic remoteness and the lack of accessibility, both to the region as well as to Soviet studies concerning it. Since the collapse of the Soviet Union, access to the region’s scientists and their data has become possible, allowing the first comprehensive studies to be undertaken. This dissertation focuses on the seismicity distribution, velocity structure, and tectonic boundaries in the region. A database of all known seismicity for the region, as well as the associated phase arrival times, was first assembled. Chapter one, “The Northeastern Russia Seismicity Database,” outlines the sources and data acquired, and discusses the intricacies and inconsistencies involved in building a useful database. The assembly of this database would not have been possible without a direct knowledge of the operational procedures used in the region, as they explain many of the little quirks and oddities discovered in the data. One of the problems encountered in the seismicity database was that of explosion contamination, discussed in chapter two, “Explosion Contamination in the Northeast Russia Seismicity Catalog.” Some regions in northeastern Russia show high levels of seismic activity, but with most of the reported events being of anthropogenic origin. This can easily result in erroneous interpretation of plate boundaries, active faults, seismic hazards, etc. On the other hand, known explosions are helpful for developing better travel time curves, which can be used to improve epicenter locations and investigate crustal structure. .85 >95. «imam 528055: of mo 8:83 ._ A 833m «8.80 Shock S890 SEQ .Emmsm 385.5: E was beam 2: mo ASE x09; NA oSmE \. 0&l \ W. 479 n I s 1 N a .. .. a «agate $0 is a «M Re 3 . a . W n a» wzfigmm .wfl c 6mm . a «ta :mnmmm m a. 464/ . m D Q oat/60+ . . 4%.» A7 \AV/vwv 2v axe . buy .., paw/WWW emu 38=88a2 .moxmsggo OMEESBB 5:5 «63% “£058: mo mace 3:808 83m .mA 833.; i I >. .........,... ,_. 03.1.3...- <—mm:m t ...... w... E a +95. .932 , $666 .waz 02.... a...”— a.m.c.m .wm—Z 33.2 31:. .waz to be due to far-field effects of the India-Eurasia collision, which results in the eastward extrusion of the Amur plate relative to the Eurasian plate (Tapponnier et al., 1982; Peltzer and Tapponnier, 1988). The Bering block, encompassing the Bering Sea, is driven by subduction of the Pacific Plate and extrusion of southwestern Alaska causing a clockwise rotation relative to the North American plate (Mackey et al., 1997; see Chapter 5). The western portion of the study area contains the eastern portions of the Siberian platform The Siberian platform generally consists of Precambrian basement overlain by a few kilometers of flat lying Riphean, Cambrian, and Jurassic sedimentary materials (Parfenov, 1991). The southern edge of the Siberian platform is separated from exotic terranes to the south by the Hauterivian to Aptian (131-110 Ma) Mongol-Okhotsk Suture (Nokleberg et al., 1998). Bounding the eastern edge of the Siberian platform is the Verkhoyansk range, which is a fold-and-thrust belt of Mesozoic age associated with the terrane accretion in the central portion of the study area occurring during that time period (Nokleberg et al., 1998). The central portion of the study area is composed of a series of exotic terranes and associated island arcs which accreted in the Mesozoic forming the Kolyma-Omolon Superterrane and Kolyma Structural Loop (Figure I-4; e. g. , Parfenov, 1991 ; Zonenshain et al., 1990; Nokleberg et al., 1994). The central part of the study area, in the Chersky Range, participated in an extensional episode in the Pliocene which resulted in the formation of the Moma rift system (Grachev, 1973; Fujita et al., 1990a). Within the past 05 my., the pole of rotation for the NA-EU plates (Figure I-2) is suggested to have moved north and extensional activity along the Moma rift ceased (Cook et al. , 1986). In Chukotka, in the north-eastem portion of the study area is the South Anyui Suture, which probably represents the closure of the South Anyui Ocean coincident with the opening of the Canada Basin at Swarm 3358: Ho 92: 59: Ewe—80 41 cuswfi m :95 580.32 050.3 - 5:. good RESP—Hm «Ebom - >2, about 120 Ma (Nokleberg etal., 1998). Beyond the South Anyui Suture lies the Chukotka Terrane, the Bering Strait and the terranes of Arctic Alaska. Superimposed over the southern edge of the Mesozoic accretionary terranes is the Okhotsk-Chukotka Volcanic Belt, dated at 67-89 Ma (Fujita et al., 1997). Southeast of the Okhotsk-Chukotka Volcanic Belt is the Kamchatka-Koryak Accretionary Zone consisting of a number of Cenozoic accreted terranes, which form the Kamchatka Peninsula and the Koryak highlands (Stavsky et al., 1990; Cook, 1988). Presently, the Pacific plate is subducting along the Aleutian Arc and the eastern edge of the Kamchatka peninsula. REFERENCES Cook, DB, 1988, Seismology and tectonics of the North American plate in the Arctic: northeast Siberia and Alaska: PhD. Dissertation, Michigan State University, East Lansing, xi + 250 pp. Cook, D.B., Fujita, K., and McMullen, CA, 1986, Present-day plate interactions in northeast Asia: North American, Eurasian and Okhotsk plates: Journal of Geodynamics, v. 6, p. 33-51. Chapman, ME, and Solomon, SC, 1976, North American - Eurasian plate boundary in Northeast Asia: Journal of Geophysical Research, v. 81, p. 921-930. Drachev, S., Savostin, L.A., Groshev, V.G., and Bruni, LB, 1998, Structure and geology of the continental shelf of the Laptev Sea, Eastern Russian Arctic: Tectonophysics, v. 298(4), p. 357-393. Fujita, K., Stone, D.B., Layer, P.W., Parfenov, L. M., and Koz'min, B.M., 1997, COOperative program helps decipher tectonis of northeastern Russia: Transactions of the American Geophysical Union (Eos), v. 78, p. 245, 252-253. Fujita, K., Cambray, F.W., and Velbel, M.A. , 1990a, Tectonics of the Laptev Sea and Moma rift systems, northeastern USSR: Marine Geology, v. 98, p. 95-118. Fujita, K., Cook, D.B., Hasegawa, H., Forsyth, D., and Wetrniller, R., 1990b, Seismicity and focal mechanisms of the Arctic region and the North American plate boundary in Asia, in Grantz, A., Johnson, L., and Sweeney, J. F., eds, The Arctic Ocean Region; The Geology of North America v. L: The Geological Society of America, Boulder, p. 79-100. Grachev, AF, 1973, Moma continental rift (Northeast USSR): Geofizicheskie Metody Razvedki v Arktike, v. 8, p. 56-75 (in Russian). Imaev, V.S., Irnaeva, LP, and Koz'min, B.M., 1990, Active Faults and Seismotectonics of Northeast Yakutia: Yakut Science Center, Yakutsk, 138 pp. (in Russian). Kim, BL, 1986, Structural continuation of the rift valley of Gakkel’ ridge on the Laptev shelf, in Egiazarov, B. K., and Kazrnin, Y. B., eds., Struktura i Istoriya Razvitiya Sevemogo Lodovitogo Okeana Sevmorgeologiya: NIIGA, Leningrad, p. 133-139 (in Russian). Koz'min, B.M., 1984, Seismic belts of Yakutia and the focal mechanisms of their earthquakes: Moskva, Nauka, 125 pp. (in Russian). 10 Mackey, K.G., Fujita, K., Gunbina, L.V., Kovalev, V.N., Imaev, V.S., Koz'min, B.M., and Imaeva, LR, 1997, Seismicity of the Bering Strait region: evidence for a Bering block: Geology, v. 25, p. 979-982. McMullen, CA, 1985, Seismicity and tectonics of the northeastern Sea of Okhotsk: MS. Thesis, Michigan State University, East Lansing, vi + 107 pp. Nokleberg, W.J., Parfenov, L.M., Monger, J .W.H., Norton, I.O., Khanchuk, A.I., Stone, D.B., Scholl, D.W., and Fujita, K., 1998, Phanerozoic tectonic evolution of the circum-north Pacific: U. S. Geological Survey Open-File Report 98-754, 125 pp. Nokleberg, W.J., Parfenov, L.M., Monger, J.W.H., Baranov, B.V., Byalobzhesky, S.G., Bunbtzen, T.K., Feeney, T.D., Fujita, K., Gordey, S.P., Grantz, A., Khanchuk, A.I., Natal'in, B.A., Natapov, L.M., Norton, I.O., Patton, W.W., Jr., Plafker, G., Scholl, D.W., Sokolov, S.D., Sosunov, G.M., Stone, D.B., Tabor, R.W., Tsukanov, N.V., Vallier, T.L., Wakita, K., 1994, Circum-North Pacific tectonostratigraphic terrane map: U. S. Geological Survey Open-File Report 94-714, 221 pp., 5 plates. Olson, DR, 1990, The Eurasian - North American plate boundary through the area of the Laptev Sea: MS. Thesis, Michigan State University, East Lansing, viii + 65 pp. Parfenov, L. M., 1991, Tectonics of the Verkhoyansk-Kolyma Mesozoides in the context of plate tectonics: Tectonophysics, v. 199, p. 319-342. Peltzer, G., and Tapponnier, P., 1988, Formation and Evolution of Strike-Slip Faults, Rifts, and Basins During the India-Asia Collision: An Experimental Approach: Journal of Geophysical Research, v. 93, p. 15,085-15,117. Riegel, SA, 1994, Seismotectonics of northeast Russia and the Okhotsk plate: MS. Thesis, Michigan State University, East Lansing, ix + 70 pp. Riegel, S.A., Fujita, K., Koz'min, B.M., Imaev, V.S., and Cook, DB, 1993, Extrusion tectonics of the Okhotsk plate, northeast Asia: Geophysical Research Letters, v. 20, p. 607-610. Seno, T., Sakurai, T., and Stein, S.,- 1996, Can the Okhotsk plate be discriminated from the North American plate?: Journal of Geophysical Research, v. 101, p. 11305-11315. Stavsky, A.P., Chekhovitch, V.D., Kononov, M.V., and Zonenshain, LR, 1990, Plate tectonics and palinspastic reconstructions of the Anadyr-Koryak region, northeast USSR: Tectonics, v. 9(1), pp.81-101. Tapponnier, P., Peltzer, 6., Le Dain, A.Y., Armijo, R., and Cobbold, P., 1982, Propagating extrusion tectonics is Asia: new insights from simple experiments with plasticine: Geology, v. 10, p. 611-616. 11 Zonenshain, L.P., Kuz’min, M.I., and Natapov, L.M., 1990, Geology of the USSR: A plate tectonic synthesis: American Geophysical Union, Geodynamics Series, v. 21, 242 pp. 12 CHAPTER 1 The Northeastern Russia Seismicity Database INTRODUCTION Prior to the beginning of this study, there was no comprehensive database covering continental seismicity in northeastern Russia. In the past 30 years several regional networks including Yakutsk, Magadan, and others, in Russia and the Western Alaska network in the US, have operated in the study area. Unfortunately, data were not exchanged between them resulting in incomplete data sets being used for epicenter locations, resulting in artificial discontinuities at network boundaries (Figure 1-1). To better understand the neotectonic setting of northeastern Russia, it is necessary to combine as much data as possible from all sources. The combined data can be re-evaluated to improve travel time curves, hypocenter parameters, and tectonic models; better understand seismicity levels, determine regions of anthropogenic sources, etc. There are two major sections to the developed database: a complete as possible catalog of hypocenters for northeast Russia and western Alaska, and a database of arrival times from combining data of all sources and networks. Supplemental information includes industrial explosions and seismic station parameters. In assembly of the database, several published and unpublished sources were used. A section which discusses each of the seismic networks considered, covering data problems, some network procedures, and data sources for each network is also included. A combined alphabetized list of seismic stations and parameters can be found in Appendix A. 13 .2323 E03358: E 85858 x838: outflow 4; 853m 55:3. -wgmzx . m a. _ mm M. . 53:32 2v. a 1.... L .flzggé m .m A Semi 17 Ergs. For determining K-class, the maximum ground motion amplitude Am, is determined using :fifi‘; A. max T (1-1) where A, and A, are the respective maximum amplitudes of the P and S arrivals (in microns), and T is the period (in seconds) of the wave. If no amplitude is available for the P phase, only the S is used. Then, using S-P time difference or distance, the K-class value is read off a nomogram calibrated for each particular region (Solonenko, 1974; Pustovitenko and Kul’chitskii, 1974: Gounbina, pers. com). A sample nomogram for the Magadan region is given in Figure 1-4. Unfortunately, the nomo grams calibrated for different regions or networks vary significantly, resulting in inconsistent size determinations between networks. For example an earthquake having a magnitude of 3.5 may be reported to have a K-class of 9.0 in one network, and a K-class of 10.5 from a neighboring network. To better understand the sizes of earthquakes reported in the compiled seismicity catalog, linear regressions were calculated relating K-class to magnitude for each network. Magnitudes used are ISC reported Mb values for magnitudes up to 5.5. Events larger than magnitude 5.5 use ISC or NEIC reported Ms values as body wave magnitude begins to saturate (Lay and Wallace, 1995). Network boundaries shown throughout the study are taken from the Materialy po Seismichnosti Sibiri and Zemletryseniya v SSSR bulletins, which are discussed below. For some networks, there are one or two seismic stations that fall outside the official boundaries 18 833358353 3‘; e, N- B Amplitude Q5 0‘! 003 OD! Axu‘ (on) (101 Kilometers Figure 1—4. Norno gram for determining K-class values of earthquakes in the Magadan region using a SKM seismometer. Horizontal axis is distance in km, while vertical axis is the amplitude of ground motion (maximum F wave amplitude plus maximum S wave amplitude) in microns. The K-class value is read off the diagonal lines. of the network. On maps of individual netwoks, seismic stations visible but belonging to a different network are labeled in gray. Seismic station codes and coordinates have been compiled for stations that have operated in northeast Russia. Most seismic station codes for Russian regional stations are internal to this study and do not necessarily correspond to international designations. Accuracy of station coordinates may vary considerably. Seismic station parameters (name, coordinates, elevation, and open and close dates of Operation) were compiled primarily by K. Fujita (pers. comm). Most station coordinates were obtained from Russian published sources or directly from the seismic station networks. Russian publications usually give station coordinates to 001°, but sometimes to only 0.1°. Some stations were simply located in the appropriate town on the map and coordinates read. For stations in small towns, this is usually accurate to better than 002°. Other stations were visited by the author and GPS coordinates obtained. Quality levels (column ‘Qu’ on seismic station tables) for seismic stations are as follows: G - Location determined with GPS (accuracy to within 100 m). 1 - Coordinates from Russian sources checked on Russian Military or TPC topographic maps and looks reasonable. Stations in small towns may also be assigned this quality level (accuracy to within 2 km). 2 - Some questions on exact location and/or two or more competing locations possible (accuracy may exceed 10 km). 3 - Location guessed or estimated; could be significantly in error. Note that quality levels have not been assigned to all stations. Some stations are known to have been moved slightly but new coordinates are not known; in this case the coordinates are shown in brackets. 20 DATA SOURCES ngletrvseniya v SSSR (1963-1991; Earthquakes of the USSR; hereafter Zemlet) and Zemletrvseniya Severnoi Evrazii (1992; Earthquakes of Northern Eurasia; also referred to as Zemlet). This publication contains a yearly bulletin listing event parameters for the larger earthquakes which occurred within each regional network. In general, only events of K-class 8.5 and larger are listed, although this has varied from year to year and from network to network. In addition, the cutoff was often raised for large aftershock sequences. Zemlet is published in Moscow and has historically been available in the US. Materialy p0 S eigmichnpsti S ibiri (1970-1990; Materials on the Seismicity of Siberia; hereafter Materialy). This is a bi-monthly publication produced in Irkutsk containing both epicenter lists and phase data for each of the seismic networks in Siberia that investigate seismicity of continental regions (Irkutsk, Magadan, Yakutsk, Amur (1979-1990), and Altai). The epicenter list provided here is generally complete, although isolated events listed in the F or East Bulletin and in the unpublished data seem to be missing. The largest fraction of the assembled epicenter list comes from this bulletin. This bulletin also contains phase data and arrival times for events equal to or larger than a K-class of 9.5 occurring within a network. Materialy issues were published with a low print run of 50 or 75 per issue, with distribution generally restricted to the seismic networks in Siberia. Seismologicheskii Bulleten - Dalnie Vogo_k (Intermittent years; Seismological Bulletin - Far East; hereafter Far East Bulletin). This quarterly bulletin published in Yuzhno Sakhalinsk is similar in format to M aterialy, and contains both epicenter lists and phase data. 21 The Far East Bulletin covers the Magadan (listed as separate networks of the “Far East” and “Chukotka”), Amur, Sakhalin, Kamchatka, and Kurile networks. The epicenter list provided here is generally complete for Magadan and Amur, although isolated events found in the unpublished data are missing. For the Kamchatka network, only events of K-class larger than 9.0 are listed. Coverage of phase data varies from network to network. For both portions of the Magadan network as well as the Kamchatka network, phase data for all earthquakes listed are included. For the Amur network, phase data is listed only for events of K-class 8.5 and larger. Phase data from the Sakhalin network are included for shallow events larger than K-class 7.6. The Far East Bulletin was also published with a low print run of approximately 60 copies per issue. Complete sets of the Far East Bulletin were obtained for 1980-1983 and 1985-1988, and partial sets for 1972-1979 and 1984. Unpublished Magadan Network Bullet'm (1977-1998). Data from 1977 through 1990 were acquired as photoc0pies, while 1991 through 1998 were computerized files. The epicenter lists and phase data for located events is identical in content and format to that found in the F or East Bulletin, although the unpublished bulletins contain much additional material on explosions and unlocated events. The unpublished bulletins are well organized into sections as follows: -Epicenters of the Far East -Phase data and arrival times for located earthquakes of the Far East -Phase data and arrival times for small, unlocatable events of the Far East Epicenters of Chukotka -Phase data and arrival times for located earthquakes of Chukotka 22 -Phase data and arrival times for small, unlocatable events of Chukotka -Industrial explosions of the Far East -Industrial explosions of Chukotka -Phase data and arrival times for events that occurred within the neighboring Yakutsk network (no epicenter given) -For 1991, there is a small supplement of arrival times for events which occurred in Alaska Unpub1i_shed Yakitsk Network Draft Materifi (Intermittent years). Unpublished data from the Yakutsk network was acquired in varying degrees of completeness. The material includes epicenters and phase arrival times from all earthquakes within the Yakutsk network. Also included are epicenters and phase arrival times for some events that occurred in the Magadan, Sakhalin, Amur, and Irkutsk networks. Explosions with associated arrival times are also given but not always located. Most of the Yakutsk data was acquired as photocopies, although some data was entered into the computerized database in Yakutsk. Data entered in Yakutsk does not contain explosions or other supplemental information. Format of the Yakutsk data is generally rough, being handwritten with pencil in script Russian, and not quite in chronological order. For some years of the Yakutsk draft material, a supplement of unpublished Irkutsk network data is included. This supplement contains Irkutsk determined hypocenter parameters and phase data for events that occurred within the Yakutsk network boundaries but were well recorded by Irkutsk stations. 23 Umiublished Kamchatka SeismicitLCatalo g. An epicenter catalog was obtained from PetrOpavlovsk listing all seismicity in the Kamchatka network from 1962 - 1996. The catalog contains over 55,000 located earthquakes. Only events north of 56° were incorporated into the database developed here. Western Alaska NetworlQata Tapg. The epicenter catalog from the Western Alaska network was taken from Biswas et al. (1983). The complete computerized phase and arrival time listings were downloaded from archive tapes at the Geophysical Institute, University of Alaska, Fairbanks. Seismograms. Supplemental arrival times were hand picked by Mackey, Fujita, Riegel, Gounbina, and Koz’min from seismograms in Yakutsk and Magadan, as well as develocorder film in Alaska by Mackey. gm Miscellaneous data were acquired from various other publications. Earthquake data for 1920-1999 were obtained from the International Seismological Centre, the International Seismological Summary, the US. Geological Survey Preliminary Determination of Epicenters (PDE) and Earthquake Data Report (EDR), Alaska Earthquake Information Center (AEIC), Kondorskaya and Shebalin (1982), Andreev (1967), Kovachev et al. (1995), Avetisov (1996), and Starovoit et a1. (1995). Additional information on explosions was obtained from Godzikovskaya (1995). Data of the SSR Catalog were obtained from the USGS CD-ROM hypocenter database. The SSR Catalog is based on data from Zemlet and Kondorskaya and Shebalin (1982). 24 SEISMIC NETWORKS AND DATABASE ASSEMBLY Yakutsk Regional Network The first station opened in Yakutsk in 1958, with the Yakutsk network being established in 1960's to monitor regional seismicity. The Yakutsk network initially worked closely with the Magadan EMSD in the collection and analysis of data. In 1982, the cooperative relationship with Magadan EMSD deteriorated and each network began Operating independently with little exchange of data. The Yakutsk network has Operated a large number of permanent seismic stations in the region, with the maximum number in the late 1980's and early 1990's (Table 1-1). Beginning in 1993, economic problems stemming from the collapse of the Soviet Union resulted in drastic cutbacks by the network and the closure of several stations. In 1993 and 1995, Global Seisrnograph Network (GSN) stations were opened by IRIS in Yakutsk and Tiksi respectively. The station in Ust’Nera was moved in 1992, and converted to 24 bit digital acquisition in the June of 1999 (Appendix C). There have been many short-term deployments of seismic stations in the Yakutsk network since the mid 1960's (Table 1-1). In addition, four temporary stations were deployed in 1971 for aftershock studies of the magnitude 7.0 Artyk earthquake, and five stations were deployed in 1989 for aftershock studies of the magnitude 6.6 South Yakutia earthquake (Table 1-2). All earthquake locations determined by the Yakutsk network are done by the historical arcs on map method. For location purposes, the Yakutsk network is broken into northern (Figure 1-5) and southern regions (Figure 1-6). The northern region locates earthquakes on a 1:5,000,000 scale map, and includes stations north of 60° N, while the 25 Table 1-1. Seismic stations operated by the Yakutsk network. Station Name Aku 56.46 120.91 700 --.68 «.68 AMMS Ammonl'naya 64.55 143.18 540 --.61 --.62 AYKS AP Artyk 64.18 145.13 700 —.88 BTGS BTT' Batagai 67.653 134.630 127 --.75 Bazovskii 56.53 123.42 1080 --.70 --.70 CGD tITII Chagda 58.75 130.60 185 --.68 CES ‘IPC Cherskii 68.75 161.33 10 --.79 --.88 Chil'chi 56.06 122.33 500 --.70 --.70 CLlS IHIM Chul'man-l 56.85 124.90 650 --.62 --.86 CLNS IHIM Chul'man-2 56.84 124.90 760 --.86 DUYS III-I Dunai 73.92 124.49 5 11.89 Dyrynmakit 56.60 121.13 460 --.67 --.67 Imangra-l 56.75 121.24 395 07.67 08.67 IMNS I/IMT Imangra-2 56.62 120.71 540 --.75 --.79 ULZS KMH Kamenistyi 65.41 144.83 670 «.88 --.88 KHG XHII Khandyga 62.65 135.56 125 «.69 «.94 KHNS XI-I Khani 57.04 121.01 390 --.67 -.67 «.75? «.76? KHY XTC Khatystyr 55.71 121.57 475 «.68 «.68 --.75 --.82? Kurul'ta 56.90 121.1 1 495 --.68 --.68 Kyubyme 63.38 140.95 950 --.74 --.74 KYUS KCP Kyusyur 70.68 127.37 20 --.85 08.89 Lapri 55.69 124.91 640 --.72 --.73 MKUS MOMA Morna-Khonuu 66.47 143.22 192 --.83 26 Table 1-1 (cont’d). Nagomyi 55.95 124.92 840 --.69 «.69 HFP Nagornyi Sta 55.92 124.97 920 --.77 --.77 NAYS I-[B Naiba 70.85 130.73 5 --.85 NYGS I-IPF Neryungri 56.68 124.66 760 --.77 --.7 8 «.80 «.82 NZDS IDKJ], chhdaninsk 62.50 139.06 603 --.80 SAYS CII Saidy 68.70 134.45 88 --.80 SSYS CCP Sasyr 65.16 147.08 580 «.86 Slyuda 56.33 124.12 1080 --.7 3 --.7 3 SOTS CTB Stolb 72.40 126.82 10 --.85 Sutam 55.96 127.59 700 --.69 --.69 TBKS TBJI Tabalakh 67.54 136.52 200 --.80 TB TMLS TMJI Tairnylyr 72.61 121.92 60 --.86 TMP TasYuryakh-l 56.64 121.33 395 02.67 03.67 TasYuryakh-2 56.62 121.41 415 07 .67 08.67 TLIS THK Tenkeli 70.18 140.78 110 —-.84 --.93 'I‘IK TKC Tiksi 71.632 128.863 38 3.56 --.93 TIXI Tiksi-GSN 71.64 128.87 30 --.95 Tokarikan 56.10 126.42 800 --.72 --.73 TUGl Tungurcha-l 57.33 121.48 440 --.70 --.70 TUG TI-II" Tungurcha-Z 57.27 121.48 315 --.78 ULlS TEE Tyubelyakh 65.37 143.15 380 --.88 --.88 UNRl Y-I-IP Ust' Nera-l 64.566 143.230 485 «.62 «.92 UNR Y-H Ust' Nera-2 64.565 143.242 485 «.92 USZ Y-H Ust' Nyukzha 56.56 121.59 415 --.64 27 Table 1-1 (cont’d). UURS YPK Ust' Urkima 56.31 123.16 540 --.81 YAKI Yakutsk-1 62.015 129.722 90 10.57 «.62 YAK 51K Yakutsk-2 62.030 129.677 91 --.62 YUBS 101311 Yubileniya 70.74 136.10 10 «.86 --.93 Iosr ZYRS 3PH Zyryanka 65.72 149.82 120 --.82 «.90 28 Table 1-2. Temporary seismic stations from the Yakutsk region. The 1989 South Yakutia and the 1971 Artyk deployments were aftershock studies. English Russ. Station Name Lat. Long. Elev. Date Date 1989 South Yakutia Deployment ACHS AMII Amedichi 57.03 122.85 930 05.89 08.89 CKHS HK‘I Chokchoi 57.65 121.72 240 --.89 --.89 KBKS KBK Kabaktan 56.68 122.42 1010 05.89 08.89 KBT SYLS CJD( Syllakh 57.12 121.86 600 --.89 «.89 YRGS HPF Yaruoa 57.49 123.07 780 --.89 --.89 AYKS AP Artyk 64.18 145.13 700 06.71 --.71 AYIS Kobdi 64.20 145.51 800 06.71 --.71 AY2S O3P Ozernaya 63.75 146.1 1 875 06.71 --.71 AY3S Tungusskii 64.20 146.38 1080 --.71 --.71 29 .3325 an :32: 2a oxen—Ego xb< :3 2: coca 3.83% 3233 oEEom €82.th £838: x83a>Eostoc 05 ..o 25:33. 0:56» 98 gogflom .m-_ 233m 663 .. .. msflaaema. .mwa m. ac. . n a.... a... o 12.93.”. I 1 .-.p. . .. ..S...4.3N..L. . )6 am the Q we. ...». . Asa $¢0747Wu4€. . . .>m.m .z40 E 4 . 0564 m8 4 +es§2mu 3 .3 me: O 3 -3 ma: 0 a... -3. a: o oevaz 30 Mag < 4.0 0 Mag 4.0 - 4.9 O Mag 5.0 - 5.9 O Mag 6.0 - 6.9 O Mag 7.0 + Figure 1-6. Seismicity and seismic stations of the southemYakutsk network. Temporary seismic stations deployed after the 1989 south Yakutia earthquake are depicted as squares. 31 southern region includes stations south of 63° N. Stations at Yakutsk, Khandyga, and Nezhdandinsk are included in both regions. Sometimes, for the Lena River Delta and Laptev Sea regions, a 1:2,400,000 scale map is used. In many cases, data are acquired for stations which are physically off the map used in determining locations. Data from such stations are omitted from the location process; only those stations within the bounds of the map on which the arcs are drawn are used. Thus, data from the northern region are not used when locating earthquakes from the southern region, as the northern stations do not fit on the map, and vice versa. Epicentral distances are determined by S g-Pg time differences for each station. Many earthquakes have phase arrival times from only three or four stations, and only one P arrival (usually Pg). In this case, the one available Sg-Pg time is used to define the origin time and other distances are determined using the S g - Origin time difference from remaining stations. Locations in the northern region use a Pg velocity of 6.1 km/sec. S g velocities used are 3.5 km/sec or 3.6 kin/sec, whichever works best for the particular earthquake. Generally, 3.5 krn/sec works best in the Laptev Sea and Lena River Delta region around Tiksi. In the southern region, Pg velocities used are 6.0 km/sec or 6.1 km/sec, whichever is best for the particular earthquake. The S g velocity used is 3.6 kin/sec, except for the Aldan shield, where 3.7 km/sec works best (Koz’min, pers. comm). There were much data acquired by stations in the Magadan, Amur, and Irkutsk networks which were not exchanged with the Yakutsk network, and subsequently not included in any of the published bulletins. Historically, the Yakutsk network had access to data from the Magadan stations of Susuman, Seimchan, Kulu, and Debin (See Magadan Network below), and occasionally others. However, overall, the unpublished Magadan network bulletin added a significant amount of phase and arrival time data for events in the 32 Yakutsk network. Of stations in the Amur region, generally only Kirovskii was used by the Yakutsk network. In comparison of Kirovskii arrivals reported in the Yakutsk unpublished bulletin with those in published sources, there are some differences in reported phases and arrival times. The discrepancy is suspected to be a result of Yakutsk receiving the preliminary Kirovskii station bulletin with times and phases determined by the station Operator. The phases and times of arrivals in published sources of the Amur data (Materialy, and Far East Bulletin) represent the final interpretation, which would have presumably been done in conjunction with other records by the Amur network. When possible, what is believed to be the final interpretation for Kirovskii phases and arrival times is used in the database. Unfortunately, there were no additional unpublished data from the Amur regional network available for Yakutsk region events beyond that which was already contained in the Yakutsk bulletin. Additional data mo st certainly does exist in the Amur network. For earthquakes in the Yakutsk region that are located by the Irkutsk network from Irkutsk stations, a printout of the hypocenter, phases, and arrival times is supplied to Yakutsk. From this printout, Yakutsk normally uses only the data from Tupik (TUP), Srednyi Kalar (SRK), and Chara (CRS) in their locations. Other stations will not fit on the map used in the locations, thus are not used. For the database assembled here, all the available Irkutsk data were included. There are some problems with data supplied to global databases, such as ISC, for events occurring in the Yakutsk region. Data reported for stations in Yakutsk network (UNR, YAK, TIK, USZ, UUR, and CLN) are preliminary time picks by station operators and often have high residuals. This can result from poor time picks, or a secondary Pg phase being reported as a first arriving Pn phase. In these cases the phase and time from the 33 unpublished data are used in the database assembled here, as they are not derived from preliminary seismo gram analysis. For this database, hypocenter parameters were primarily taken from Zemlet and Materialy (1963-1990) and unpublished draft material (1991-1996). Phase data and arrival times for events larger than K-class of 9.5 were taken from Materialy (1970-1990). Phase data and arrival times for smaller events are from unpublished Yakutsk network bulletins (1982-1990). Unpublished data from the 1971 Artyk and 1989 South Yakutia aftershock studies are also included in the database. All data since 1990 are from unpublished network bulletins. For several events along the boundary with the Magadan network, supplemental arrival times were read from Magadan network seismograms. In comparing data from the three primary sources of Materialy, Zemlet, and the unpublished network bulletin, no major variations were found. The unpublished draft material contains hypocenters which sometimes differ with those found in Materialy and Zemlet. The reason for this is not entirely clear. The seismicity catalog in Zemlet is identical to that in Materialy, except it does not list events smaller than K-class 7.5. For all years, events with statistically poor locations listed in Materialy and Zemlet have origin times rounded to the nearest second and coordinates to nearest 0.1 degree. For these events, the unpublished draft material gives origin time to 0.1 second and coordinates to 0.01 degree. In many cases, the hypocenter parameters were originally entered into the database using Materialy or Zemlet, and retain the less precise coordinates. A linear regression relating K-class for the Yakutsk network to ISC magnitude is shown in Figure 1-7. The relationship determined is K = 2.05 + 2.15 (M) (1-2) 34 20 18 - / o’ . 16 - a) 14 2 m L5 9’ X 12 a 10 - / Regression: —-— K=2.05+2.15(M) / Goodness of fit = 57.9% 8 -// _._.. 95% Regression confidence ---------- 95% Value prediction 6 I I I 3 4 5 6 7 ISO Magnitude Figure 1-7. Relationship between K-class and ISC reported magnitude in the Yakutsk network. 35 where K is K-class and M is magnitude. In general, it was found that events occurring in the Laptev Sea tend to have a lower K-class value than continental events for a given magnitude. There are two possible reasons for this. First, many of the Yakutsk epicenters are further south than the ISC locations. This may put the events erroneously close to the Yakutsk stations, which for a given amplitude will tend to lower the K-class value assigned. Second, most of the raypaths from Laptev Sea events to Yakutsk network seismic stations travel down the axis of the Laptev Sea rift. This may result in preferentially greater attenuation of signals at Yakutsk stations relative to teleseismic stations used to calculate magnitude. Terrygorartsgtations in the Lena River Delta, Laptev Md New Siberian Islands The Laptev Sea region has had several temporary stations or networks deployed. The All-Union Research Institute of Ocean Geology (VNIIOkeanologiya) operated one to three stations per summer in the New Siberian Islands from 1972 through 1976 (Figure 1-8; Table 1-3; Avetisov, 1983, 1996). Three stations were operated just west of the Lena River Delta from July to September, 1975 (Table 1-3; Avetisov, 1983, 1996). Several local networks of stations were also deployed during the summers in and around the Lena River Delta between 1984 and 1988 (Figure 1-9; Avetisov, 1996). With a few exceptions, the events located in studies by Avetisov were not located by the Yakutsk regional network. In July and August, 1989, the P. P. Shirshov Institute of Oceanology deployed a small five station array of ocean bottom seismometers (OBS) in two locations within Buorkhaya Bay, in the Laptev Sea (Figure 1- 10; Table 1-3; Kovachev et al., 1995). A toral of 26 events were located from the two OBS deployments, two of which were also located by the Yakutsk regional network. 36 1300 E Figure 1-8. Summer seismic station deployments in the New Siberian Islands region from Avetisov (1996). Stations shown as triangles and located events as circles. 37 l1 Table 1-3. Temporary stations in the New Siberian Islands, west of the Lena River Delta (Avetisov, 1983; Avetisov, 1996) and the Laptev Sea (Kovachev et al., 1996). English Russ. Station Name Lat. Long. Elev. Date Date Cc _ 7 - ,. i _ - ,, nO-n Closed New Siberian Islands Stations Kotelnyi 75.760 137.600 8.72 9.72 - Kigilyakh 73.367 139.867 6.73 9.73 - Dirnnoe 73.233 142.400 3.74 4.74 - Zemlya Bunge 74.8330 142.583 4.75 6.75 - Novaya Sibir 75.0500 147.000 4.75 6.75 - Mys 74.2670 140.883 4.76 6.76 - Khvoinova Mys Nerpichii 75.8330 143.333 4.76 6.76 - Mys Diring- 75.9500 139.917 5.76 6.76 - Ayan | Stations west of the Lena River Delta - All dates are 1975 Udzha 71.25 117.17 08/27 09/19 - Kalgannakh 71.83 114.33 0701 08/ 10 - Chochurdakh 72.83 1 16.25 08/13 09/26 - Laptev Sea Ocean Bottom Seismometer Deployment l - All dates are 1989 # 1 71.75 131.40 -18 07/28 08/9 - # 2 71.85 131.22 -20 07/28 08/9 — # 3 71.90 130.42 -17 07/29 08/9 - # 4 71.77 130.82 -17 07/29 08/9 - # 5 71.62 130.78 -16 07/29 08/9 - 38 Table 1-3 (Cont’d) Laptev Sea Ocean Bottom Seisrnometer Deployment 2 - All dates are 1989 # 1 72.283 131.30 -18 08/10 08/22 # 2 72.45 130.35 -10 08/10 08/22 # 3 72.466 131.20 -19 08/10 08/22 # 4 72.616 130.60 -16 08/10 08/22 # 5 72.267 130.65 -13 08/10 08/22 39 70° N 122° E 1 35° E Figure 1-9. Summer seismic station deployments in the Lena River Delta region from Avetisov (1996). Stations shown as triangles and located events as circles. Stations deployed for refration profiles shown as lines. Numbers indicate years of operation. 40 73° N Laptev Sea 70° N 1 250 E 135° E Figure 1-10. The 1989 summer deployments of ocean bottom seismometers shown as hexagons and located seismicity shown as circles. Events and stations from Kovachev et al. (1995). Yakutsk network stations in gray for reference. 41 The temporary OBS, New Siberian Islands, and the Lena River Delta seismic stations were operated independently from the Yakutsk network stations. Events located and listed in Kovachev et al. ( 1995) and Avetisov (1996) are included in the database. Phase data were not available. Marian Regional Network The first station opened in the region was Magadan in 1952. In the late 1960's following the deployment of the northeast Russia test network, installation of permanent seismic stations began. These stations were operated in conjunction with stations in the Yakutsk network. The Magadan Experimental Methodological Seismological Division (EMSD) network was established as a separate entity in December, 1979 to monitor regional seismicity. The Magadan EMSD initially worked closely with the network in Yakutsk in the collection and analysis of data. In 1982, the cooperative relationship with Yakutsk deteriorated and each network began operating independently with little exchange of data. The reason for this is not clear. The Magadan EMSD has operated a total of 15 permanent seismic stations in the region, excluding those in the Chukotka network, which were also operated by the Magadan EMSD (Figure 1-11; Table 1-4). All stations in the Magadan region operate at least one complete three component set of instruments. Horizontal instruments deployed in the Magadan region are oriented with respect to local magnetic north, not geographic north (Savchenko, pers. com). The largest number of stations were active in the late 1980's and early 1990's. Three temporary stations, Obo, Orotukan, and Yagodnoya were operated in 1977. A few arrival times are also reported for a station Obo 42 Mag < 4.0 0 Mag 4.0 - 4.9 O Mag 5.0 - 5.9 O Mag 6.0 - 6.9 O Mag .70 + YAKUTSKNETWORK 67° N Sea of Okhotsk z‘ I g KURILE , KAMCHATKA - < - OKHOTSK NETWORK 54° N WK ° flirt" 165..., 145° E Figure 1-1 1. Seismicity and seismic stations of the Magadan network. 43 Table 1-4. Seismic stations operated in the Magadan region. All stations were operated by the Magadan EMS D. in English Russ. Station Name Lat. Long. Elev. Date Date Qu Code Code (In) Open Closed DBI IIBH Debin 62.339 150.751 332 «.74 «.92 G EVES 3BH Evensk 61.92 159.23 22 «.80 7.93 1 KU-S KJI Kulu 61.889 147.431 655 1.80 10.92 G KJIY MAG MFII Magadan 59.560 150.805 78 1.52 1.92 1 MA2 MA2 Magadan-GSN 59.575 150.768 339 9.93 1 MYAS MKT Myakit 61.407 152.093 670 «.83 «.88 G NKBS HJIB Nel'koba 61.34 148.81 531 9.83 6.97 1 61.338 148.813 6.97 9.99 G OBO Obo 61.80 149.77 440 «.77 —.77 1 OMCH OM‘I Omchak 61.67 147.87 820 9.99 1 OMOS OMJI Omolon 65.23 160.54 260 6. 82 7.93 1 OMS OMC Ornuskchan 62.52 155.77 527 12.67 1 ORT Orotukan 62.26 151.34 470 «.77 «.77 1 SEY CMH Seimchan 62.93 152.38 211 4.69 1 SNES CHI“ Sinegor'e 62.09 150.52 400 «.76 «.88 1 MGD MAI Stekol'nyi 60.046 150.730 221 3.71 —.94 1 MFII-l (GO-046) (150.730) «.94 1 CTK SUUS CMI-I Susuman 62.78 148.15 640 8.69 «.95 1 CCM (62.78) (148.15) «.95 «.98 1 62.779 148.163 «.98 G TTYS TXT Takhtoyamsk 60.20 154.68 11 9. 87 1 TL-S TJIA Talaya 61.134 152.398 730 1.89 G TLAS TJI USO Y-OM Ust' Omchug 61.13 149.63 580 «.68 «.83 1 YAG Yagodnoe 62.53 149.62 480 «.77 «.77 1 44 in 1981. Beginning in 1992, economic problems stemming from the collapse of the Soviet Union resulted in drastic cutbacks by the network and the closure of several stations. By the mid 1990's, the Magadan network was reduced to Operational stations only in Susuman, Omsukchan, Seimchan, Magadan, and Stekolnyi. Two other stations, T alaya and Nelkoba, were mothballed. The economic situation for the network was sufficiently bad that vegetables were sold out of the stations to keep them open (Savchenko, pers. comm). Today, seven stations remain open, with Magadan (MA2) being a GSN station and Seimchan (SEY) being a Geo scope station. Equipment problems in Seimchan have resulted in a significant amount (years) of downtime for the Geoscope station. In the summer of 1999, stations in Nelkoba, Seimchan, Susuman, and Talaya were converted from photo paper to PC based 24 bit resolution digital recording in conjunction with M.S.U. (Appendix C). The station in Seimchan uses the existing Streckeisen seismorneter installed by Geoscope. In late September 1999, the town of Nelkoba was abandoned, and the station was moved to Omchak, approximately 70 km to the northwest of Nelkoba. Several stations of the Magadan regional network were also moved slightly in the mid to late 1990's. In 1994, Stekolnyi was moved approximately 0.6 km north from its original location. Nelkoba was moved approximately 150 m north in the summer of 1997. Susuman was moved twice. First, in 1995 the station was moved 100 - 200 m, but the location remained near the center of town. In the fall of 1998, the station was moved approximately 1 km to the east to the meteorological station outside of town. For these stations, codes or coordinates were not changed or updated in any of the available data sources, published or unpublished. 45 There have been three methods of determining earthquake locations in the Magadan network, each using Sg minus Pg times. Prior to 1982, locations were computed primarily by hand, and occasionally using a “Besmas 6" electronic calculating machine. In 1982, they began using computers for locations, which were compared to are on map epicenters. The computer determined epicenters were often “adjusted” to better agree with a paper location if there was a discrepancy between the two. The adjustment procedure was dropped in the early 1990's, when the network switched to computer only hypocenter calculations. In the location procedure, the travel time curves used were derived from the 1959 Magadan- Ust’Srednikan DSS (Deep Seismic Sounding; refraction) profile (Ansirnov et al., 1967; Davydova et al., 1968). The model for calculation of the travel time curves uses a three layer crust (Figure 1-12). The uppermost layer is 6.0 km thick and has a P velocity of 5.3 km/s. The velocity at the base of layer one is 6.0 km/s, which is used for the Pg velocity. The second layer extends from 6.0 km to a depth of 20.0 km and has a P velocity of 5.8 km/s. A refraction surface at the base of the second layer has a P velocity of 6.7 km/s and is used as the Conrad discontinuity for P*. The third crustal layer lies between 20 and 35 km depth and has a P velocity of 6.1 km/s. The base of layer three is the Moho, with a Pn velocity of 8.1 km/s. Table 1-5 lists the travel times used in the Magadan network for Pg, Sg, P*, 8*, Pu, and Sn phases. These travel times are derived from this model and are cahbrated for a hypocenter depth of 5 .0 km. The travel times are also plotted in Figure 1-13. The crustal model used here is counterintuitive from a geologic standpoint, as high velocity planes are sandwiched between thick low velocity layers (Figure 1- 12). Vertical velocity profiles with thin high velocity zones between much thicker low velocity layers is typical of those determined from DSS profiles in the former Soviet Union (Gal’perin, 1974). Slopes of travel 46 .5505 0.8 33053 Eo>o acct Ev. m a he 222:8 cofismmx‘ define 53me of E moxasgtao $532 e8 com: oan .9629 3320 .NT— oSmE gee. S u ..> 010: Ex o.mmr .... gee. B u Q> / we; 3 u Q> v // Era, in w\Ex w.m H Q> /x/ Q aex 3 u > v .... a 1. so... an. was. me u > let ZO.._.<._.m 47 Table 1-5. Travel time curve used for locating earthquakes in the Magadan network. This table depicts travel times for a hypocentral depth of 5 km km Pg (5) S g (s) P* (s) S* (3) PD (8) Sn (5) 0.0 0.7 1.3 4.3 7.5 8.7 15.1 10.0 2.4 4.2 5.8 10.1 9.9 17.3 20.0 4.1 7.1 7 3 12.7 11.2 19.4 30.0 5.7 10.0 8.8 15.3 12.4 21.6 40.0 7.4 12.9 10.3 17.9 13.6 23.7 50.0 9.1 15.8 11.8 20.5 14.9 25.9 60.0 10.7 18.7 13.3 23.1 16.1 28.0 70.0 12.4 21.6 14.8 25.7 17.3 30.2 80.0 14.1 24.5 16.3 28.3 18.6 32.3 90.0 15.7 27.4 17.7 30.9 19.8 34.5 100. 17.4 30.3 19.2 33.5 21.0 36.6 110. 19.1 33.2 20.7 36.1 22.3 38.8 120. 20.7 36.1 22.2 38.7 23.5 40.9 130. 22.4 39.0 23.7 41.3 24.7 43.1 140. 24.1 41.9 25.2 43.9 26. 45.2 150. 25.7 44.8 26.7 46.5 27.2 47.3 160. 2 .4 47.7 28.2 49.1 28.4 49.5 170. 29.1 50.6 29.7 51.7 29.7 51.6 180. 30.7 53.5 31.2 54.2 30.9 53.8 190. 32.4 56.4 32.7 56.8 32.1 55.9 200. 34.1 59.3 34.2 59.4 33.4 58.1 210. 35.7 62.2 35.7 62.0 34.6 60.2 220. 37.4 65.1 37.1 64.6 35.9 62.4 230. 39.1 68.0 38.6 67.2 37.1 64.5 240. 40.7 70.9 40.1 69.8 38.3 66.7' 250. 42.4 73.8 41.6 72.4 39.6 68.8 260. 44.1 76.7 43.1 75.0 40.8 71.0 270. 45.7 79.6 44.6 77.6 42.0 73.1 280. 47.4 82.5 46.1 80.2 43.3 75.3 290. 49.1 85.4 47.6 82.8 44.5 77.4 300. 50.7 88.3 49.1 85.4 45.7 79.6 320. 54.1 94.1 52.1 90.6 48.2 83.9 340. 57.4 99.9 55.1 95.8 50.7 88.2 360. 60.7 105.7 58.0 101.0 53.1 92.5 370. 62.4 108.6 59.5 103.6 54.4 94.6 380. 64.1 111.5 61.0 106.2 55.6 96.8 400. 67.4 117.3 64.0 111.4 58.1 101.1 420. 70.7 123.1 67.0 116.6 60.5 105.3 440. 74.1 128.9 70.0 121.8 63.0 109.6 460. 77.4 134.7 73.0 127.0 65.5 113.9 480. 80.7 140.5 76.0 132.2 68.0 118.2 500. 84.1 146.3 78.9 137.4 70.4 122.5 520. 87.4 152.1 81.9 142.5 72.9 126.8 540. 90.7 157.9 84.9 147.7 75.4 131.1 560. 94.1 163.7 87.9 152.9 77.8 135.4 580. 97.4 169.5 90.9 158.1 80.3 139.7 600. 100.7 175.3 93.9 163.3 82.8 144.0 650. 109.1 189.8 101.3 176.3 88.9 154.8 700. 117.4 204.3 108.8 189.3 95.1 165.5 750. 125.7 218.8 116.3 202.3 101.3 176.2 800. 134.1 233.3 123.7 215.3 107.5 187.0 850. 142.4 247.8 131.2 228.2 113.6 197.7 900. 150.7 262.3 138.6 241.2 119.8 208.5 950. 159.1 276.8 146.1 254.2 126.0 219.2 990. 165.7 288.4 152.1 264.6 130.9 227.8 48 .92 m .8 Econ .8288»: a 8.. 385:8 Ba 323 805. 38250: cacawflz 2: E 25:82 oxazgtao be new: $23 2:: .02“; .2; oSwE ES 8:93 com cow com o P _ _ O time curves depicted in Figure 1-13 reflect only the thin high velocity planes. It is unclear how the initial time offset of the seismic phases at a distance of 0.0 km is determined. The time offset at 0.0 km is not calculated using the layer velocities in the model applied to a vertical path. At 0.0 km distance, there should be no arriving P* or Pn phases but only reflections from the boundaries. Reflections on the travel time curve refractions branching off asymptotically, not merely be extensions of the refraction travel time curve as depicted. In the computer locations, the depth is usually constrained if the first depth estimate is greater than 30 km In the published bulletins with Magadan network data, if no depth is listed, it is assumed to be 5 or 6 km. In the location procedure, no station corrections are used and station elevation is not considered (Gounbina, pers. com). The unpublished Yakutsk bulletin provided a significant amount of additional data for earthquakes in the Magadan Region. This supplemental data is not included in any of the published bulletins containing Magadan network data, as it was not exchanged with the Magadan EMSD. Historically, the Magadan network only had access to data from the Yakutsk network station in Ust’N era. However, the reported phases and arrival times for Ust’Nera listed in the Yakutsk unpublished bulletin were often slightly different (within 2.0 sec.) than the data reported to Magadan. The discrepancy is a result of Magadan receiving the preliminary Ust’N era station bulletin with times and phases determined by the Ust’Nera station operator. The phases and times of arrivals in the unpublished Yakutsk data represent the final interpretation, which is done in conjunction with other records by one interpreter at the network headquarters in Yakutsk. The final interpretation for Ust’Nera phases and arrival times is used in the database. 50 There are some similar problems with data supplied to global databases, such as ISC, for events occurring in the Magadan region. Often preliminary time picks are reported for stations in the Magadan network (MGD, MAG, and SEY) with high residuals. This is often a result of a secondary Pg phase being reported as a first arriving Pn phase. In these cases the phase and time from the unpublished data is used, as they are not derived from preliminary seismograrn analysis. For data, hypocenter parameters were primarily taken from Zemlet and Materialy (1963-1990) and unpublished network bulletins (1991-1998). Phase data and arrival times for events larger than K—class of 9.5 were taken fromMaterialy (1970-1990). Phase data and arrival time for smaller events are from the Far East Bulletin (1972 and 1974) and unpublished Magadan network bulletins (1977—1990). All data since 1990 is from unpublished network bulletins. For several events along the boundary with the Yakutsk network supplemental arrival times were read from Yakutsk seismograms. In comparison of data from the four primary sources, Materialy, Zemlet, Far East Bulletin, and the unpublished network bulletin, there are some variations found. From 1972 - 1982, the unpublished network bulletin and the Far East Bulletin contain identical origin times and hypocenters, which are sometimes different than those found in Materialy and Zemlet. This may be a result of events being relocated by those who produce the Materialy bulletin. The seismicity catalog in Zemlet is identical to that in Materialy, except for not listing events smaller than a K—class of 8.5. Since 1983, all four sources contain identical event parameters. For all years, events with statistically poor locations listed in Materialy and Zemlet have origin times rounded to nearest second and coordinates to the nearest 0.1 51 degree. For these events, the unpublished network bulletin and Far East Bulletin give origin time to 0.1 second and coordinates to 0.01 degree. A linear regression relating K-class for the Magadan network to ISC magnitude is shown in Figure 1-14. The relationship determined is K = 2.84 + 2.03 (M) (1-3) where K is K-class and M is magnitude. This is somewhat different than the relationship K: 4.3 + 1.8 (M) (1-4) which is cited by Andreev et al. (1967), which does not fit the data (Figure 1-14). Data of Station Iul’tin (ILT) and the Magadan EMSD Chukotka Network Small events occurring in the Chukotka region were undetected until 1966, when seismic station Iul’tin (ILT; Figure 1- 15; Table 1-6) opened in Chukotka. ILT was operated by the Institute of Physics of the Earth from 1966 until mid-1995. Chukotka epicenters reported in Kondorskaya and Shebalin (1982; NCSE) from 1966 to 1974 and Zemlet (1966-1982) are single station locations from ILT. These single station locations determine epicentral distances from S-P time differences and obtain azimuth from the polarization of the first arrival on both horizontal components (Lazareva, 1975). This has led to some difficulties for interpretation of the data. First, there is a clear unnatural looking linear trend of epicenters to the north and south of the station (Figure 1-15), indicating possible poor locations. Second, analysis of the epicenter cluster to the northwest of ILT has shown that they are explosions misidentified as tectonic events (Figure 1-15; See Chapter 2). These events are listed as explosions in the compiled seismicity catalog. Analysis of some of these 52 16 /l // 15 7 / 14 ~ 13 - a) 8 T.) 12 - x 11 — // / Regression: 10 ‘ — K=2.84+2.03(M) Goodness of fit = 67.0% 9 _ _ _ 95%. Regression / ~ confidence // ---------- 95% Value prediction 8 ” . T 3 4 6 ISC Magnitude Figure 1-14. Relationship between K-class and ISC reported magnitude in the Magadan network. The heavy gray line depicts the K-M relationship cited by Avdreev et al. (1967) for the Magadan region. 53 :32: 02a 25 30:82 02.: 8.82 AN fiasco oomv 223283.80 c2833 38.: ~88 2 2:3 Ho “nuance: of 2 22823 .5 5520 032 of. .moncoe 88.1915 223 c237. .3 6282 5.223% 2 _ ouswfi O J $ ode m , . O O O O O +2 O o 0 ~... ... _.’a 0 w 0 ...... 9 @oo o 000 . o O Qv o ”no.0 09 O O 00.....=8 o o¢nm>cchEwEo do 0 O 1 a_._v.”um.o...... ...... O O m w. om..wu....._.....o o o o %o m ..o. o....__xmm2mc_co._..o _>Em>_oumo on o o o l o o . ' 8m 32:5. ‘ movE 54 Table 1-6. Permanent seismic stations to operate in Chukotka All but station lul’tin were operated by the Magadan EMSD. Station Uelen was abandoned shortly after Opening because noise prevented the return of any useful data. English Russian Station Name Date Date LCode ,. -1 Ce --W fl 7 . ‘ Elm)‘: O . 11 Closed ANSS AHII Anadyr-1 64.77 177. 57 11. 80 1. 89 ANYS AHII Anadyr 64.734 177.496 55 4.89 7 .93 1 9.96 BILS Ble Bilibino 68.059 166.449 283 8.81 4.92 1 BIIH BILL BILL Bilibino GSN 68.065 166.452 299 8.95. l EGVS 3FB Egvekinot 66.323 179.127 W 18 -.9O -—.94 l ILT l/UIT lul'tin 67.87 178.74 W 235 3.66 7.93 1 MKI MAP’I Maiskii 68.97 173.71 261 8.82 6.94 1 MKVS MPK Markovo 64.68 170.41 25 10 .86 4.92 1 MKN PVD TIPB Provideniya 64.424 173.226W 25 9.80 12 .93 Uelen 66.16 169.84 W 5 «.81 —.82 55 epicenters are of reasonable quality given the difficulties of single station locations (Table 1-7; See the Western Alaska network section later in this chapter). The Chukotka regional network contained six seismic stations which were operated from 1981 to 1993 by the Magadan Experimental Methodological Seismological Division (MEMSD; Figure 1-16: Table 1-6). However, the Chukotka network only Operated as an experimental network in 1981 and 1982 and phase data were not compiled. Data were recorded using a galvanometer and photographic paper. A seventh station, located at Uelen (Figure 1-16), operated intermittently for about three months in 1981 but was abandoned due to extreme noise. The station at Uelen also operated with somewhat different instruments (Artamonov and Mishina, 1984). Fifteen epicenters located in Chukotka have been identified as possible explosions from the mining around Polyarnyi, Leningradskii, and Plamennyi (Godzikovskaya, 1995; see Chapter 2). These events are flagged in the database as possible explosions. Focal parameters from the Chukotka network were determined using the same procedures and seismic velocities as described below for the Magadan network. Arrival times from station ILT supplemented data from the Chukotka network in analysis and were included in the Russian phase bulletins. Unfortunately, in 1993, all seismic stations in Chukotka were closed because of financial constraints, although Anadyr (ANY S) was reopened in 1996 with assistance from the Chukotka regional government. In 1995, a new GSN station was opened in Bilibino by IRIS. Hypocenteral parameters were primarily taken from Zemlet (1966-19 82) and Materialy (1983-1990). Occasionally, the Zemlet and Materialy bulletins disagree on whether longitude is east or west. When compared with the unpublished network bulletin 56 Table 1-7. Comparison of origin times and epicenters for earthquakes located by ILT, the Western Alaska network (WAK), and those relocated in this study. Date ILT Time and WAK Time and Relocation Time _ ,. . _ _E :icenter E uicenter and . icenter April 06,1981 23 33 39. 23 34 06.5 23 33 48.4 67.1 N 66.310 N 67.37 N 174.0 W 169.871 W 172.48 W April 06, 1981 23 45 36. 23 46 23.9 23 45 45.0 67.1 N 64.089 N 67.40 N 174.0 W 168.019 W 172.55 W April 07, 1981 O7 08 29. O7 09 50.6 07 08 37.61 67.1 N 65.943 N 66.68 N 174.0 W 168.642 W 174.15 W April 07, 1981 07 14 52. Data Acquired But 07 16 03.0 67.1 N No Epicenter 66.65 N 174.0 E Calculated 174.08 W January 16, 1982 12 35 15.6 12 35 08.0 12 35 09.9 64.0 N 64.774N 64.80 N 1695 W 171.405 W 170.84 W 57 0:250: 8383.5 2: ...o 2223 2:28 23 52228 .22 Semi a 3 m \xmoEmz 5.219236 1 Z 65$ 4. 2.6 o )/V«U/ 6M“? 0 0 .V ¢& 00 4.4! ... . . so . . o. .....66 . .. .O . MAOSQ 6 ED. . 2224 O. 0% .35 +3 82 0 >354. . 6.6 - 3 we: 0 < . 3-3 82 o 2.2 3.3 m6: 0 . ' oevfiz .2638: 8:325 65 go 3233 2528 use b2520m .24 Semi 169$ 49 IV 0 O V )K/ 66. o o . é o b7 .. . 8 6+ . . o . .4 me . . .. ....&o . .. .O . ““23. 6 3D. . 20:24 .35 + c.” we: 0 >354. 6.6 - 3 we: 0 4 . 3-3 we: 6 is. 3.- 3. we: 0 . O on. v 82 58 and a close look at the arrival times, Zemlet is usually found to be incorrect. Materialy (1983-1990) contains phase data and arrival times only for events larger than K-class of 9.5. The unpublished Magadan network bulletins (1983-1990) contain phase data for all events. All data since 1990 is from unpublished network bulletins. For approximately 50 events, phase data and arrival times from the Chukotka network were supplemented with data from Alaskan stations (primarily Anvil Mountain; AVN). In comparison of data from the four primary sources which contain Chukotka data, Materialy, Zemlet, Far East Bulletin, and the unpublished network bulletin, all are found to report the original hypocenter parameters determined by the Magadan network. The only exception being events with statistically poor epicenters. For these events, the unpublished network bulletin and Far East Bulletin give hypocenter parameters to 0.1 second for origin time and 0.01 degree for coordinates. Origin time is rounded to nearest second and coordinates to nearest 0.1 degree in Materialy and Zemlet. A linear regression relating K-class to ISC magnitude for the Chukotka region is shown in Figure 1-17. The relationship determined is K: 802+ 1.00 (M) (1-5) where K is K-class and M is magnitude. This relationship is anomalous when compared to other regions discussed below. However, the poor fit to the data should be noted. Kondorskaya and Shebalin (1982) citing Rautian ( 1960) report a relationship of K = 6.5 +1.5 (M) (1-6) between K-class and magnitude for Chukotka, which was also considered anomalous (Figure 1-18). Addition of magnitudes from the Western Alaska network and K-class determinations from station ILT for events located by both (Table 1-7) yield 59 K-class / —-—K=8.02+1.00(M) / Goodness of fit = 24.2% 10 _. / / _ _ 95% Regression / confidence / ------- 95% Value prediction T I 3 4 5 ISC Magnitude Figure 1-17. Relationship between K-class and ISC reported magnitude in the Chukotka network. 60 lif— IJ- i7»- //»- MP Figure 1-18. Relationship between local magnitude and K-class as reported by Kondorskaya and Shebalin (1982). 61 K = 7.35 + 1.13 (M) (1-7) (Figure 1-19). However, the poor locations by the Western Alaska network result in an underestimation of magnitude for all but the January 16, 1982 event, as the epicenters were assumed to be too close to the stations. Use of only the January 16, 1982 event in the regression yields K = 4.58 + 1.73 (M) (1-8) which is probably the best determination possible given the available data set (Figure 1-20). This relationship is closer to the regressions determined for other regions discussed below. Overall, the difficulty in determining a relationship between K - class and magnitude for Chukotka may be a result of poor data. North_eas¥tRussi_a Test Network In the mid 1960's (1962 - 1967) a number of experimental seismic stations were established throughout northeast Russia to determine background seismicity levels to aid in developing permanent seismic networks (Mishin, 1967; Table 1-8). The distribution of seismicity located using this “test network” (Figure 1-21) was instrumental in site selection for future seismic stations, particularly in the Magadan region and Chukotka. Most of the temporary stations were deployed between six months and one year. Because only a few stations were deployed at any given time, the test network did not locate a large number of events. Hypocenters obtained from the test network are from Andreev (1967) and are included in the database. Arrival times and phase data for the recorded events are not available. 62 K-class 16 / Regression: 8 _ / —— K=7.35+1.13(M) / Goodness offit=71.5°/o / __ 95% Regression ,«- confidence .......... 9504’ Value prediction 1 2 3 4 5 6 Magnitude Figure 1-19. Relationship between K-class and magnitude in the Chukotka network. Magnitudes greater than 3.5 are from the ISC catalog, while those less than 3.5 are from the western Alaska network. The January 16, 1982 event is depicted as a square. ISC magnitudes are Mb, while those from the Western Alaska network are M1. 63 18 10a Regression: — K = 4.58 + 1.73 (M) Goodness of fit: 60.0% _ _ 95% Regression confidence 95% Value prediction l 5 .......... Magnitude Figure 1-20. Relationship between K-class and magnitude in the Chukotka network. Magnitudes greater than 3.5 are from the ISC catalog, while the remaining magnitude 3.1 event is from the Western Alaska network. Table 1-8. Seismic stations and station parameters from the temporary network established in northeast Russia in the mid 1960's. Parameters from Mishin (1967). . English f__°d Russ. Station Name Elapv. Date Date Code Open Closed AMG AMP Amguema 67.05 17 8. 88W 1 l. 65 4. 66 ANC AHC Anyuisk 68.34 161.56 10 6.64 3.65 BLG BJII" Balygychan 63.91 154.09 139 7.63 6.64 BL] BILS Bilibino-1 68.04 166.44 260 8.64 1.65 GRM FPM Garmanda 62.18 159.08 140 12.66 5.67 ILR I/UIP Ilimei 67.26 167.96 350 10.64 10.65 LMT JIMT Lamutskoe 65.54 168.85 178 4.65 10. 65 SMT IIIMT Mys Shmidta 68.88 179.38W 5 4.65 1.66 NKl HJIB Nel'koba 61.34 148.81 531 6.63 1.64 0L1 OMJI Omolon-l 65.25 160.52 260 12.63 1.65 081 OMC Omsukchan 62.52 155.77 527 1.63 1.64 VRN BPH O. Vrangelya 70.94 179.62W 10 2.66 4.66 PVK l'IBK Pevek 69.70 170.27 20 5.65 1 1.65 PVl I'IPB Provideniya-1 64.45 173. 18W 20 1.65 6.66 S-K C-K Srednekolymsk 67.46 158.71 30 4.64 12.64 STK CTK Stekol'nyi 60.046 150.730 221 7.64 5.66 U-B Y-B Ust‘ Belaya 65.51 173.28 20 11.66 5.67 SRD CPI! Ust' Srednikan 62.44 152.32 580 12.62 11.63 VNK BHK Vankarem 67.84 175. 85W 10 3.66 6.66 ZYl BPH Zyryanka-l 65.74 150.89 37 1.64 10.64 65 .Ahoozamz x838: ©8882 £3.3— amaosto: 2: .8 2.28% 3:58 95 azugflom ‘8 ._N-_ 2:5 66 Western Alaska Network The Western Alaska Network was Operated from January 1977 through June 1982 (closed for 1980), in the region of the Seward Peninsula, Alaska (Figure 1-22). The network contained 20 field seismic stations, and two permanent stations, Anvil Mountain (AVN) and Granite Mountain (GMA; Table 1-9), which were recorded on develocorder film in Nome, Alaska (Biswas et al., 1980, 1983). Granite Mountain was closed in April of 1978. A total of 1,010 earthquakes were located by the network, indicating high levels of microseismicity (Biswas et al., 1983). The largest earthquake located by the network was a ML 5.2 near Kotzebue (station KTA). Most events located are in the southern half of the Seward Peninsula and in the Kotzebue Sound region, with a lesser number in the Bering Strait and Chukotka. Focal parameters were determined using first arrival P times and S-P time differences. From July 1982 until 1998, the only seismic station operating in western Alaska was AVN (Figure 1-22). In 1998, a new seismic station was opened at Tin City, on the western tip of the Seward Peninsula. For database assembly, an attempt was made to supplement western Alaska data from 1981 and 1982 with arrival times from the Chukotka network, which was being set up at the time. Although seismo grams for more than 100 events were investigated, only a few useful arrival times were acquired, with most events showing only weak emergent arrivals. This is a result of the Chukotka stations being poorly optimized for good recording (amplification, noise, etc.). A few events located by the Western Alaska network were also located by the IPE station at lul’tin (ILT). Comparison of the epicenter locations indicates that the differences in locations are consistently greater than 100 km and in one case approaches several hundred 67 Mag < 4.0 o Mag 4.0 - 4.9 O Mag 5.0 - 5.9 O Mag 6.0 - 6.9 O Mag 7.0 + CHUKOTKA oft). ' ' -.NETyvoRK O 174° w Figure 1-22. Seismicity and seismic stations of the western Alaska network. 68 Table 1-9. Seismic stations and station parameters of the Western Alaska network. Parameters from Biswas et a1. (1980), and Biswas et al. (1983). Station Code Latitude Longitude Elevation Satellite Site Geologic Formations Name (m) Delay (560) Alder Creek ACK 66.083 162.195 377 Permafrost Metamorphic Anvil Mt. AVN 64.56 165.37 323 Metamorphic Besboro Is. BBO 64.12 161.30 244 Volcanic Cape Darby CDY 64.34 162.79 335 Volcanic Candle CDL 66.1 161.66 75 0.54? Siliceous Metasedimentary Creek Christmas CRK 64.67 160.53 680 Sedimentary Creek Devil Mt. DMA 66.30 164.52 238 0.54 Volcanic Ear Mt. EAM 65.92 166.24 701 0.54 Igneous Granite Mt. GMA 65.43 161.23 858 Volcanic Kogog KGR 63. 16 162.05 320 Volcanic River Kanguksam KGS 63.30 168.99 488 0.54 Volcanic Mt. Kookooligit KKL 63.59 170.37 655 0.54 Volcanic Mt. Kotzebue KTA 66.84 162.59 24 0.54 Permafrost Sedimentary North River NRA 63.89 160.51 107 0.54? Metamorphic Poovook PVK 63.44 171.55 411 0.54 Volcanic Remote REM 65.95 164.58 358 0.54? Basalt Savoonga SOV 63.65 170.45 198 0.54 Volcanic Stuart Is. STM 63.59 162.43 140 Volcanic Tin City TCY 65.56 167.95 72 0.54 Igneous Teller TLR 65.32 166.21 122 0.54 Metamorphic Topkok Pt. TPK 64.55 163.99 122 Volcanic Unalakleet UNL 63.89 160.67 122 Volcanic 69 kilometers. Because epicenters determined from the Western Alaska network data were determined using several stations, the assumption would be that locations from the Western Alaska network are probably better than the single station locations calculated from ILT data alone. This assumption is shown to be false when the arrival times from the Western Alaska network are used in relocating the events. Relocated events fall closer to the single station ILT locations than the Western Alaska network locations (Table 1-7). Origin times from the relocations are also closer and more consistent with ILT origin times. The relocations use original network picked Pn arrivals supplemented with secondary Pg and S g arrivals picked fi'om the original develocorder film seismo grams. Output from the relocations is given in Appendix D. Several things may have contributed to poor epicenter determinations in the Western Alaska network. First, only the first arriving P and S phases were picked. For close events, the first arrivals were not differentiated as to crustal phases (Pg and Sg) or mantle refraction phases (Pu and Sn). This ignores the second arriving, but useful, Pg and Sg phases. Second, many of the arrivals picked as first arriving P or S are actually mispicked later arriving Pg or S g phases. The mispicked phases were not identified in the location process and sometimes result in large errors when using a small number of recording stations. Third, many earthquakes have origin times that are later than the first arrival. This could be a result of bad time picks or a problem with the location procedure used. It appears that when determining locations, all events were run through the location algorithm and the resulting focal parameters were used without attention to individual events or regard to whether the solutions were reasonable. The preceding discussion calls into question the overall quality of the Western Alaska network locations. 70 Kamchatka Peninsula Network Klyuchi was the first seismic station to open on the Kamchatka peninsula in 1948, followed by PetrOpavlovsk in 1951. Most other stations in the network opened in the early 1960's and later (Figure 1-23; Table 1-10). Seismic station histories and parameters given here are less well researched than for the Magadan network because of their lesser impact on the study area. Because of subduction of the Pacific plate under the Kamchatka peninsula, this network monitors the most seismically active region in northeast Russia, as well as activity on many of the peninsula’s active volcanoes. The Kamchatka network also records seismic activity to the north of the subduction zone into the southern portion of the Koryak Highlands. As this study is interested primarily in non- subduction related crustal events, only earthquakes in the region north of 56° N (the northern edge of the Pacific subduction zone) are included. Approximately 5,000 hypocenter locations for the Kamchatka network north of 56° N were taken from the unpublished Kamchatka seismicity catalog for 1962 to 1996. For the Kamchatka network, Zemlet generally lists only events having a K-class greater than 8.5, although this does vary for some years. Earthquake locations were computed by hand up until 1978, when location procedures were computerized (Gordeev, pers. comm). Along the boundary with the Magadan network in the Koryak Highlands and in Shelrkhov Bay, many events were independently located by each network. There seems to have been essentially no exchange of phase data for anything but the largest teleseismically recorded events in these regions. Given a choice, the Magadan network hypocenter is usually preferred over that from the Kamchatka network because the Magadan network generally had better azimuthal station coverage. 71 O .1 2 AP 5‘ 1 ‘GR é? Ml A I‘AAnus a 35 RA” BER Mag < 4 O c? o Mag 4.0 - 4.9 51 fil/‘SKR Pacific Ocean 0 Mag 5.0 - 5.9 E O Mag 6.0 - 6.9 E O Mag 7.0 + 49° N 154°)? ‘ 114° E Figure 1-23. Seismicity and seismic stations of the Kamchatka network. Only seismicity north of 56° north are included. A few stations associated with volcano monitoring are omitted for clarity. 72 Table 1-10. Seismic stations and station parameters of the Kamchatka network. Apacha 52.925 157.131 2.90 APN AI'IX Apakhonchich 56.00 160. 84 700 —.64 —. 80 AVH AB‘I Avacha - old 53.07 158.5 —.63 10.76 AVH AB‘l Avacha - new 53.265 158.738 900 7.76 BER BP3 Berezovaya 52.27 158.433 —. 81 -.94 BKI BPH Bering 55.195 165.99 40 —.62 BPF (Nikol’skoe) BGC Bogachevka 54. 850 160.900 —.64 -. 65 BLC 53.193 158.800 —.76 CIR 56.127 160.725 1600 10/98 ESO 3CC Esso 55.925 158.700 490 -. 65 GNL FHJI Ganali 53.942 157.620 1200 1.88 GRL FPJI Gorely 52.552 158.080 1250 7.80 INS Institut 53.066 158.605 175 11.81 Vulkanologii KMN Kamenistaya 55.76 160.240 1 100 10.90 KAM Kamenskoe 62.456 166.210 —.94 KBT 56.208 162.819 200 10.97 KII KAP Karymski - old 54.030 159.480 790 7 .74 —.86 KPM KRY Karyrnski -new 54.036 159.449 900 9.89 KLY KJI‘I Klyuchi 56.313 160.852 80 —.48 2. 89 Kolokol'chik —. 67 -. 67 KRT Korito 55.966 160.222 1000 10.97 KRK KPK Koryak 53.292 158.636 1050 7.75 KZL K3JI Kozelskaya 53.201 15 8. 894 950 -.76 —. 84 73 Table 1—10 (cont’d). KOZ K3P Kozyrevsk 56.05 8 159. 872 40 -.61 9. 89 KZY Kozyr 56.070 159.900 450 11.89 KPL 56.592 161.296 1700 --.86 —.90 KRS Krestovskii 56.214 160.558 1200 7.87 KRI KPH Kronoki 54.596 161.134 5 8.66 KBG KBF Krutoberegovo 56.255 162.705 10 -.68 KB KPB LGN 56.083 160.690 2500 9.99 MED MIIH Mednyi 54.786 167.556 —.7 3 -.75 MIP Mal. Ipelka 52.276 156.758 370 8.97 MLK MJIK Milkovo 54.700 158.630 155 -. 62 -. 63 10.89 —.93 NLC HJI‘l Nalychevo 53.171 159.345 5 -. 67 —.67 3.69 12.69 3.84 OSS OCC Ossora 59.25 163.065 10 -.7 3 OZR O3P Ozero 54.692 160.392 10.66 -.7 7? PCH Pakhach 60.558 169.125 —.92 -.94 PAL Palana 59.093 159.963 -.94 -.96 PAU l'DKT Pauzhetka 51.467 156.810 110 11.61 PET I'IT P PetrOpavlovsk 53.024 158.650 150 -.51 PDK HIIK Podkova 56.140 160.780 800 —.83? RUS PYC Russkaya 52.432 158.507 75 12.87 SDL Sedlovina 53.278 158.884 1235 9.91 SEL CMJI Sernlyachik 54.12 159.98 11.61 7.74 SMA 53.263 158.801 1235 2.91 SPN IIIPH Shipunski 53.107 160.011 170 4.85 SRD 56.317 159.717 800 1.92 SVL IIIBJI Tsiveluch 56.583 161.225 900 10.80 74 _ Table 1-10 (cont’d). IE1. Tilichiki 60.433 166.075 —.94 -.96 TOP T011 Topolovo 53.230 158.041 155 11.61 -.93 TIUI I UGL 53.209 158.824 1140 8.92 UBL Ust' 52.842 156.308 20 11.61 —.64 Bolsheretsk I VDP BlIP Vod0padnii 55.770 160.220 1060 —.77 -.91 VKM Verkhene 54.627 158.473 170 10.66 —.71 Kamchatsk LZLN 56.018 160.804 1100 8.88 75 In comparing events listed both in Zemlet and the unpublished Kamchatka catalog, it is found that many events prior to 1968 and after 1983 sometimes list different hypocenter parameters for the same event. Events since 1983 have been relocated by the Kamchatka network to improve the uniformity of the locations and remove errors, which results in the differences between the catalogs for those years (Gordeev, pers. comm). Events prior to 1 968 may also have been relocated at some time in the past, resulting in a similar situation. '1‘116 velocity structure used for the Kamchatka network locations is taken from Kuzin (1973) and Kuzin et al. (1974). A linear regression relating K—class for the Kamchatka network to I SC magnitude was found to be K=3.74+ 1.61 (M) (1-9) Where K is K-class and M is magnitude. The regression and data are shown in Figure 1-24. Phase data for Kamchatka network events are not included in the database. \Arrlllr Regional Network The Amur network monitors seismicity between the Eurasian plate and the Amur plate- The Sakhalin network monitors seismicity along the boundary between the Amur plate and Okhotsk block. Unfortunately, this study has not worked directly with the the Amur network, thus first hand knowledge of operational procedures and unpublished data were not available. As reported in the Far East Bulletin, earthquakes are located graphically. \7 e10<:ities used by the Amur network for locating earthquakes are based on average \r e10 Cities for the Irkutsk region, assuming a crustal thickness of 40 km Velocities used are: Pg = 6.15 krn/s Sg = 3.55 lcrn/s Pn = 7.9 krn/s Sn = 4.4 krn/s 76 K-class / 0 Regression: / . —— K=3.74+1.61 (M) Goodness of fit = 57.4% .— _ 95% Regression confidence .......... 95% Vame prediction fir I 3 4 5 6 7 ISC Magnitude Figure 1-24. Relationship between K-class and ISC reported magnitude in the Kamchatka network. 77 Permanent seismic stations in the Amur network region first opened in the early 1970's (Table 1-11; Figure 1-25). Seismicity data from the Amur network comes primarily from the SSR catalog and the Far East Bulletin for events prior to 1979. Since 1979, event parameters are taken from the SSR catalog, Materialy, Zemlet, and the Far East Bulletin. Since 1982, many additional events, not found in other sources, which occured in the Amur region are available in the unpublished Yakutsk network bulletin. As with other networks, statistically poor events have origin times rounded to the nearest second and epicenters rounded to 0.1 degree when listed in the SSR catalog, Zemlet or Materialy. In the Amur region, SSR catalog reports event parameters the same as Zemlet, except K-class is converted to magnitude according to the relation K = 3.99 + 1.80(M) (1-10) where K is K-class and M is magnitude. This has essentially the same slope, but a slightly lower K-class axis intercept than the relationship between reported K-class and ISC magnitude (Figure 1-26) which was determined in this study to be K=4.69+ 1.83 (M) (1-11) Phase data from the Amur region is taken from the Far East Bulletin (1972, 1974, 1985-1988), Materialy (1979-1990), and unpublished Yakutsk network bulletins (1982- 1995). Some phase data from the Sakhalin, Yakutsk, and Irkutsk network stations are given in the published bulletins of Amur region events. The unpublished Yakutsk network bulletins also add a significant amount of phase data to that available in published sources for the Amur region. Hypocenters calculated by the Yakutsk network often differ greatly from those determined by the Amur network, which are found in published sources. Upon 78 Table 1-11. Seismic stations and station parameters for the Amur network. 325 Date Date 0611:. Cosed - 1 BMKS BMH Bomnak 54.705 128. 847 3.74 EKI EKM Ekimchan 53.067 132.945 485 1 1.79 GRZ FPT Gornotaezhnoe 43.70 132. 15 220 7.90 GRT GRD FPB Gornovodnoe 43.70 134.733 270 7.88 GRV GNY FPH Gornyi 50.762 136.455 500 12.78 KNN XHT Khingansk 49.122 131.192 520 7 .80 4.84 KNG KIRS KPC Kirovskii 54.428 126.983 440 4.74 KLDS K1111 Kul'dur 49.205 131.642 425 --.-— --.-— LZR .113P Lazarev 52.2 141.493 120 12. 80 NKL HKJI Nikolaevsk-Amur 53. 142 140.7 83 25 9.70 RMNS PMH Romny 50.855 129.4 210 10.78 «.87 TEI TPH Ternei 45.067 1366 30 7. 82 VLA B1111 Vladivostok 43. 12 131.893 75 «.29 --.31 «.34 YASS HCH Yasnyi 53.29 127.983 310 1.75 ZEA 3E3 Zeya 53.755 127.293 270 6.76 ZMN 3MB Zimniki 45.475 134.258 150 7.88 UNYS YHIO Unknown 79 121°E Mag < 4.0 o Mag 4.0 - 4.9 O Mag 5.0 - 5.9 O Mag 6.0 - 6.9 O Mag 7.0 + A OTEI ‘-45°N Figure 1-25. Seismicity and seismic stations of the Amur network. 11- / Regression: . // — K=4.69+1.83(M) / Goodness of fit = 67.1% / 10 ~ / .— _ 95% Regression ///’ confidence / """"""" 95% Value prediction 9 / l . 3 4 5 ISO Magnitude Figure 1-26. Relationship between K-class and ISC reported magnitude in the Amur network. 81 relocation of the events with the combined data set, parameters consistently match better with the Amur network locations. Sakhalin Island Network Seismic stations were first established on Sakhalin Island in the early part of the century by the Imperial Japanese government. These stations were closed in 1945 when Sakhalin was occupied by the Soviet Union following the end of World War 11. Russian network stations were first deployed in the late 1960's, which led to the establishment of the Sakhalin network headquartered in Yuzhno Sakhalinsk (Table 1-12; Figure 1-27). The Sakhalin network monitors seismicity along the boundary between the Amur plate and Okhotsk block. Seismicity for Sakhalin was obtained from Zemlet, the Far East Bulletin, and the SSR catalog. The Far East Bulletin contains many smaller events not found in the other catalogs. Deep focus events under southern Sakhalin Island due to subduction of the Pacific plate are not included in the database. Unfortunately, this study has not worked directly with the Sakhalin Island network, thus first hand knowledge of operational procedures and unpublished data were generally not available. As reported in the Far East Bulletin, earthquakes are located graphically with Circles on maps. The crustal model used for calculation of travel time curves uses a two layer Crust over the Moho. Total crustal thickness is 33 km, with the Conrad discontinuity at a depth of 15 km. Velocities are reportedly taken from J effreys (assumed J effreys and Bullen, 1940. based on context). Velocities used are: Pg = 5.6 km/s Sg = 3.3 km/s P* = 6.4 km/s S* = 3.9 km/s Pn = 7.75 km/s Sn = 4.42 krn/s 82 Table 1-12. Seismic stations and station parameters for the Sakhalin Island network. English Russ. Station Name Long Date Date _Code (in)V O . - Closed Bykov 47.317 142. 567 6. 68 10.68 ESU Esutoru 49.083 142.033 100 12.39 «.45 (Uglegorsk) Firsovo 47.65 142.567 20 8.79 11.79 Gornozavodsk 46.567 141.85 50 12.71 6.72 Korsakov «.51 1.52 Kotikovo 49.133 144.25 10 7.69 9.70 Lesogorsk 49.442 142.2 40 7.69 10.69 Lopatino 46.6 141.825 40 4.69 10.69 Moneron 46.258 141.25 40 9.71 5.72 Nizhnii Armudan 50.817 142.533 150 9.66 4.69 HKJI Nogliki 51.817 143.15 25 10.64 12.64 «.88 NW I-IBP Nyvrovo 54.317 142.617 5 11.81 Ogon'ki 46.775 142.383 70 6.68 9.68 OKH OXA Okha 53.55 142.933 24 12.58 «.65 Okha (New) «.65 OCT Ootomari 46.65 142.767 36 «.09 «.45 (Korsakov) OTI Otiai (Bykov?) 47.325 142.783 20 2.34 «.45 Ozhidaevo 47.033 142.392 220 «.76 «.77 Pravda 46.942 142.008 40 9.71 11.71 Shebunino 46.433 141.858 40 9.71 11.71 SKK Shikka (Shikuka) 49.233 143.117 2 «.28 «.45 (Poronaisk) Sovetskaya Gavan 48.967 140.283 50 6.69 11.70 83 Table 1-12 (con’t). Tikhrmnevo 49.2 142.9 150 6.69 10.69 Toyohara «.43 «.45 (Y uzhno) TYV TMC Tyrnovskoe 50. 85 142.65 100 4.69 ms 3 UGL YI‘JI Uglegorsk 49.083 142.083 20 -.51 Utesnoe 46. 6 143.075 20 7.73 9.79 Vzmor'e 48.85 142.517 20 7.82 12.82 Yablochnyi 47.167 142.067 20 6.68 9.68 YSS 1 Yuzhno 47.02 142.717 40 10.47 «.57 Sakhalinsk (Novo Aleks-androvsk) YSS IOCX Yuzhno 46.958 142.762 100 «.57 Sakhalinsk 84 L . 57° N YAKUTSK NETWORK Mag < 4.0 0 Mag 4.0 - 4.9 ""'"'" O Mag 5.0 - 5.9 O Mag 6.0 - 6.9 O Mag 7.0 + X C. .39. r' rn 0 7: I O ..r (.0 X Z - m .4 E O 33 X 45° N 139° E 1470 E Figure 1-27. Seismicity and seismic stations of the Sakhalin network. 85 A linear regression relating K-class for the Sakhalin network to ISC magnitude was found to be K=3.84+ 1.31 (M) (1-12) where K is K-class and M is magnitude. The regression and data are shown in Figure 1-28. Phase data, although available in the F or East Bulletin, was not entered into the database of this study. Earthquakes located by the Sakhalin network use data from the Amur network, and generally have a large number of P* and 8* phases reported, indicating that Sakhalin Island may have a well developed Conrad discontinuity. Lrl_9 6o: com: m co: m— oo: m— ..on 4454.1 113.434... 3 _ 2880 Shock $909.00. «notcfie 2 cm ...»o .90»? z . .4coi._ . .1. mm. mm I I . .o 1 .... wuuunufin r .... «1.3930 . Wuwwaovratpwt . .. .9 . . mEo>m >3. «6 . . . . -. . Sm. 3er 2.81 -0-.- 1.11 <¥m< < ‘1' 2880 News; /r 104 Number of Events 180 TOTAL NUMBER OF EVENTS DAY = 1677 NIGHT = 1815 5 10 15 20 ~,11LOCALDAY _. LOCAL NIGHT Hour of Day (UTC) Figure 2-3. Histogram of event origin times from aftershocks of the 1989 South Yakutia earthquake. Note a slight bias towards "nighttime" events. 105 fl 1ij . ,g several seismic stations. Black areas on Figure 2-2 represent regions where more than 65% of the seismicity occurs during local day. Many of the regions with predominantly “daytime” events are associated with discrete clusters or trends of seismicity, most of which can be associated with mining or construction related blasting. Several clusters of reported seismicity in the Amur region have more than 90% of the events occmring during local day. A few cells with predominantly “daytime” seismicity do not correlate with obvious identifiable explosion sources. These locations are probably a result of random statistics of small numbers as these cells generally are close to the 10 event cutoff. We have examined several regions of “daytime” bias which can be positively related to explosion contamination. As a cautionary note, it should be mentioned that some events occurring at night may also be explosions. Russian law requires that explosives loaded into boreholes can not be kept overnight, but must be detonated. This may result in some “nighttime” blasting, depending on the work schedule of the particular mine. This is supported by a limited number of “nighttime” blasts listed in the unpublished network bulletins. The numbers of “nighttime” blasts are small when compared to those occurring in the “daytime”. Numbers for the regions listed below correspond to numbered locations on Figure 2-2. 1. Amur District The clearest region of explosion contamination is the Amur District. When plotting “daytime” and “nighttime” epicenters separately (Figures 2-4 and 2—5 respectively), we see some distinct differences. There are several large clusters of “daytime” seismicity that correlate geographically with specific mining regions. For these regions, we also identify contamination level changes with season. Figures 2-6 through 2-18 show the number of events that occur during 106 .8380 some owes 5:3 388% Be oé 8:: .88on oeeamemeE .«o 9:05 @2882 33288823. 8:: mew .93: 08 £3, 82388 mm 33:8 .=8<-_8=am 05. .8382: w=_e:o%otoo .6 88mm E 8.16% as moxoe >an 8 888880 .8 823.0 05 oo 8585 3588a. 82%: SE< on. .8 568mg $85.30.. dim 883m a own: a 6.82 momma 107 Z com 0 018930 o 8m dammed 38¢. 05 no 56888 ..oEEfiE: .n-m 8% E moow~ ......... 108 specific hours of specific months of the year. Figure 2-6 analyzes a reported cluster of seismicity in the northern part of Figure 2-4. The seismicity in this region is believed to be tectonic, as the region is unpopulated, and there is no known mining or development in the area. The analysis of this region can therefore be used as a baseline to which other analyses can be compared. The area analyzed contains 399 located earthquakes, with 189 having occurred during local “daytirre” and 205 during local night. This corresponds to a day/night ratio of 0.922, which is essentially the same as that found for the 1989 aftershock sequence discussed above. The numbers of sumrrer and winter events occurring in this region are also similar. In southern Amur, analysis of reported seismicity around the Raychikhinsk coal mining region shows activity only during winter months and daylight hours (Figure 2-7). This is somewhat different from the nearby Khingansk mining region, where events occur throughout the year instead of being restricted to winter months (Figure 2-8). Figure 2-9 corresponds to mining or quarries in the region around Komsomolsk’ na Amur. On average, origin times of events in the Raychikhinsk, Khingansk, and Komsomolsk’ na Amur center around 05 hours UTC, which corresponds to 1 pm local time. Mining also occurrs about 5 km to the north of Chegdomyn, which accounts for the high level of “daytime” seismicity in the region (Figure 2- 10). However, here origin times average slightly earlier at around 04 hours UTC, or 12 noon local time, and there is a shift towards summer months. It is unknown what is being mined here. Temporal analysis of the reported seismicity in the region of Svobodniy (Figure 2-11) is also consistent with sources of anthrOpogenic origin. However, the existence of mining in this region is unclear from the available maps. Events here may also show a slight bias toward summer months and slightly later origin tines averaging 07 hours UTC, or 3 pm local time. To the north of Svobodniy are mining regions near Shimanovsk (Figure 2- 12) and Oktyabrskiy (Figure 2-13). 109 Tog] Events = 399 Winter Day = 90 Winter Night = 105 Summer Day = 99 Summer Night = 105 _L N _L O siueAa 10 iequmN Figure 2-6. Temporal variation of probable tectonic earthquakes. 110 Total Events = 121 Winter Day = 109 Winter Night = 2 Summer Day = 9 Summer Night: 0 ewe/El 1° ieqtunN _. Figure 2-7. Temporal variation of reported seismicity in the Raychikhinsk mining region. 111 Total Events = 161 Winter Day = 80 Winter Night = 8 Summer Day = 68 Summer Night = 5 ewe/G 10 JeqwnN CD Figure 2-8. Temporal variation of reported seismicity in the Khingansk mining region. 112 Total vents = 63 Winter Day = 36 Winter Night = 2 Summer Day = 19 Summer Night = 6 A siueAa 10 requmN N 10 001/7 12 Figure 2 9 Temporal variation of reported seismicity in the Komsomolsk' na Amur mining region. 113 Total Events = 53 Winter Day = 22 Winter Night = 3 Summer Day = 24 Summer Night = 4 O) urnN rs C" (D q C -h 2“ 02 23 ....- (D Figure 2-10. Temporal variation of reported seismicity in the Chegdomyn mining region. 114 Total Events = 43 Winter Day = 18 Winter Night = 1 Summer Day = 22 Summer Night = 2 0) mm] .h siueAa 10 19C] N 084‘} 8 " ’ 15 K . . 8‘1 '14 , ' 20 0‘0 00’7) 12 _ afifiidfi Figure 2-11. Temporal variation of reported seisrrricity in the Svobodniy region. 115 Total Events = fl Winter Day 42 Winter Night 0 Summer Day = 22 Summer Night = 0 UJnN 03 A SIUGAB 10 ieq N Figure 2—12. Temporal variation of reported seismicity in the Shimanovsk mining region. 116 Total Eveng = 143 Winter Day = 106 Winter Night = 15 Summer Day = 14 Summer Night = 8 siueAa 10 WWW. Figure 2-13. Temporal variation of reported seismicity in the Oktyabrskiy placer mining region. 117 Mining near Shimanovsk occurs approximately 40 km to the northeast of the town, along the west bank of the Zeya river. Events located in this region occur exclusively during daylight hours, but with a slight bias towards mid winter. For Oktyabrskiy, extensive placer mining has occurred in the vicinity of the town, primarily to the north, southeast, and along the Gar’ River valley to the south. Analysis of origin times for the Oktyabrskiy mining region indicate that almost all blasting occurs during the winter “daytime”. Each of the reported seismicity regions analyzed in Figures 2-6 through 2-13 appear as isolated or partially isolated discrete clusters or trends of events. Based on the strong biases of origin times, it is believed that essentially all reported earthquakes in these clusters are of anthropogenic origin due to mining. To the northwest of Oktybrskiy is a seismicity cluster near the town of Taldan (Figure 2- 14). Prior to 1981, a small number of earthquakes occurred in this region with no bias in origin times, indicating this cluster probably contains some natural seismicity. Beginning in 1981, the numbers of events rose drastically, and a strong “daytime” bias was introduced into the origin times. Although this is strongly suggestive of the commencement of mining operations, no mines are indicated on the available maps, which are dated 1986. Godzikovskaya (1995) reports two explosions as having occurred within the bounds of this chrster. The low level of natural seismicity appears also to have continued, with a magnitude 5 .0 event occurring during the night of January 31, 1985. Explosion contamination due to dam construction in the Zeya basin region to the east (approximately 54° N x 127° E) has been noted and is discussed at length by Godzikovskaya (1995). Temporal analysis of the region around Ekimchan is also consistent with a mix of tectonic events and explosions (Figure 2-15). There are a total of 283 events in this region, with 92 occurring during the “nighttime”. If we assume the number of“ ytirre” tectonic events is 92% of the “nighttime” level (85 events; percentage based on the temporal variation of 118 Total ven = 224 Winter Day = 119 Winter Night = 19 Summer Day = 105 Summer Night = 14 glue/\a 10 JeqUJnN C) Figure 2—14. Temporal variation of reported seismicity in the Taldan region. 119 Ifgtal Even§ = 3 Winter Day = 123 Winter Night = 47 Summer Day = 68 Summer Night = 45 _.L 00 O N O) srueAa :0 JeqwnN _. A Figure 2-15. Temporal variation of reported seismicity in the Ekimchan mining region. 120 tectonic events discussed above), then the total number of tectonic events in this region is about 177. Therefore, the nmnber of explosions is 106, representing a 37 % contamination of the database. In the north-central portion of Figure 24 there is a northwest-southeast trend of predominantly “daytirre” seismicity extending several htmdred kilometers. The temporal distribution of these events is shown in Figures 2-16 for the central segment and 2-17 for the northern segment. This trend correlates with the route of the Baikal-Amur Mainline railroad (BAM). Explosions associated with its construction in the 1980's are listed as earthquakes in the seismicity catalogs. Note also that most events are located to the west of the track in the central portion, and to the north of the track in the northem portion, indicating systematic errors in the location procedure. Tle region covered by Figure 2- 18 is somewhat more problematic. This region probably does has tectonic earthquakes, as there are a reasonable number of “nighttirre” events, but there is a clear bias towards “daytime” events. Here, tlere seem to be two possible explanations. A branch of the railway extends south from the town of Tynda Construction along this segment of the railway may account for the “daytime” events. However, tle Tynda region events are biased more towards early winter than those along the mainline railway, which tend more towards summer (Figures 2-16 and 2—17). This may suggest a mining origin as opposed to railroad construction, but maps do not show any apparent mining activities. Overall, as explosion contamination appears to be primarily confined to daylight hours, “nighttime” seismicity should better reflect the natural tectonic distribution of earthquakes (Figure 2-5). If one compares “daytime” and “nighttime” seismicity, a different, more northerly trend appears in the plotted ‘highttirne” earthquake epicenters. This probably delineates an active 121 tal Events = 140 Winter Day = 73 Winter Night = 7 Summer Day = 51 Summer Night = 9 mm a, .b srueAa to 130 N Figure 2-16. Temporal variation of reported seismicity along the central segment of the Baikal-Amur railway. 122 T Even = 6 Winter Day = 80 Winter Night = 14 Summer Day = 75 Summer Night = 17 o JeqwnN m A O) srueAa l N Figure 2-17. Temporal variation of reported seismicity along the northern segment of the Baikal-Amur railway. 123 T tal ven = 339 Winter Day = 162 Winter Night = 33 Summer Day = 114 Summer Night = 30 srueAa 10 JeqwnN 0) Figure 2-18. Temporal variation of reported seismicity in the Tynda region. 124 tectonic feature that was previously obscured by clusters and trends of explosions. When observing locations of the larger teleseismically located events, it is found that they fall almost entirely within the regions where seismicity occurs at night. Clusters and trends of primarily “daytime” occurring events have very few earthquakes of magnitude 4 or larger. In summary, for the Amur region it is clear that any future studies of seismicity or neotectonics must consider the extent of explosion contamination in the seismicity catalog. 2. Polyarnyi-Leningradsky—Plarnennyi Polyamyi and Leningradsky are placer gold deposits located along the coast of the Chukchi Sea in Chukotka. Plamennyi was a mercury deposit about 100 km south of Polyamyi which was mined from 1967 to 1972 (Pilyasov, 1993). From 1966 to 1982, most of the events located in this area were single station locations obtained by the three- component seismic station at lul’tin (ILT; Figure 2-19). A clear bias towards winter and “daytime” is evident for the cluster of events in the mining region (Figure 2-20). The ILT data analyzed here also contains 15 events located by the Magadan Chukotka network in the 1980's which are also consistent with explosions both in origin time and location. Several of the located events near the Plamennyi mine occured after the 1972 close date of the mine. There are two possible explanations for this. First, the location accuracy of the ILT epicenters is on the order of 50 or 100 km, thus events from Polyamyi or Leningradsky may be mislocated to the south Second, the unpublished Magadan bulletin contains explosions as having originated at Plamennyi, suggesting on—going exploration work or a limited resumption of mining activities. Comparison of origin times of ILT located events with the more recent known explosions from the same mining region yields a nearly identical 125 .888 Emto 8e>e oo max—«8w 8382 8 wow: :2on 82888 .38 woes—E .8238— 088 :85 58853 A p.15 8:3 .2-N Emmi PBS . ...»... o .. sauce—WEE.— .. o a... M ' k. 126 —L N SerAE 10 JeqwnN To 1 v n = 103 Winter Day = 61 Winter Night = 20 Summer Day = 11 Summer Night = 11 -.L O 10 , 20 .45“ 2 0’7’6 1 Figure 2-20. Temporal variation of reported seismicity in the Polyamyi Leningradsky, and Plamennyi mining region. 127 temporal distribution, with blasting primarily in the daylight hours of late winter and spring (Figure 2-21). Note also the low level of “nighttime” blasts. The complete lack of teleseismically recorded events around Polyarnyi-Leningradsky-Plamennyi as compared to the region a few hundred kilometers to the southeast is also consistent with explosions (Figure 1-26). Previous authors have attempted to use the explosions reported in tectonic models (Lander, 1996) and assessment of seismic risk (V. Kovalev, pers. comm), both of which illustrate the magnitude of the contamination problem in the region. Other earthquakes located by ILT (Figure 2-19) do not show any temporal bias, thus probably generally represent tectonic events. 3. Kolyma Gold Belt A cross of predominantly “daytime” seismicity lies along the Kolyma gold mining belt (Figure 2-2). Tectonically, this region is extremely complex in that it is located just south of the Ulakhan Fault system (Figure 2-22) along which motion occurs between the Okhotsk block and the North American plate (Imaev et al., 1994), thus statistical separation of anthropogenic sources from tectonic events is more difficult. Mining in this region is primarily placer gold but also includes coal and other minerals. “Daytirre” and “nighttime” seismicity are shown in Figures 2-22 and 2-23 respectively. Temporal analysis of the large cluster of events in the area northwest of the town of Susuman indicates a bias towards local day and winter/spring (Figure 2—24). This bias is consistent with the distribution of known explosions from the Magadan Seismic Bulletin for the Susuman region (Figure 2-25). Based on the Day/Night seismicity plots in Figures 2-22 and 2-23, the eastern half of the seismicity cluster is entirely explosions, while the western half probably contains some tectonic events. 128 ...—..-—- ’ ‘ ANNNNNODOD momeomom CDCDO NmoN-h-O) g SlUGA'fl 10 JGQUJHN Figure 2-21 T . ' . ernporal Variation of e 1 ' and Plamennyr mining regiOns. xp osrons from the Polyamyi, Lenm' E 1 ky 129 88:25 858a 38888 10 wet 302 80388888 824.298 he meflwoe 650 can 38: Ewto 805 me 33:88 8383 8 tom: 82w8 2868 983 “6825 33 Bow «8.20M 05 mo .9688“; $83.35.. .NN-N oeswfi m OWWM l1 0 e m OMV~ z 00 O 00 O. I... 00 O O O 0 fl .0 O O\ 0” v O ‘ O O O .0 O. O . O to” co 0. o o 0 END 0 o .. ”no... 0 .. w .. «EEO! gomobemzd. q; o o no a 0 6.3.0.“... .. .8 I D 0.00001. a q... . 8 ... c .. . . ... . 0000 e. o. e o 0.00. o o V\V 130 .NN-~ 083m 3 magma; 335 2385 don Eow «830M 9: no 36833 $88.32.. .mN-N 25wE mom: ZOOO \ in o o o. u . .....u... . ...“... w... .. ~ .. O 0000 0 ...-I . 39.32 8 a. «826. . «4 —‘. kw .88.: 3m m cm: 131 .m~-m 25wE 8 82w“: 33% 23800 .zon Bow «Ex—GM 05 .8 58838 ..oEEFEZ: .mN-N ouswi m owe m cm: zooo x ..w . . .. 131 Total Eveng = 278 Winter Day = 124 Winter Night = 46 Summer Day = 92 Summer Night = 16 siueAE] i0 JeqwnN C” Figure 2-24. Temporal variation of reported seismicity in the region northwest of Susuman. 132 glue/\a l0 JequmN 120 f ..... 11o " , 100 - mCO 8300 Figure 2-25. Temporal variation of explosions in the Susuman region. 133 Note also the increased numbers of “daytime” events within a 100 kilometer radius of Susuman, which is also probably a result of mining. About 200 kilometers southeast of Susuman is the Kolyma hydroelectric station and dam Blasting during construction of this dam resulted in contamination of the earthquake catalog (Godzikovskaya, 1995). Temporal analysis of explosions in the Kolyma damregion is shown in Figure 2-26. There are also highly elevated levels of “daytime” seismicity for approximately 100 km upriver from the dam, and a diffuse cluster of primarily “daytime” events approximately 120 km to the east. The cluster to the east is in the vicinity of the Ust’ Srednikan dam project, where Godzikovskaya (1995) also notes explosion contamination. One puzzling cluster of seismicity is associated with the region around the town of Kulu (Figures 2-22 and 2-23). This cluster is biased towards winter months, but only slightly towards daylight hours (Figure 2-27). However, this seismicity cluster is from events which occurred in the 1970's and 1980, with a few events occurring each year. In 1980, when the seismic station at Kulu opened, the earthquakes essentially stopped, with only a few events located since (See yearly plots in Appendix B). This would be consistent with explosions, as the local operator could distinguish the events as explosions and they would be removed from the catalog. For comparison, temporal analysis of known explosions from the Kulu region are shown (Figure 2-28). Explosions in the Kulu region are probably a result of mineral exploration studies. On site inspection of the Kulu region showed no evidence of mining. The closest mining noted was at Matrosova, about 40 km to the south (Figure 2-22). From Matro sova, there is a small lineation of “daytime” events extending about 70 km to the southeast (Figure 2-22). This trend parallels an extensive placer mining operation extending 134 flfgtal Eyegts = 36 Winter Day = 22 Winter Night = 4 Summer Day = 6 Summer Night = 4 umN 4s siueAa l0 ieq ‘{ CA 8 1 5 5" I140 10 fig ' 0&9?" 0’7) 3600‘ Figure 2-26. Temporal variation of seismicity from the region of the Kolyma dam. 135 Total Events = 136 Winter Day = 62 Winter Night = 43 Summer Day = 13 Summer Night = 18 .5 siueAEl l0 JeqwnN Figure 2-27. Temporal variation of reported seismicity in the Kulu region. 136 SIUGAE 10 leqU-mN Figure 2-28. Temporal variation of explosions in the Kulu region. 137 from Matrosova to Nelkoba, which follows the main highway through the region. This may indicate a location bias for events in this region. Explosion contamination of the seismicity catalog has clearly affected analysis of seismic hazards in the region. Vazhenin et al. (1997), citing T. A. Andreev, shows increased seismic hazards in the regions north of Susuman, around Kulu and the trend extending south, and near the Kolyma hyodoelectric station, all of which are a result of explosion contamination. 4. Ust' Nera Placer gold deposits around Ust' Nera are on the northwest extension of the Kolyma gold belt. As indicated on Figure 2-2, seismicity reported in the vicinity of Ust' Nera is temporally biased towards “dayt'nne”, with 10 “daytime” events, versus 2 “nighttime” events, although statistics of small numbers may be a factor. The region immediately to the southeast of Ust' Nera is very active tectonically, where events of up to magnitude 7 have occurred. 5. Lazo Lazo is a gold placer deposit between the Adycha and Nel'gese Rivers at 66.5° N, 1370" E (Figure 2-2). “Daytime” and “nighttime” seismicity of Lazo and northern Yakutia are shown in Figures 2-29 and 2-30 respectively. The reported cluster of seismicity near Lazo is almost entirely confined to the “daytime” in the early part of the year, thus it is believed they are almost exclusively explosions (Figure 2-31). A similar looking cluster of seismicity about 100 km south of Lazo was also analyzed for temporal variation (Figure 2- 138 .88: Ewto 803 he $9228 8882 8 new: 82on 2868 mg non—«5 883:; 8055: Ho 36888 $8839. .m~-N oSwE m owfl o. ‘oo 0 139 .88: 8E8 80>». .8 max—«8“ 8888 8 wow: 82on 2368 32a 3925 due—«V 808.5: Mo 56888 $8832.. .om-m Sawfi m 06% non . WOWN~ Emu—3.8%“ ,. . .y. 140 Total Events = 47 Winter Day = 33 Winter Night = 7 Summer Day = 4 Summer Night = 3 LUHN 4s ewe/G l0 ieq N 4400”) 10 12 Figure 2—31. Temporal variation of reported seismicity in the Lazo mining region. 141 To Events = 49 Winter Day = 14 Winter Night = 11 Summer Day = 13 Summer Night = 11 UJHN 4:. glue/G] l0 leq N Figure 2-32. Temporal variation of reported seismicity in the cluster of epicenters south of the Lazo mining region. 142 32). This cluster of seismicity was suggested to be of mining origin (V. Imaev, pers. comm), though the temporal variation is more consistent with tectonic activity. 6. Deputatsky The Deputatsky tin mining region is completely biased to winter events, with 13 events reported during winter day and 2 events during winter night (Figure 2-33). There are no events reported in the summer months, which is a good indicator of an anthropogenic origin However, caution must be taken as the number of events is small. Temporal analysis of an additional small cluster of seismicity immediately to the north is more consistent with tectonic events. 7. Kular The region around Kular and to the north is an extensive placer gold mining region. Coal has also been mined southeast of Kular along the banks of the Yana River. The coal mining operations closed in the past few years but in the mid-1990's gold mining Operations began south of Sevemyi, about 25 km southeast of Kular (V. Imaev, pers. comm). Reported seismicity in the Kular region forms an elongated north-south trend (Figure 2-29), which shows an excellent correlation with explosion locations (Figure 2-1). Temporal analysis of reported seismicity in the Kular region shows a strong bias to winter “daytime”, which is consistent with explosion contamination from mining (Figure 2-34). 143 Total Events = 15 Winter Day = 13 Winter Night = 2 Summer Day = 0 Summer Night = 0 UJnN A srueAa 1019‘] Figure 2-33. Temporal variation of reported seismicity in the Deputatsky mining region. Total Events = 45 Winter Day = 23 Winter Night = 5 Summer Day = 14 Summer Night: 3 O) umN A ewe/El lo req [0 N _. . 10 or (A _ . ’K 8 ' ‘ r 15 \l\0 I14 10 ‘ ‘ \ < \96 0% ’ 20“\6:(o°‘ ° Figure 2-34. Temporal variation of reported seismicity in the Kular mining region. 145 8. Stolb Reported seismicity in the Stolb region is probably contaminated by “daytime” explosions. From 1988 to present, the geological survey in Moscow has been conducting explosions in this region along the Lena River looking for placer deposits of diamonds (V. Imaev, pers. comm). Although there are only 12 events that fall within this region, 9 occur during daylight hours, and 8 occur between 1989 and 1994, which is consistent with the exploration work (Figure 2-35). 9. Yugorenok Yugorenok is a mining region along the Yudoma and Allakh-Yun rivers (V. Imaev, pers. comm), which is geographically coincident with a discrete cluster of earthquake epicenters. Of the 25 earthquakes located in the vicinity, 20 occur during winter “daytime” hours (Figure 2-36). In addition, there are no large events associated with this cluster of seismicity. Two other small seismicity clusters to the north are both associated with teleseismic events and do not show any strong temporal biases in their origin times, which suggests tectonic origins. 10. South Yakutia South Yakutia is similar to the Kolyma gold belt in that there are tectonic events occurring in the vicinity of mining regions. Several cells in this area show strong “daytime” biases, each of which is associated with mining. Figures 2-37 and 2-38 plot the “daytime” and “nighttime” seismicity of southern Yakutia. Three regions are of note in south Yakutia. Aldan is a mining region with extensive deposits of gold and phlogopite mica (Shabad, 146 um srueAa lo ieq m N Total Events = 12 Winter Day = 5 Winter Night = 3 Summer Day = 4 Summer Night = 0 Ont/7 10 ,. r‘ S 7 10 966‘} 15' ’1 Oa‘lk . . ‘\ 12 20§eio°w Figure 2-35. Temporal variation of reported seismicity in the Stolb region. 147 MM Winter Day = 20 Winter Night = 2 Summer Day = 2 Summer Night = 1 UJHN A srueAa l0 ieq N o 5 .7 . 10 0‘*4 Ch K 15 9'“ KO 0’7) 12 990‘ Figure 2-36. Temporal variation of reported seismicity in the Yugorenok nining region. 148 .88: 888 8o>o mo 29:85 8888 8 com: 82%: 2888 £88 888m .888; 80:88 .8 58883 $8880.. Swan. 8&5 149 .38: 888 803 .8 mam—«ca 88888 8 wow: 82on 8888 much 888m .83—Sr 808:8 8 80883. 888882.. .wm-m oSwE O o, no. =c>oxommoo . O 150 1969). The region is associated with a diffuse cluster of predominantly “daytime” seismicity (Figure 2-39). Although the seismicity occurs throughout the year, its concentration during daylight hours is more consistent with an anthropogenic source than tectonic. Explosions from the Aldan mining region are located and listed in the unpublished Yakutsk bulletin (Figure 2-1). Approximately 200 km south of the Aldan mining region is an extensive coal mining region near Chul’man. The Chul’man mining region produces many explosions (Figure 2-1), some of which are located by the Yakutsk network and listed in the unpublished data. The seismic station at Chul’man seems able to identify and filter the explosions. A plot of the temporal distribution of reported earthquakes (Figure 2-40) shows a bias towards ‘nighttime” events similar to that found for the tectonic event test area and the 1989 South Yakutia earthquake aftershock sequence (Figures 2-3 and 2-6). To the northeast of Chul’man is a dense cluster of seismicity near the settlement of Spokoyny (Figure 2—37). Temporal analysis of the cluster shows a strong bias towards winter “daytime” events, which would be consistent with placer mining (Figure 2-41). Soviet military 1:200,000 scale tepographic maps (dated 1986) show extensive mine workings in the region, but list all settlements as uninhabited. This is inconsistent, as the events located here occur from the 1970's through the mid 1990's. The published literature makes no mention of any mining activity in this region, nor does the unpublished Yakutsk network bulletin locate anything identified as an explosion in the region. Overall, the nature of activity at this location remains unclear, but is suspect. 151 Tota Events = 51 Winter Day = 18 Winter Night = 4 Summer Day = 21 Summer Night = 8 O) o ieqwnN A srueAa l N Figure 2-39. Temporal variation of reported seismicity in the Aldan mining region. 152 Total Events = 103 Winter Day = 25 Winter Night = 25 Summer Day = 20 Summer Night = 33 _L N O 00 O) 1 Z r: 3 cr 9, g r _ J. .. C O O O. o z Gm «Q a oo o O 0 0V : O .... “...... . . D... ...... ..... a...” .. O .0 ”000‘... 00. O z 8 ... .... . 158 with three magnitude 4 events. The two eastern events may correlate with the N-S trend, while the western event could be a large explosion associated with the nearby mining. This western event was a “daytime” occurring magnitude 4.1 event, but is listed as such only in Zemlet, with no entry in the ISC Bulletin. In the central part of the Amur region, the South Tukuringry fault, as mapped, also shows no correlation with seismicity. However, an extension of the northern portion of the fault is co—linear with a well defined trend in the “nighttime” seismicity. Overall, mapped faults in the southern portion of the Amur region do not correlate with tectonic microseismicity and thus may not be active. Faults mapped in the northern Amur region and southern Yakutia generally correlate with “nighttime” seismicity, consistent with tectonic activity. Clear seismicity trends are associated with the Atugey-Nuyam fault, various unnamed faults, and the intersection between the Atugey-Nuyam, Gilyui, and Avgenkur faults. Many of the faults mapped in this region show clear lineations on topographic maps and meteor satellite images. Given the distribution of seismicity in the region, it is clear that many additional active faults exist; lineaments associated with some of them are visible on Figure 2-45. Focal mechanisms from the Amur region are found in Koz’min (1984), Parfenov et al. (1987), and the Materialy and Zemlet catalogs (Figure 2-45; Table 2-2). Focal mechanisms in the northern part of the Amur region are generally consistent with a left- lateral transpressional boundary. Focal mechanisms in the central Amur region are concentrated along the north— south seismicity trend between 132 and 133 degrees east. These mechanisms are predominantly northeast - southwest thrusting events, although individual mechanisms vary somewhat. In some cases, the orientations of the planes can be adjusted as they are constrained with little data. The north - south seismicity trend may therefore 159 deco: >228£8 S cows 238 BEBEBEB new gogflom 895$: :23 :2on 5:14. 2: no 958288 Econ firm Bamfi Zowsv \. m owm: m Gwyn“ a com: a coma 8r womfi < ...—0 o 0 one. U 00' o o o o o . .... no 0V- 0 O of ..v o 160 Table 2—2. Focal mechanisms of the Amur region Planes are given as Strike - Dip - Rake. References are: KOZ - Koz’min (1984), PAR - Parfenov et al ( 1987), CHU - Chung et a1 (1995), PDE - USGS Preliminary Determination of Epicenters, ZEM - Zemlet, and MAT - Materialy. DATE ORIGIN LAT LONG MAG PLANEl PLANE2 REP 78 08 21 10 15 54.4 55.22 124.80 . 292 63 - 065 36 — KOZ 89 07 23 12 01 30.0 54.50 125.05 4.7 006 32 118 154 62 74 ZEM 73 11 02 07 31 32.9 54.04 125.75 4.9 354 79 - - 248 35 - — KOZ 86 08 15 20 20 34.4 48.93 126.45 5.2 116 63 — 12 212 79 -153 PDE 72 06 13 10 45 03.2 54.91 126.46 4.9 026 80 — 296 89 — KOZ 86 02 28 17 07 24.4 48.64 126.66 4.9 179 84 97 309 9 41 ZEM 72 08 09 20 51 51.8 56.84 127.41 4.7 116 30 122 260 65 63 KOZ 77 08 16 13 56 59.8 53.93 128.7 4.2 022 90 - 112 86 - PAR 79 04 27 19 38 18. 55.94 130.17 4.6 234 63 59 106 41 134 KOZ 77 11 01 03 54 26. 55.41 130.52 4.5 225 90 152 135 62 180 KOZ 63 06 21 13 44 20. 47.91 130.61 5.3 134 85 8 043 82 174 CHU 82 03 10 20 33 . 51.80 131.90 . 015 48 137 138 60 52 ZEM 53.07 132.11 304 66 - 134 24 - KOZ . . 132.64 127 58 148 235 64 36 MAT 83 07 30 15 42 12. 53.24 132.64 107 68 44 357 50 150 ZEM 71 O4 09 11 O2 49. 56.9 133.1 327 76 128 075 39 22 KOZ 70 08 29 14 59 23.9 51.08 135.30 072 70 - 188 40 - KOZ (I) \l O DJ 0 U1 N O U) KO N N \lw U1 N .b (D U'Hbihlbtb 0515QO 161 represent a thrust boundary, and the main active tectonic feature in southern and central Amur. There are also a few mechanisms in China near southern Amur. One of these strike- slip mechanisms falls near the extension of the north-south trend through Amur, indicating right-lateral motion. The north- south trend mapped here may be the active extension of the right-lateral Tanlu fault in eastern China. From eastern China, the mapped active segment of Tanlu fault crosses the Russian border near 48° N x 131° E (Huang et al., 1996), correlating almost exactly with the trend visible in the Amur region. The traditional location of the Tanlu fault in Russia is usually further to the east, along the Amur River valley, which is not supported by the seismicity evidence (Figures 2-43 and 2-44). It is suggested here that the right-lateral Tanlu fault of eastern China has an active extension into the Amur region of Russia In the Amur region, the strike of the fault gradually changes from northeast-southwest in the south to slightly northwest-southeast further north. As the strike of the fault changes, motion changes from right-lateral strike-slip to northeast-southwest directed thrusting, which is consistent with focal mechanisms for the region (Figure 2-45). CONCLUSION Based on a very simplistic temporal analysis, it is evident that the seismicity catalog of northeast Siberia is heavily contaminated with industrial explosions. Overall, it appears that the majority of contamination in the seismicity catalog results from “daytime” mine blasts. As a first step in removing contamination, a map of only “nighttime” events (Figure 2-46) provides a better idea of the level and distribution of natural background 162 ‘ 163 Figure 2-46. "Nighttime" seismicity of northeastern Russia. microseismicity in the region. In northeastern Russia, “nighttime” seismicity plots are better for identification of active tectonics and faulting. 164 REFERENCES Agnew, DC, 1990, The use of tinn-of-day seismicity maps for earthquake/explosion discrimination by local networks, with an application to the seismicity of San Diego county, Bulletin of the Seismological Society of America v. 80, p. 747—750. Chung, W.-Y, Wei, B.-Z, and Brantley, 8]., 1995, Faulting mechanisms of the Liyang, China, earthquakes of 1974 and 1979 from regional and teleseismic waveforms - evidence of tectonic inversion under a fault-bounded basirr Bulletin of the Seismological Society of America, v. 85, p. 560-570. Godzikovskaya, AA, 1995, Local emlosions and earthquakes: Rossiskoe Aksionemoe Obshchestvo Energy and Electrification "EES Rossii", Moscow, p. 55-66 (In Russian). Figura, P., Faust, T., Fujita, K, and Koz’min, B.M., 1996, Seismotectonics of the Amur Region, Eastern Russia: Transactions, American Geophysical Union (E OS), 1996 Fall Meeting, v. 77, # 46, Supplement, p. 521. Huang, W., Gao, W., and Ding, G., 1996, Neogene volcanism and Holocene earthquakes in the Tanlu fault zone, eastern China; Tectonophysics, v. 260, p. 259-270. Imaev, V.S., Imaeva, L.P., Koz'min, B.M., and Fudzhita, K, 1994, Active faults and modern geodynamics of the Yakutia seismic belts: Geotektonika, p. 59-71 (in Russian). Lander, AV., Bukchin, B.G., Droznin, D.V., and Kiryushin, A.V., 1996, The tectonic environment and source parameters of the Khailino, Koryakia earthquake of March 8, 1991: Does a Beringia plate exist?: Computational Seismology and Geodynamics, v. 3, p. 80-96. Koz'min, B.M., 1984, Seismic belts of Yakutia and the focal mechanisms of their earthquakes: Moskva, Nauka, 126 p. (in Russian). Materialy po Seismichnost’ Sibiri, 1970-1990: Academy of Sciences of the USSR, Siberian Branch, Irkutsk (bi-monthly, in Russian). Odinets, MG, 1996, The problem of polluting the earthquake catalog with industrial blasts in northeastern Russia, in Lin'kova, T. 1., and Bobrobnikov, V. A , eds. , Geophysical Models of Geologic Processes in Northeast Russia: NEISRI, Magadan, p. 90-99 (in Russian). Parfenov, L.M., Koz’min, B.M., Imaev, V.S., and Savostin, LA, 1987, The tectonic character of the Olekma-Stanovoy seismic zone: Geotectonics, 21 (6), p. 560-572. 165 Pilyasov, A. N., 1993, Regularities in the mining-industrial mastery of northeast Russia: Kolyma, 1993 (8), P. 5- 12 (in Russian). Riegel, SA, 1994, Seismotectonics of northeast Russia and the Okhotsk plate: MS. Thesis, Michigan State University, East Lansing, ix + 70 pp. Shabad, T., 1969, Basic industrial resources of the USSR: Columbia University Press, New York, 393 pp. Vazhenin, B.P., Mishin, S.V., Sharafudinova, L.V., 1997, Earthquakes in the Magadan Region: NEISRI, Magadan, 44 pp. Zemletryaseniya v SSSR, for 1963-1989: Nauka, Moscow (annual, in Russian). Zemletryaseniya v SSSR, for 1990-1991: Russian Academy of Sciences, Moscow (annual, in Russian). Zemletryaseniya Severnoi Evrazii, 1992: Geoinforrnmark, Moscow (in Russian). 166 CHAPTER 3 Relocations of Northeast Russia Earthquakes INTRODUCTION The quality of hypocenter locations is of utmost importance when earthquake data are used in seismicity distribution studies as well as in developing better velocity or tomographic models. Of course, high quality locations depend on using the correct travel time curves in the location procedure. The existing database contains several thousand events that have been located by several seismic networks, each employing different location methodology, travel time curves, and phase data. Hypocenter determinations throughout the study area can be improved by using a single methodology, calibrated travel time curves, and combined phase data from adjoining networks. Location proceduresused in individual networks are discussed in Chapter 1. In this chapter, the larger events in the study area will be relocated in conjunction with deve10ping best-fit crustal travel time curves. PREVIOUS WORK In effort to gain a better understanding of the Russian computed locations, travel time curves based on the original epicenter and origin times are plotted. Figures 3-1 through 34 show travel time curves for Pn, Pg, Sn, and Sg phases respectively. Note significant scatter as well as trends of misassociated phases. The misassociated phases are most evident on Figure 3-3 where many Sg arrivals are cataloged as Sn arrivals. On the Sg travel time curve (Figure 3—4) there are parallel trends of arrivals. These are a result of the minute of the 311in times either recorded or typed incorrectly, as they delineate multiples of 60 second 167 .wcozsm 383m 5880520: Soc 38 2 E505 «:5 23.5“ Nde mEano BE 2:. flow: 825 SE: Emto can 88:88? Eton“: cfimmsm .flmwsm Ecumaonto: 88 o>So 9:: EB“: _a>.:.=~ 893 an ._-m oSmE E8 8865 oovm comm ooom 009 com: 003 OONF 003 com com 09. com _ _ _ _ r _ _ _ _ _ _ _ om 00F om? oom 0mm oom 0mm (3) awn |8ABJJ_ 168 .2838: 53.? 25:8 Boa 2E. 68: 803 88: Ewto :8 88:83? 888%: 563% .383“ 5282.8: :8 9.56 2:: 388 32.5 823 mm .N-m anfi oovw OONP _ AExv oocfimfi 82 com 08 8:. com o _ _ _ _ i _ _ I O - om H. .. 8: m o. w. 9 - 09 ® . r oom omm 169 .3058» «and 529358: :5: 3:0 fl 5505 San .mfizba mhwé 25:8 83 2F flow: 825 88: Ewto 23 38:88»: 3:38 553% .Emmam Eminence: 8m vino 0:5 .03“: E>Em 0223 cm .m-m Eswfi ES 85% oovm comm ooom com? com: 83 com? 002 com com oov com o _ P . _ — _ _ _ L — P _ .l o I om I o? . . r o2 .. a .. am e. 2,. . t . . - 08 w. s a . L ...,. MW . . . .. .. omm [cw ... l 8v . . - Sq , . ,. 8m 0mm 170 A\ a; ,fiv .w_m>Ea :33 23:8 SE 2:. flow: 23, 3:5 Emto 95 22:83? cocoa“: cfimwsm .Emmzm Eogmaoztoc 8.“ ciao 2:: 6%: Rita 823 mm .v-m 053m Ev; 8:920 ooom 003 com: 003 oomw ooow com com oov com o _ _ _ _ _ _ p _ L _ o om 09 of 8m SN 08 . . omm . .... wax... V I oov . .. 1 8v . I So - omm oom (s) awn |eAeJ_|_ 171 offsets from the main Sg trend. Similar trends are visible to a lesser extent on the Pg and Sn travel time curves (Figures 3-2 and 3-3). A composite reduced travel time curve is shown in Figure 3-5. The only previous study attempting a systematic relocation of events in the study area looked at a set of 75 events in the southern Magadan and northern Yakutsk networks (Mackey, 1996; Mackey et al., 1998). In this study, events were located using only the Pg phase, assuming a crustal velocity of 6.00 km/s. High residual arrivals were omitted from the location routine and several misasSociated phases were corrected. Travel time data were then inverted to solve for the best fitting Pg and Pn velocities and new origin times for the events. Although the primary focus of this study was to determine crustal thickness based on seismic velocities and Pg/Pn crossover points, the regional best-fit P wave velocities were also determined (5.992 km/s crust, and 7.961 km/s upper mantle). This crustal velocity is less than the 6.1 km/s crustal P velocity used for the original Russian locations in the region (Figure 3-2). The relocations and inversion of the data resulted in a significant reduction in data scatter on the travel time curves (Figures 3—6 through 3-8). DISCUSSION Relocations computed here are intended to build on relocation work presented in Mackey (1996), and Mackey et al. (1998). The basic location routine is a least squares best fit routine called ‘HYPZDT,’ originally from Caltech Originally, this program used only one travel time curve, thus hypocenter parameters could be computed using only one phase. The routine was modified for this study to accept multiple phases, Specifically Pg, Pn, and Sg. Sn arrivals are not used as the data are very noisy. 172 .5505 0.8 0:050; «madam 52805.8: :55 3:5 3:0 .0895 50235.5: :8 05:0 055 .035 500300: 0:89:80 .m-m 0::wE :5; 8:32: ooom ooom coop o b . L r. JJo-otfi‘flmw. o g u v. . o o u o. ...H ... ”c. o. 0 ”Es . ... engmwarhv ... . . . r . ... . . .u. ... ....1...oc..s.n a. al....HKSHutal . diam . .. . com Iv‘sloso no. I h. o C O 0 .0 0 I ' Ian Q u A O POrlo-‘lo o I 00* on n no. no. 0 n o .0 u ‘0' o I 0 O- ' o o f o- o n v. n o o o 0.. a o. a t o . pun... -- I o 0‘ a o. o. o .0 a. c , ‘ . .. 1 «V. . Dru”? .fimgiv. ... .. . 1 .... Influwa.’ . . fin ..m m ~'. ~ . .c . . u. ... ...” ML r ......{VW .~._ g... .5. 41H. .l. o o g- .\ o u...‘ fififi q g?“ ’00 Meat. 3 Co _. ’3‘“, ~ .I v. H .. \ . , . a o. . ... ... .. Mu». new WW‘rw. .. . I. o \ a a oi o o‘ ... can... a a o o o. ‘9. . . a 1.. . a o s I on. \.‘h.o.b int“. ,«...,.x. .....VM.».~.UVVW.£.R .1“ Wu . 9. a . . . . . . s . . . ...3 T .. .. . . ” .....:......... .3.» .x:at,. . .. . II. I o a o _ . . . e . .. . ‘ . . O o. o. - ... ,. . a . ~ ... o . . u . . Y. .... . h. . s. . .o a . .. M.,. ....u ...»...fic... . ... . .u. ... ....w.....§r .. .. afar. ...o. no a . o s o a. a. fi.’ . , o. I I o “no. .0 a a s pa 0 .. f~ IS ... 1.”.5 . O 0 0.0.1. ,3... . \.... A. 0 O. ... . ._ 40 .n. b . , . ‘u o . . .....r ... {Mr .1 .4“. .. a . . H 0' $ '37.. . o F»\~o o O to 0‘ ._v I. . f0. . . .. . .... : . . p ., ... one: o F O o o o ’ . (0% c on o o. C a n u i O O. I. a .o . u I, o a I I I I o O c ..I T t on. o n o. a c o no.- ... . .t can... a to o .3 . a no . n. o D. o a . ... xii... ...u . e o o a V I. . O o o c D . o n a In! vb” ..ruflnlo .0000 o u o o c u o o a. o . coco ... o I 0 o v pm..." “0 8 h o .5“ I .... u. .. . O o. o #0 0' ~ w ... Q o / CO c ( 173 20 Vr= 6.00 km / s X .g o X A W 25. .. » ’2' .-.:.'- ,12- I: a. .,. . Yr {lid 0 tn . J‘c 'x‘r..'7;>.‘.? 3.1-8.3" ..--.:".-;-~‘.-'»°.‘<. a;.'?.cfr-’~ 11"..” '. on“. 0 . (no a x v - 79::47’: -1;..-- use. taxi» L-9‘:".'T;2‘Corol 1‘ b: ' 0’ a e on". . no; 3315' ‘1“! v A 2 r ; o 4‘5 . ‘5'»? '\ 1“ ’3 3:33;” 7'3 ‘3 )d)% y Y O x o . Q + x A 0 o o x0 Q X )1 )1 + + A + + + + _20 o 0. x (km) 1000.0 20 Vr= 6.00 km / s A (I) v Y V 0 L l— —20 O. X (km) 10000 Figure 3-6. Pg reduced travel time curves for 75 events used in Mackey (1996) and Mackey et al (1998). Upper curve uses original Russian determined hypocenter parameters and reflects a velocity of 6.1 km/s. Lower curve plots the same data after all events were relocated and inverted for velocity and origin time. Reduction velocity is 6.0 km/s. Figure from Mackey (1996). 174 20 Tr (s) Vr= 8.00 km/s 0 -20 X (km) 1000.0 20 Tr (s) Vr= 8.00 km/s :20 x (km) 1500.0 Figure 3-7. Pn reduced travel time curves for 75 events used in Mackey (1996) and Mackey et aL (1998). Upper curve uses original Russian determined hypocenter parameters. Lower curve plots the same data after all events were relocated and inverted for velocity. Reduction velocity is 8.0 km/s. Figure from Mackey (1996). 175 .833 50x00: 505 0::wE .052 ow a: 5020.» :050:50m £50— 3.: m: 5020:, :m 5:: £50. mad 0: 50:03 £029 mm .5: um 50 0005055 53:50 50:23: a 555 50:06:00 0: :50: 0:60:50 :m-wm 05H .50>050: 0:03 5:35: 3:560: :wE :05: 8:5 05 :8 55:02: 50.5 50555805 0055 Ewto 5:: 050050 508020: 0005: mm 05 00: 50:2: 8:5 =< .883 .5: :0 >830: 5:: 633 30.002 5 500: 8:05 m: 505 02:0 055 025: 0:00:58 500:50m .w-m 0::me 0.08: Ace; x .o 00 ' ‘ . 3 I a .' IIJ‘JJ O 1 .... \ i ..‘ll. . .. . u. . ..~ .. ...J.....v|. he‘lnd... . . u v 0.- x u I. #5. (8) it 0.: 176 (2 Throughout the study area, many geologic and tectonic environments exist, which combined with the physical vastness of the region makes it likely that no single velocity or travel time curve will reflect actual seismic velocities for any particular phase. In order to overcome this problem, the study area was broken into cells, and the best fitting velocities were determined by minimizing the sum of event RMS residuals through trial of multiple travel time curves. Cell sizes are generally 3° north-south by 5° east-west, although this was adjusted in areas with sparse activity in order to increase the number of useable events (Figure 3—9). For any given block, only events containing Pn phase arrivals (generally 2 or more) were used. This selects only the larger events, which contain more arrivals and have better azimuthal coverage of receiving stations. Travel time curves for Pg and Sg phases were calculated assuming a flat-earth- straight—ray approach. This is reasonable because Pg and S g phases are confined to the crust, thus the distance traveled between epicenter and station is equivalent to the surface distance. For hypocenters at depth, the travel path is assumed to be the hypotenuse of the triangle made by the depth and the surface distance. Event depths are restricted to a maximum of 33 km, as all events are assumed crustal in nature. Events for which depth tends above the surface are restricted to 0 Ian In order to determine the best crustal velocities for locating events within a given cell, the selected events are first located with crustal velocities ‘guessed’ for the region. The velocities used in the guess were generally the best fit velocities from an adjacent cell. The resulting output is analyzed for high residual arrivals, generally those greater than 3.5 seconds. High residuals are generally a result of either typographical errors, misassociated phases, or bad time picks. Typographical errors and misassociated phases are corrected if 177 50:55:05 0:03 00200—9 .8030 50:05:00 0:023 m:0_m0: 3:53:55 :0 550 .o-m 0::mE at- 00: at 00: _ _ _ _ q q d d a lq‘q Alfi d u - d d d d u 1 2300 0.005000: rd... 0» I s .. 3x T a 00m. 002.20% t 07 Av - l V Ext: V min/«fl Q S0000 0.20:5. ”gm/Va 000. >033 - n — p . . b — r p : F — . p b h . p L LI — F b p . — p p . — r . - 178 possible, while bad time picks are flagged to be omitted from use in further locations. Unstable events having many high residual arrivals such that it was impossrble to get a reasonably good location and events with four or fewer recording stations are omitted from further analysis. Overall, less than 5% of the originally selected events were omitted due to stability problems. In order to determine the better fitting travel time curve to use for Pn arrivals in the location procedure, the Jeffreys—Bullen (JB; Jeffreys and Bullen, 1970) P wave travel time curve was compared to the IASPEI 91 (1-91) curve by Kennett (1991). Results for test regions in the Amur, northern Yakutia, Magadan, and Chukotka regions indicate that the J B table does a better job of fitting the PD arrivals in the region The Pg and Sg velocities used in each comparison were the same. Overall, when comparing RMS residuals for events located using the two travel time curves, approximately 80% of the eventshave a lower RMS residual when the J B table is used as opposed to the I-91 table. Therefore, the JB table was used in all relocations computed in this study. This is reasonable as the J B table was constructed at a time when continental observations determined the data set, as Opposed to the I-91 table which incorporates more oceanic data. Following removal of large residual time picks and correction of misassociated phases, the remaining selected events for a given block are located multiple times using different crustal velocities. Crustal velocities tested for each block range from 5.875 km/sec to 6.350 km/sec in 0.025 km/sec increments for Pg, and 3.470 km/sec to 3.650 km/sec in 0.020 krn/sec increments for Sg. In this manner, the crustal velocities which best fit the events in the cell are found. The newly found best fitting velocity for each cell is then used to relocate the events a second time. After locating with the new velocity, any additional 179 high residual arrivals are omitted or corrected. Arrivals with residuals over 3.0 seconds were removed. Arrivals with lower residuals were occasionally removed if the station was very close to the epicenter, or if a single residual had a value several times larger than all others for a particular event and ‘stood out’. If any arrivals were removed or had phase associations changed, the events in the cell were again subjected to a search for the best fit crustal velocities. Three-dimensional plots of a cell’s cumulative residuals for varying Pg and Sg velocities are useful to illustrate how the residuals change with differing velocities. Figures 3-10 and 3-1 1 illustrate how the residuals change for specific cells in the Magadan and south Yakutia regions respectively. Note the absolute minimum in each plot is different, which shows lower velocities are better in the Magadan cell, while higher velocities work better in the south Yakutia cell. Final crustal velocities for each cell are shown on Figures 3-12 and 3-13, and are given in Table 3-1. A total of 1311 earthquakes were relocated (Appendix E) in 45 geographic cells. Crustal velocities in northern Yakutia are somewhat problematic, with adjacent cells alternating between high and low velocities. This region is re-evaluated with a slightly different method discussed below. In this study, the Pn phase controls the depth determination of the hypocenter, while the crustal phases better constrain the origin time and coordinates. This is a result of the geometry of the raypaths. The crustal phases are not sensitive to depth because a change in depth has a negligible effect on the raypath length for distant stations. For example, a change in depth from 0 to 10 km results in an increase of the raypath length of 0.5 km for a station 100 km distant. This corresponds to a difference in Pg travel time of less than 0.1 second, which is less than timing or picking errors. Because few earthquakes in this study have more than one station within 100 km of the epicenter, the crustal phases exert little influence in 180 Number of Events: 47 Best Fit Velocities: Pg = 6.025 km/s 89 = 3.510 km/s Cumulative Event Residual Figure 3-10. Pg-Sg velocity residual graph for the region 60 - 63° N x 145 - 150° E. 181 Cumulative Event Residual Number of Events: 109 Best Fit Velocities: Pg = 6.275 km/s 89 = 3.630 km/s .00:0:0:0: :0: 550:0 0:850:00 50535 02:00.? mm 50:05:00 :0 5.:0 .NTm 0::wE 02. 00: 0:: 00: 00: 03 00: 00: _ __..___...__“ vi.._“.__“___l 0:5. 006 £ 800.06 0:00.050: >._._OO.._m> 0 . 0:5. 8.0 I ... a 3%” Mg: mm «lamwvlemwafi l 20006 0.5:: 9.000 o ABM: 101.. ... . (u. {.0 . o 0. on. on o o '00. o 02 o a 00m. >030: _ — p l— — m5 om mm on mu 183 00:80.0: :0: :32? 0:850:00 .0Ewt0 02:00.? mm 50:05:00 :0 5:0 .me 0::mE 0t- 00: 0t 00: 00: 03 00: 00: _ 1...“.._.“lll_¥. _ _ ._"._mv 0:5. 2.0 . ... :000Q 0.0050000: >._._OO._m> 0 /. 03:00:05 flex 00.0 I ... r a 00% 002.20% 30000 0.20:: I 0‘0: I . My :1 mn AMMQQ 00m. :03: plLu—__—__y.——._..—L_P-r—-:lr_i:—:_ . _ 184 Table 3- 1. Best fit velocities and number of events per geographic region. Region 48-51N 48-51N 48-51N 48-51N 48-57N 51-54N 51-54N 51-54N 51-54N 54-57N 54-57N 54-57N 54-57N 57—60N 57-6ON 57-60N 57-60N 57-60N 57—60N 57—60N 60-63N 60-63N 60-63N 60—63N 60-63N 60-63N 60-65N 60-65N 60-66N 63—66N 63—66N 63-66N 63-66N 63-66N 63-69N 65-69N 66—69N 66-69N 66-69N 66—69N 66-69N 69-72N 69-72N 72-75N 72-75N lZO-lZSE 125-130E 130-1358 135-1403 140-14SE lZO-lZSE 125—1303 130-135E 135-140E 120-125E 125-130E 130-135E 135-1403 120-125E 125-1303 130-14OE 140-14SE 145-lSOE ISO-1558 155-1603 135-1403 l40—l45E 145-150E lSO-lSSE 155-160E 160-16SE 165-170E 170-180E 120—13SE 135-140E 140-145E l45-lSOE ISO—1558 155-160E 180-160W 165-180E 125-130E 130-13SE 135-140E 140-14SE l45-lSOE 125-1358 135-1458 120-130E 130-1353 Pg=6. Pg=6. Pg=6. Pg=6. Pg=5. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. .100 Pg=6 Pg=6. Pg=6. Pg=6. Pg=6. Pg=5. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=5. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=6. Pg=5. 040 025 100 100 875 100 125 125 075 275 175 150 275 300 275 075 950 040 025 050 050 025 025 06 05 975 000 225 050 050 025 025 025 050 060 275 075 200 175 040 050 200 225 900 Best Velocities km / sec Number of events Sg=3.530 Sg=3.530 Sg=3.550 Sg=3.550 Sg=3.470 Sg=3.570 Sg=3.570 Sg=3.570 Sg=3.550 Sg=3.630 Sg=3.590 Sg=3.590 Sg=3.570 Sg=3.630 Sg=3.650 Sg=3.630 Sg=3.570 Sg=3.510 Sg=3.510 Sg=3.510 Sg=3.570 Sg=3.530 Sg=3.530 Sg=3.510 Sg=3.51 Sg=3.51 Sg=3.470 Sg=3.470 Sg=3.610 Sg=3.550 Sg=3.530 Sg=3.510 Sg=3.530 Sg=3.510 Sg=3.530 Sg=3.530 Sg=3.610 Sg=3.570 Sg=3.550 Sg=3.570 Sg=3.53 Sg=3.550 Sg=3.590 Sg=3.570 Sg=3.510 2 12 28 185 determining the depth of the earthquakes. On the other hand, the same shift in depth reduces the path length of the Pn phase a couple of kilometers, as well as allowing the Pn phase a longer time in the higher velocity upper mantle, and shorter time in the lower velocity crust, all of which reduce the travel time. Overall, variations in depth of an earthquake from the surface to 33 km can change the Pn travel time about 3 seconds, thus depth determinations are primarily a result of Pn phase travel times. A problem can arise with both hypocenter parameters and origin time if the actual seismic velocity along the Moho is different than in the travel time curve used in the location procedure. An incorrect Pn velocity can affect the origin time and coordinates (particularly depth) in a drastic manner if the azimuth window of recording stations is small. With a small azimuth window of recording stations, a shift in origin times as well as distance from stations to epicenter can easily accommodate incorrect crustal velocities. In the process of locating earthquakes in the northern Yakutia region, such a problem was suspected of developing. Note fi'om Figures 3- 12 to 3-13 and Table 3-1 that velocities determined in adjacent cells in northern Yakutia vary drastically, with no smooth gradient throughout the region It was thought that a drastically incorrect Moho velocity was causing sufficiently large shifts in origin times, or locations of events, such that the best fitting crustal velocities were erratic. Unfortunately, exclusion of Pn arrivals from the location procedure removes the primary depth constraint for hypocenter parameters. This led to the development of an alternate method of depth determination without using the Pn phase in the location procedure (Appendix F). It was found, however, that omitting all Pn arrivals from the location Procedure and solving for best fitting crustal velocities resulted in essentially the same best fit Crustal velocities in northern Yakutia, with the erratic velocity shifts in adjoining cells 186 remaining. As a result of this, the alternate method of depth determination was not used in the final analysis. The method does illustrate the dependence of depth on Pn arrivals when the crust is assumed to be one layer with one velocity for crustal Pg and Sg arrivals. The final analysis of hypocenter locations for Yakutia north of 66° was done using a 5° x 3° moving window, shifting the window in 1° increments. For each cell, the best fitting crustal velocities were determined by trial of multiple Pg and S g velocities as discussed above. This resulted in a similar velocity structure as determined earlier, but with greatly smoothed velocity shifts (Figures 3-14 and 3-15). The overall pattern of Pg velocities is similar to that of the Sg velocities. RESULTS Crustal velocities determined in the location process correlate well with the regional tectonic provinces. Generally the highest velocities (Pg velocities ranging from 6.225 to 6.300 km/s and Sg from 3.61 to 3.65 km/s) occur in the western portion of the study region, which is associated with the Siberian platform Elevated crustal velocities in the Siberian platform are consistent with seismic studies conducted by Suvorov et al. (1999), where Pg velocities were generally found to range from 6.2 - 6.3 km/s. South of the Siberian platform, velocities decrease across the Mongol - Okhotsk suture (Figure L4). This velocity decrease is consistent with with the results of Suvorov and Komilova (1985). Velocities also drop Sharply in the Verkhoyansk foldbelt, along the eastern edge of the Siberian platform From the Verkhoyansk foldbelt and east through the Mesozoic terrane assemblages (Kolyma - Omolon superterrane) to the Bering Strait, crustal velocities are consistently in the 6.00 - 6.05 km/s range, and Sg velocities in the 3.51 - 3.55 km/s range, with only a few cells 187 7f; .3055? m:_>0:: 0 9:0: 3 5055805 053:0> 505:0: :0: 02:00:? w: .3-m 0:09”: omw ~ an 3.5— om.m >._._UO._m> m:Ev_ and I mm .5: on 188 .30553 m5>0:: 0 55:: 3 5055:805 330% 505:0: :0: 00500—3 mm .me 0:05”: w wank» 0:5:— Dim >._._OO._m> 0:50. mod I . mm 00 50 mm mm 0:. K N: MK 5h mm 189 deviating slightly. The final analysis for velocities in northern Yakutia indicate that the highest velocities are associated with the Siberian platform along the western edge of the region. The lowest velocities occur in the Laptev Sea and correspond to active rifting along the extension of the Arctic Mid-Ocean Ridge. The velocity shifts in northern Yakutia are probably a result of rapidly changing velocity gradients associated with presently active rifting adjacent to the Siberian platform and other older tectonic structures. The low velocity region in the Laptev Sea extends into the continent, where it generally follows the strike of the grabens outlined in Fujita et a1. (1990). Crustal velocities determined for Sakhalin are greatly reduced, although this determination was done with only 2 events, and is thus not statistically reliable. The newly determined velocities in each cell were used in the final relocation of events. Plots comparing original and relocated epicenters show clear improvement of relative locations. In the Amur region, there is clear improvement on a seismicity trend extending through the Zeya basin and clear tightening of seismicity clusters throughout the region (Figure 3-16). It is expected that an improvement in event locations will result in better defined lineations in the seismicity as earthquakes occur on faults, which are planar. It is also expected that clusters of events would concentrate into smaller areas for point sources, such as aftershock sequences. Relocated epicenters in the Magadan region show a tightening of several clusters of seismicity and a slightly improved lineation of events along the trace of the Ulakhan fault (Figure 3- 17). The cluster near 62° N x 157° E is due to an aftershock sequence following an event on February 11, 1987. Clusters of seismicity in the eastern portion of Chukotka are reduced to much smaller lineations (Figure 3-18). It is difficult to judge whether epicenters in the Koryak Highlands are improved as there are not 190 05:0: 5:0 £8020 50:50:00 0500 :0 50:55:05 50>0::.5 :0 305000: 0:00:55 030::< .:0_w0: ::::< 05 :0: 205053 520020: .m> .05wtO 57m 0:03.: 50:000_0m .0595 a 066% W0¢N~ . a 065% WefiNH . .m. r.) w. EEwtO .me Emmi >7 :95 z .3 z .2. 8802mm m a: .mEmto >7 :2? z .3 193 enough events, particularly if seismicity here is partitioned on several faults. Original and relocated epicenters for northern Yakutia are plotted in Figure 3-19. Although epicenters clearly moved in the relocation process, the lack of clear clusters and trends makes it difficult to evaluate any improvement in relative locations. Overall, depths determined in the location process are reasonable. A histogram of event depths shows a clear peak centered around 10 km (Figure 3-20). There are also peaks for depths of 0 and 33 km, which were the confining limits for the depth determinations. As depths determined in this study are primarily dependent on Pn arrivals, any variation in Moho depth will affect event depth A thickened crust will increase the travel path for the Pn phase. Because the path is longer, it will take longer for the Pn phase to reach the station, giving it a positive residual relative to the assumed thinner crust model. To reduce the residual, the Pn path length must be lengthened in the thinner crust model, which will move the event to a shallower depth Of course, this may trade off with Moho velocity. Thicker crust may have higher Moho velocities which will result in a earlier Pn arrival, thus offsetting the shallow event depth Likewise, a thinner crust and lower velocity Moho would have the Opposite effects. It is difficult to account for either of these variables without significant prior knowledge of Moho depth and velocity. To further improve hypocenters, it would also be necessary to develop corresponding travel time corrections for all stations and all possible source locations. Travel time curves resulting from the relocated events show a distinct improvement. A composite travel time curve derived from all relocated events (Figure 3—21) shows a decrease in level of scatter when compared to those derived from the original Russian locations (Figure 3-5). Figure 3-21 is a compo site travel time curve for relocations computed 194 120° E Relocated o 0 . . 660$ 150°E 15°“ 0 o . . 66o“ 150°E Figure 3—19. Original vs. relocated epicenters for northern Yakutia. 195 Number of Events 100 - 80 - 60 - . ~ 40 - F 20 - o + , -1o 0 1o 20 30 Depth (km) Figure 3-20. Histogram of relocated event depths. Most event depths are around 10 km. 196 40 comm ooom oom— r coo? — .BEEO 2a 8% cm .303822 :8: 820883 3282;: was: Emmsm “weep—to: e8 0:3 2:: .38. @0032 :22on Mormon—Eco 4N4“ 223m Ea; 8.555 com — o 9.6531!!! an. T cow om? ovw oo— om? com CNN OVN 0mm owN com. (S/wx 0'8 - peonpea) spuooes 197 in all cells. Because each cell used distinct calibrated crustal velocities, the composite travel time curve is somewhat smeared, as it reflects all velocities used in the relocation process. Sn data are omitted, as they were not used in the relocation process. Travel time curves from individual cells better show the improvement obtained. Figures 3-22 and 3-23 compare travel time curves for individual cells in the Magadan and south Yakutia regions respectively. In each case, the level of scatter is reduced for all phases using the relocations, consistent with improved hypocenter parameters. The cells shown here are the same cells for which the 3-D velocity residual graphs are shown (Figures 3-10 and 3-11). Travel time curves from different cells can also be compared. Figure 3-24 compares travel time curves for the Magadan and South Yakutia cells, where the higher crustal velocities in the south Yakutia cell are evident. FUTURE WORK Without question, hypocenter parameters have been improved in this study, and the general knowledge of the regional crustal velocity structure has been expanded. However, there is still room for improvement of the hypocenter locations. This is particularly true in regions such as northern Yakutia, where there are large horizontal velocity gradients. In general, hypocenters located in regions of high velocity gradients will shift towards the higher velocity region, and there will be a bias in all the locations in that region. The best way to overcome this problem is to deve10p correction surfaces for each station that would account for the residuals introduced from differing velocities in all directions and at all distances (see Bondair et al., 1998). Correction surfaces can be developed for both crustal and mantle phases. 198 Travel time (Reduced 8.0 km/s) 200 q. 8 0 . 150- 0..° 09090 ~? 00 § .3}, ° 3 89 can: 0 ° $'° . . as . o 0.0 0 100- &§ 0 O 9 O 0 200 400 600 800 1000 1200 Distance - km Figure 322. Travel time curve for the region 60-63°N x 145—150°E comparing original (open circles) with relocated event parameters (closed circles). Note the significant reduction in scatter of data points. Sn data are omitted. 199 200 150 n é Travel time (Reduced 8.0 km/s) 0| 0 O Q) I I l T I O 200 400 600 800 1000 Distance - km Figure 3-23. Travel time curve for the region 54-57°N x 125-130°E comparing original (open circles) with relocated event parameters (closed circles). Note the significant reduction in scatter of data points. Sn data are omitted. 200 200 00 Q, I QO 150- 9°“ 22‘ E .3: <2 100— co "O a) O D '0 (D E (D ._E_ 79- 501 S '— O O. 0 d 0‘ O o 200 400 600 800 1000 1200 Distance - km Figure 3-24. Travel time curve comparing the regions 60-63°N x 15-150°E (solid; Magadan) and 54-57° N x 125-130° E (open; south Yakutia). Note increased velocities for relocated events in south Yakutia. Sn data are omitted. 201 CONCLUSIONS The earthquake relocation process used here has improved the quality of hypocenter locations over the original Russian determinations, as well as developed regionally calibrated, best-fit crustal velocity models. Regional crustal velocities determined are consistent with the known geologic and tectonic setting, with higher velocities associated with the Precambrian Siberian platform, and lower velocities associated with the Mesozoic terrane assembledges. Improvements in epicenter locations have clarified several seismicity clusters and fault lineations in northeast Russia. 202 REFERENCES Bondair, L, Yang, X., Wang, J., Bahavar, M., Israelsson, H., and McLaughlin, K, 1998, Tuning and calibration activities at the PIDC: 20'” Annual seismic research symposium on monitoring a comprehensive test ban treaty ( C TBT), proceedings, p. 1- 10. Fujita, K., Cambray, F. W., and Velbel, M.A., 1990, Tectonics of the Laptev Sea and Moma rift systems, northeast USSR: Marine Geology, v. 93, p. 95—118. Jeffreys, H. , and Bullen, KB, 1970, Seismological Tables: Office of the British Association, London, 50 pp. Kennett, B.N.L., 1991, IASPEI 1991 Seismological Talbes: Research School of Earth Sciences, Australian National University, Canberra, 167 pp. Mackey, KG, 1996, Crustal thickness of Northeast Russia: MS. Thesis, Michigan State University, ix + 102 pp. Mackey, K.G., Fujita, K., and Ruff, L.J., 1998, Crustal thickness of northeast Russia: Tectonophysics, v. 284, p. 283-297. Suvorov, V.D., and Komilova, Z.A., 1985 , Deep structure of the Aldan shield according to near earthquake data: Soviet Geology and Geophysics, v. 26 (2), p. 79—84. Suvorov, V.D., Parasotka, BS, and Chernyi, SD, 1999, Deep seismic sounding studies in Yakutia: Izvestia, Physics of the Solid Earth, v. 35 (7-8), p. 612—629. 203 CHAPTER 4 Tomography of Northeastern Russia INTRODUCTION This chapter attempts to develop upper mantle Pn tomography models for northeastern Russia and associate velocity variations with the regional geology. Data used in the tomography are the Pn arrivals of the events relocated in Chapter 3. Of particular interest for tomographic study is the Laptev Sea and northern Yakutia region, where the boundary between the North American plate and Eurasian plate changes from extension to compression (Cook et al., 1986). PREVIOUS WORK The shallow velocity structure of northeastern Russia is essentially unstudied by tomographic methods. The only previous attempt of Pn wave tomography was done by Wallace and Tinker (1998). This study investigated the portion of Siberia between 70° E and 1 80° B using events recorded at 11 digital broadband stations installed in the 1990's (primarily IRIS stations). This study obtained 237 arrivals from 43 earthquakes in and around Siberia. Given the geographic size of the area and the small number of raypaths, conclusive results were not obtained. 204 METHODOLOGY Tomography Code The tomography used an expanded form of the time term method developed by Hearn (1984). The tomography code used was written by D. McNamara for investigating Pn tomography in the Tibetan Plateau (McNamara, 1995). It is assumed that Pn is a head wave that propagates along the Moho. The Pn phase is therefore modeled as three segments: a segment in the upper mantle, one down going crustal segment below the event, and one up going crustal segment below the recording station (Figure 4-1). The travel time t can then be expressed by the time term equation t = a + b + Ds (4- 1) where D is the horizontal distance from the event and the station, sis slowness of the Moho, and a and b are the respective static time delays for the event and station (Hearn et al., 1991). The event and station delays (a or b) can be expressed as delay=I(sf - 32 )“zdz (M) where s, is the crustal slowness profile as a ftmction of depth (Hearn et al., 1991). For the tomographic study, the area of investigation is gridded into cells. A modified time term equation can then be used to estimate the slowness in each cell. The travel time for any individual raypath then becomes the sum of the crustal static delays plus the slowness of each Cell multiplied by the path length traveled in each cell 205 doggone—«0 05 .3 contammo Be 35 moan: 93 2mm 3.0: :25: 05 9503 80.: won— ofi. Amo— wfiowasv .8388 05 one Awe— wEowEsoE 605 05 So 33:23 08 mzooootoo 033m 0:02 05 mg? wfiaawemoa 053 30: a on 00 @0533 mm am .7... Emmi 0:02 Eo>m 206 I: a +b + Edis, (4-3) where d, and s, are the distance the ray travels and slowness in cell i, respectively. The sum is over all cells in which the ray travels (Hearn et al., 1991). In this tomography code, assumptions are made regarding average crustal thickness, average crustal velocity and average Moho velocity. From these assumptions, the static corrections (mean Pn residual) associated with each event are determined, and new residuals are calculated. The event delays calculated are dependent on crustal thickness and velocity variations, as well as errors in hypocenter depth and origin time. The individual effects of these parameters will trade off with one another, thus the actual event statics cannot be interpreted in a meaningful way (Hearn et al., 1991). Static corrections for receiving stations are next determined using. the mean residuals for each station and new residuals are again calculated. Static corrections for the stations are primarily dependent on crustal velocity and crustal thickness. Crustal thickness can affect station statics the most (about 1 second per 10 km change in crustal thickness; Hearn et al., 1991), thus they can be interpreted as such The affect of crustal thickness to station static delays is similar in concept to the alternate method of determining hypocenter depths in Appendix E. Finally, using (4-3), cell slownesses are estimated from the weighted mean of the apparent slowness of all rays passing through each cell. The weighting for slowness in a given cell is the product of the rays traversing the cell, adjusted for the fraction of each rays total length that is included within the cell. In each iteration, the full residual remaining after the static corrections is applied to the Moho leg of the travel path To remove any cells 207 with extreme slowness values due to poorly sampled cells, the model is smoothed between each iteration by averaging each cell with its eight neighbors. This is, in effect, the only dampening that occurs in the process. Once the new model is determined, an updated average Moho velocity for the entire study region is calculated. New residuals are again calculated based on the updated average Moho velocity and the process beginning with event static corrections is repeated until the data converge. Iterations are usually stopped when the change in RMS residuals from one iteration to the next becomes less than 1 % (McNamara et al., 1997). Data Selection Pn data selected for use must exclude misassociated Pg arrivals, and deeper penetrating P arrivals. There were several specific criteria used in the selection of useable Pn arrivals. 1. The Pg/Pn crossover distances in northeast Russia generally range between 100 to 150 km, thus rnisassociation of phases can be a serious problem (Pg arrivals reported as Pn) at close distances. Therefore, only Pn arrivals from a distance greater than 150 km are used. For distances between 150 and 350 km, Pn arrivals are excluded unless the Pg phase for the particular station and event is also reported. This should, in most cases, eliminate the rnisassociation problem Pg arrivals are generally not misidentified as Pn at distances significantly beyond the crossover point (> 350 km). The minimum distance was changed in some model trials. 2. For distances greater than 350 km and up to 1,500 km, all reported Pn arrivals were selected. The maximum distance varied between 1,100 and 1,500 km for different 208 models calculated. Beyond 1500 km, the rays begin to penetrate deeper material and apparent velocities increase, thus are not suitable for Pn tomography. The maximum distance was changed in some model trials. 3. Pn arrivals with too high or too low apparent velocities were excluded to remove data outliers. The cutoff velocities varied from model to model. 4. Events with fewer than 2 Pn arrivals passing criteria one through three above are also excluded from the usable data as it is not reasonable to calculate event static corrections when there is only one recorded Pn phase. This was adjusted to three or four Pn arrivals in some models. DISCUSSION Initial models The initial tomographic model developed used primarily the original Russian computed hypocenters and origin times. Some teleseismically recorded events for which Russian determinations are unavailable or of poor quality used hypocenter and origin time determinations from the ISC bulletin or the USGS Earthquake Data Report (EDR). The number of teleseismic events is small relative to regional events. Data used included only Russian stations reported in either the local bulletins, the ISC bulletin, or the EDR. Pn arrivals with selection criteria one through three above are shown in the travel finite curve in Figure 4-2. From the original database of Russian determined event Parameters, a total of 6,437 Pn arrivals from 116 stations were selected from 1,624 events. I‘lle distance range for accepted arrivals was 150 - 1500 km. Arrivals with apparent 209 A. 08: 83? o0 owcfi 8:820 @0388 05 80:0“. 8:: Eofiu> flow: 20: 03 98 35:8 360.? $5. 0.x 0:“ £60. vs 05 023:0 =8 £50m >80 $250083 5:33— EEmtc :0 683 «Big cm .~-v SawE 99080.2 Doom oomN OOON Dom _. 009. com o - _ P b o _ om.- . 1 ov. a... . . shank”. .. . E E I cm: ....3... . 0.82 0.3. 210 O ,— CND O (S/wx 0'8 peonpeu) ewu 1 O O V CO Z (3 L0 38:000.”? cflmmzm was: 6005 Fofiwofio. >355:er 05 00o owfio>00 58.3% .mé oezwi 211 velocities greater than 8.6 krn/s or less than 7.4 km/s were also excluded. The resulting raypath coverage from the selected arrivals is shown in Figure 4-3. The average crustal thickness for the area was assumed to be 37 km, and the average Moho velocity 8.0 km/s (Mackey, 1996; Mackey et al., 1998). Although the average crustal thickness and Moho velocity are known to vary considerably, these values are a reasonable estimate for average values (Mackey, 1996; Mackey et al., 1998; Suvorov and Komilova, 1985; Suvorov and Komilova, 1986). Northeast Russia was gridded into 111.2 x 111.2 km square cells for the tomography. The velocity model calculated is shown in Figure 4-4. The model shown is the third iteration computed. Contours on the model map velocity perturbations as a percent deviation from 8.02 km/s, which is the average Moho velocity calculated on iteration three. The most prominent feature is a large low velocity zone in the Laptev Sea and northern Yakutia, coincident with the Laptev Sea Rift. To the south and west of the Laptev Sea Rift, velocities increase on the Siberian Craton. Velocities decrease south of the Siberian Craton along the Eurasia — Amur plate boundary. The highest velocities are found in the Magadan region. Although the model calculated seems consistent with the tectonic setting, there are several pOtentially serious problems. First, the RMS residual error for the model decreases only two iterations before beginning to increase (Figure 4-5). The model shown in Figure 4—4 is the third iteration, with the minimum RMS residual error. This indicates that the model does not converge readily. In addition, the overall decrease of the RMS residual error is from 0.957 to 0.901 seconds, or 5- 9 %. This is significantly less than the average travel time residual reduction from 1.26 212 .mfiSS—So 3388:“ 05 mo court“ 5 2a «:31; 505.5: E 958:8 mo 35.33% omfizix define—8 203 maouafistom 80:3 25:82 :8 E8032 $58 dih— cd SHE 2:536 E023 E 0.8 20:35:?“ £029» no 85380 68:82.»: 355.8% 553M 375 £95m 583055: mo .Easuwofiou cm #1. 233m 2 cow /, \ «880 33%... . . 2 com 213 1.00 0.95 - ii 76 3 :9 (D Q) 0.90 4 I (D E E 0.85 ~ 0.80 I l I I T 1 l T 0 1 2 3 4 5 6 7 8 9 Iteration Figure 4-5. RMS residual vs. Iteration for tomography based on data from original Russian hypocenters. The minimum residual is on iteration 3. 214 to 0.55 seconds in 8 iterations for the study of the Tibetan Plateau (McNamara et al., 1997), which used the same code. Second, as evident on Figure 4-6, a shift from higher velocities in the Magadan network to lower velocities in the Yakutsk correlates well with the network boundary. Different event location procedures used in the two networks probably introduces biases in hypocenter parameters that are being resolved by the tomography. In this region, the tomography may be mapping the seismic network boundary, and not perturbations in the velocity of the Moho. Third, station static corrections are unreasonable, with static corrections ranging from minus 15 seconds to plus 12 seconds (Figure 4-7). This is an order of magnitude greater than that found in the Tibetan Plateau by McNamara et al. (1997). Although stations with the largest corrections are those with few arrivals, other stations supplying large amounts of data still show unreasonable static corrections. For example, Batagai, one of the Yakutsk network’s best stations, has an unreasonable static offset of 6.7 seconds. The distribution of static corrections does show consistent polarities in the statics. In general, the Yakutsk network stations generally have negative static corrections, while the Magadan and Kamchatka networks consistently have positive station static corrections. Models using relocations Regional Model The next study investigated Pn velocity of northeast Russia using 1,311 relocated events and their arrival times calculated in Chapter 3. For initial parameters, the average . ' Crustal thickness for the area was again assumed to be 37 km, and average Moho velocity is 215 33:28 29$ 83333 82.3 80:32 :3 Eamoaoc 3E8 «>5— o.» Sat 8:35“. «cooked 5 Ba 20:35:09 baa—9» mo £58.80 .5623 232 :8.— 8: 38 moist—Son #830: 05 mafia—a8 Emfiwoaou 2: E :32 b2: 3:050: 5952. 833029 8:82 Bogota fine—Son x85»: 05 5E» 8.29:8 x838: x$=3a> 2: E 8:62? .533 8 €950: saws—2 2: S 85823 beam—2 Set 5% < .38an x33a> Ea macaw“: 05 5233 £253 05 mo Enema—=2 :m 64V 235 m co: m omg Zowm Y¥m0>>._..m_z .Z_ , .xK/mx //,/ 2 - 1-7 J./.z/,)N//u. \ .//r\\ . - xmoi .2 v. .:x<>. /o Z owe - 216 .8808 S SE 8 €508 9 SEE So: owcfi wcozoohonc ouaum .3258st cflmwnm EEmto mEms £92382 c8 3288.50 233. cosfim 6+ oSmE >9 ace Z oNv . ..-O mucoomm m_.+ mZOFUmmm—OO o_._.<.rw ZO_.—.<._.w Z can H . m— ..E: 217 8.0 km/s (Mackey, 1996; Mackey et al., 1998). Grid spacing was again 111.2 x 111.2 km square cells for the tomography. Data selected for use in the tomography have apparent velocities between 7 .4 km/s and 8.6 km/s, and fall in the distance range between 150 and 1500 km (Figure 4—8). Removal of data outside these parameters results in the number of useable events being reduced to 1,066 having a total of 5,146 useable arrivals. Raypath coverage is shown in Figure 4-9. The model was smoothed two times between each iteration. Results of the tomography using the relocated hypocenter determinations are inconsistent with those determined using the original Russian hypocenters discussed above. Iteration three of the new model is shown in Figure 4- 10. RMS residuals drop consistently for at least 30 iterations using the above parameters and selected data (Figure 4-11). Although at first glance these results appear reasonable, there are several problems. First, station static corrections are worse than in the model using the Russian locations, with some corrections being tens of seconds even on the first iteration. Event static corrections are of similar magnitude. Static corrections generally increase with iterations. Although RMS residuals fall for at least 30 iterations, residuals never approach the lower values found when using the Russian hypocenters. This is inconsistent with the improved locations and reduced scatter in the travel time curve. Inspection of the tomographic model calculated on the 30'11 iteration show cells with velocity perturbations exceeding 100 percent, which is clearly nonsensical. Perturbation polarities in the relocated event model are often opposite of those in the model using original Russian hypocenters. Using the relocated hypocenters, the Siberian l Craton shows reduced velocities, while northern Yakutia shows higher velocities. Both 218 ooom flow: 32? mo 0mg: 853$: @2988 05 20:0: 8:: _aomto> drain b_oo_o> $.5— 9w :5“ $.5— 1: on. 5250: :3 :397. 3:8: =< $3503»: @8822 :o :83 Exits :m .w-v oSwE wt—QumsO—V— 00mm OOON 00m —. COO _. 00m 0 Ex Oomw Ex cm? I met; QF I COCO VOONF Q L0 (S/wx 0'9 peonpea) GUJLL 219 $85033 @8822 wEm: ESE 3:8on8 2: :8 092050 Sachem d4. oSmE 220 .33—:28 203 22:35:22 20:? 22:32 :3 632:2 320m .282 a.» 82.. 5325: 83.5: 2 08 2289395: 3022, no 2:850 28:30:? @8822 $22. 233— 883058: no Agar—mafia: :m 6:. RawE 3 co: m 2: 0 Z gov 2 com 221 L75 L70“ 165- 160‘ L554 L50‘ L45% RMS Residual (3) L40“ L35“ L30‘ I T I I I I I I I I I I I I I I I I I I I 0 1 2 3 4 5 6 7 8 9101112131415161718192021222324252627282930 Iteration Figure 4-11. RMS residual vs. Iteration for tomography based on data from relocated hypocenters. 222 inconsistent with the known geology and tectonics. Many attempts were made to isolate the problem with the runaway static corrections, velocity perturbations, and higher than expected RMS residuals, including: - Varying the range of acceptable velocities for data selection - Changing the distance range for acceptable data - Changing the number of smoothing passes between each iteration - Changing the minimum number of Pn phases per event or station for usable data - Varying the cell size used in the model - Removal of some receiving stations - Varying the assumed average crustal thickness - Varying the assumed average Moho velocity Variation of the above parameters yielded no improvements in the model. It is suspected that an odd distribution of events and several outlying stations results in a trade off between the event and station static corrections. The distribution of events and stations are such that the code is unable to solve for Moho velocities as static corrections applied are unreasonably large. Instead of the static corrections being reduced from iteration to iteration with more of the “time” going to perturb the Moho velocities, static corrections simply increase out of control. In one small experiment with the tomography code, several raypaths traversing an otherwise unsampled region of the Siberian Craton were intentionally added into the database. These arrivals all had known large negative residuals, which should map the area which the rays traveled through as having a higher velocity. This was essentially an attempt to force a solution in the areas sampled only by the added raypaths. The resulting model showed no high velocity perturbation where expected. The large negative residuals associated with these paths was apparently absorbed into the static corrections of the events and/or station. 223 Local Magadan Model In an attempt to overcome the problem with many receiving stations lying on the edges of the raypath coverage and the associated trade off between station and event static corrections, a smaller region encompassing only events and stations in the western Magadan and central Yakutsk regions was studied. This model showed no drop in RMS residuals (Figure 4-12), and source static corrections varied tens of seconds, even on the first iteration. Receiver static corrections were of a reasonable magnitude, with most being less than an absolute value of 1.0 second. CONCLUSIONS The seismic phase data for northeastern has proven to be very difficult to use for the development of tomographic models with this code. In a general sense, the original Russian h)Ipocenter locations seem to produce the most believable results, although significant PTOblems remain. It is unclear why improved relocated hypocenters produce such poor t01110 graphic models. It is possible that this tomography code has difficulty handling such a large area with such nonuniform event and station distributions, or raypaths at high latitudes. 224 1.5 1.4- 1.3-l 1.2d 1.1 - 1.0 - 0.9 -1 RMS Residual (s) 0.8 - 0.7 - 0.6 .. 0.5 I d Iteration 10 Figure 4-12. RMS residual vs. Iteration for tomography based on data from relocated hypocenters in the Magadan region. REFERENCES Cook, D.B., Fujita, K., and McMullen, C.A., 1986, Present-day plate interactions in northeast Asia: North American, Eurasian and Okhotsk plates: Journal of Geodynamics, v. 6, p. 33-51. Hearn, T.M., 1984, Pn travel times in southern California: Journal of Geopyhsical Research, v. 89, p. 1843-1855. Hearn, T.M., Beghoul, N., and Barazangi, M., 1991, Tomography of the western United States from regional arrival times: Journal of Geopyhsical Research, v. 96, p. 16,369-16,381. M ackey, KG, 1996, Crustal thickness of Northeast Russia: MS. Thesis, Michigan State University, East Lansing, ix + 102 pp. Mackey, K.G., Fujita, K., and Ruff, L.J., 1998, Crustal thickness of northeast Russia: Tectonophysics, v. 284, p. 283-297. McNamara, DE, 1995, Lithospheric structure of the Tibetan Plateau: PhD. Dissertation, University of South Carolina, x + 259 pp. MCNamara, D.E., Walter, W.R., Owens, T.J., and Ammon, C.J., 1997, Upper mantle velocity beneath the Tibetan Plateau from Pn travel time tomography: Journal of Geopyhsical Research, v. 102, p. 493-505. S‘-1V0rov, V.D. , and Komilova, Z.A., 1986, Thickness of the earth’s crust in the southeastern Verkhoyana-Kolyma fold system (according to near earthquakes): Tikhookeanskaya Geologiya, n. 4, p. 32-36 (in Russian). Squrcv, V.D., and Komilova, Z.A., 1985, Deep structure of the Aldan Shield according to near earthquake data: Geology and Geophysics, v. 26 (2), p. 79-84. Wallace, T.C., and Tinker, M.A., 1998, Seismic characterization of Siberia: in Proceedings of 20‘" annual symposium on monitoring a comprehensive test ban treaty (CTBT), Santa Fe, New Mexico, p. 536-541. 226 CHAPTER 5 Seismicity of the Bering Strait Region: Evidence for an Independent Bering Sea Plate INTRODUCTION Within North America, the Bering Strait region is one of the most poorly studied regions with a moderate to high level of seismic activity. Between 155° W and 180° W, and north of 64° N, at least one magnitude 7, eight magnitude 6, and tens of magnitude 5 events have occurred this century. For a comparable time period, the seismicity is more than an order of magnitude greater than the New Madrid zone. Prior to the political Opening of the eastern Soviet Union, the international boundary between Alaska and Chukotka was closed to seismological data exchanges, thus seismicity maps of either side of the Bering Strait were incomplete because only Soviet or western data were used. New cooperation has allowed data sets to be combined, and the region to be viewed as a whole to better understand the regional seismicity and tectonic framework (Figures 5-1 and 5-2). Compilation of the 6”Listing data sets (Chapter 1) produces a seismicity map which outlines a proposed Bering plate. Focal mechanism data indicate that there is northeast-southwest extension in the Bermg strait, right-lateral strike-slip motion in western Chukotka and the Gulf of Anadyr’, and northwest directed thrusting in the Koryak Highlands. These motions, along with ge010 gical evidence presented below, support the existence of a Bering Plate rotating Q10 ckwise about an Euler pole in western Chukotka (Figure 5-3). The existence of a Bering plate has been proposed previously by several workers. EVer since the first global plate motion inversion (Minster et al., 1974), there have been 227 £2on :mbm watom 05 «0 a9: bmogflom .Tm Semi Baa: mam: . o 72mg .3 term ‘.. .3 m. .3... . emm £26m w_:.mc_:mn_ . 29.20. .. $ofl¢ . . O .. .e . L8 . ... «ENHOVSIU. “ .5. can u . anew Q amen o 373. o oev 38m 02.5:me I‘I‘l 228 ADV: xawsm new Cb: wag—«M 0.8 333 332—wi— domwe :35 mini 2: mo 9:: x09: 23 68908602 .m-m ouswi Bob: 0 a co ., 77.33 0 o o ..... mcwmwmmmuxzt o 0 c5 twee: 1,30 00 o 3 :omfimo @ O $3.5 5G2”... . 9 wenEoUFoumw n . .5036 o o Em O o. \LK new: $6.6 Efzbek OO enmnaeceéo O szMEonE A... be; i no _. _. ,, 0m 0 O ‘ IfI. “WNW”; .0” 0‘“ k 4% o. c m: .. Hu 0 _E§n_vom O > LW/i/A mxuoxsco b ....® 06 WM 0 we on O HOMDEU O OO O in: > O O b mcozfiw 2.53m exam? >> 4 $5.36: 0 «.2523 «£9.30 0 mxoom o_cmo.o> I 9558:: ... Ex cam 3‘sz QJil. 229 .28 Sim 88:2. Em 3°55 Emu 025—mm mu:m_m_* 3:95 0 52. 2 £352 2.3., 82 32 SEC were 3m. 383:9 * Q 0 ea. mafia 25:55? Emoz are. .mEmEmzooE =88 023.5352 5:5 .035 mctom 2: we 8:588 Ecofiom .mim 833m Ex com Till 23mm 8.52 can wmcwucaom Em_cmo_o> 658mm umfifiqcoagxm amucnom mar :85 9:50. Ex 9:8 e>=o< 2:58.; @ * 230 various suggestions of a Bering plate. Although the reason that required Minster et al. (1974) to introduce a Bering plate (misfit of slip-vectors on the Aleutian Trench) has been shown to be erroneous (Engdahl et al., 1977), the seismicity of interior Alaska has allowed this proposal to continue (e.g., Stone, 1983). A Bering plate, nearly identical in regional extent to that proposed here, was also proposed recently by Lander et a1. (1996) to explain the seismicity of the Koryak Highlands. However, no other supporting evidence was provided and it was not placed in a regional tectonic context. SEISMICITY MAP The Bering Strait seismicity map (Figure 5-1) is a compilation of epicenters, some relocated, from earthquakes recorded teleseismically and by regional networks in eastern Russia and western Alaska. A detailed discussion of the data used is given in Chapter 1. Relocations were performed for some of the larger historic earthquakes. For the event of September 7, 1933, the relocated epicenter varied nearly 1000 kilometers depending on which stations were used in the location process. This event was excluded from the seismicity map (Figure 5-1). A few other events originally located in this region were found to move south into the Aleutian subduction zone. REGIONAL SEISMOTECTONTCS AND GEOLOGY Seward Peninsula Seismicity in the Bering Strait region is a westward extension of seismicity from the northern end of the Alaska subduction zone (Figure 5-1). In western Alaska there are a few ' areas where the seismicity concentrates along specific lineaments, but in general the 231 seismicity tends to form diffuse zones. The heaviest concentration of seismicity in western Alaska is in the southern half of the Seward Peninsula To the east of the Seward Peninsula, seismicity appears to extend almost to the Kaltag fault at the Yukon River, although there is a decrease in seismicity at the Kugruk fault zone. This does not seem to be due to station distribution, as several Western Alaska network stations operated in the immediate vicinity. North of the Seward Peninsula, seismicity occurs primarily in Kotzebue Sound. The apparent lack of seismicity in the eastern Chukchi Sea may be an artifact of seismic station distribution in the Western Alaska network. To the south of the Seward Peninsula is Norton Sound. Norton Sound is relatively free of seismicity compared to the southern half of the Seward Peninsula. Several seismic stations of the Western Alaska network were deployed in the vicinity of Norton Sound, thus the lower levels of microseismicity are likely real. In the Seward Peninsula, focal mechanisms indicate that the region is under northeast-southwest extension (Biswas et al., 1983, 1986; Liu and Kanamori, 1980; Figure 5-3; Table 5-1). Focal mechanisms east of the Seward Peninsula show a mix of normal faults of various orientations indicating generally north- south extension and right-lateral strike-slip faulting (Figure 5—3; Estabrook et al., 1988). Focal mechanism solutions for the magnitude 7.3 Huslia event of 1958, the largest event in the region, indicate normal faulting, generally with northwest to southeast tension (Ritsema, 1962; Biswas, 1983; Wickens and Hodgson, 1967, and others; Table 5-1; Figure 5-3), although the nodal planes vary significantly among the different solutions. An east-west striking rift across the southern Seward Peninsula was pr0posed by Turner and Swanson (1981), and is consistent with the north- south extension indicated by these focal mechanisms (Biswas el al., 1983). There is considerable evidence for the 232 Table 5-1. Focal mechanisms for western Alaska and Chukotka Mechanisms are listed from west to east. Those followed by an asterisk are plotted on Figure 5-3. Date Lat. Long. Mag Plane 1 Plane 2 Method Reference 85 09 10 60.43 168.86 5.6 072 46 136 196 60 53 CMT HRV 91 04 27 60.78 166.87 5.4 258 49 148 011 66 45 CMT HRV 91 03 08 60.86 167.02 6.7 037 34 84 224 56 94 CMT HRV * 92 07 17 60.86 167.32 5.3 333 47 18 230 77 136 CMT HRV 97 01 03 61.07 167.40 5.7 358 16 149 119 82 77 CMT USGS 88 10 13 61.85 169.65 5.7 070 53 164 170 77 39 CMT HRV * 086 58 130 217 42 48 P MSU 95 10 02 66.69 179.14 5.1 055 71 170 148 81 19 CMT HRV * 86 10 19 63.90 -178 69 5.3 336 68 4 245 86 158 CMT HRV 323 50 28 216 69 137 P MSU * 91 02 21 58.43 -175.45 6.6 276 52 -52 045 52 -128 CMT USGS * 71 10 05 67.38 -l72.57 5.2 267 55 -145 142 50 ~40 SYN MSU 96 10 24 67.13 —172.84 6.0 251 60 -138 136 54 -38 CMT HRV * COMPOSITE 65.9 —166.2 319 50 -52 088 54 -126 P BIW 64 12 13 64.88 -165.75 5.3 240 36 ~143 124 69 —58 P BIW * 66 08 26 66.71 -162.7 5.0 280 84 -l70 190 80 -5 P COL COMPOSITE 65.0 -162.0 102 16 -150 330 80 —11 P BIW 81 O7 12 67.71 -161.20 6.2 154 79 -17 247 73 —168 CMT HRV * 096 21 -108 296 70 -84 P BIW 65 04 16 64.69 -160.23 5.8 305 66 -93 137 25 -82 SYN LIU * 73 04 11 64.61 -160.04 4.2 250 70 ~60 010 35 -144 P C00 58 04 13 65.82 -155.65 341 79 -65 095 29 -154 P BAL 58 O4 07 66.03 -156.59 7.3 058 64 —95 250 26 —80 PS RIT * 072 66 -78 222 27 -116 P WIC 80 10 14 66.93 -155.17 4.3 124 66 178 214 88 24 P GED 80 10 06 66.97 —155.15 4.6 243 81 169 335 79 9 P EST 80 10 06 66.86 -155.05 4.2 124 85 -163 031 73 -5 P GED References are: BIW- Biswas, 1983; COL - Coley, 1983; HRV - Harvard Moment Tensor, USGS - USGS Moment Tensor, LIU; Lin and Kanamori, 1980; C00, Cook. 1988; WIC - Wickens andHodgson, 1967; EST - Estabrook, 1988; GED — Gedney and Marshall 1981; BAL - Balakina, 1962; RIT. Ritsema, 1962; MSU - Fujita (pers. com). 233 existence of this rift. There is a series of generally east-west striking Quaternary normal faults offset by short north-south striking strike—slip faults, which is similar to basin and range type extension (Plaflrer et al., 1993). In addition, sediment filled grabens as much as 1.2 km deep, hot springs (Turner and Swanson, 1981; Till and Dumoulin, 1994), and Quaternary and older late Cenozoic basaltic volcanism have been mapped (generally < 5.8 Ma; e.g., Moll-Stalcup, 1994). South of the Seward Peninsula, east-striking faults of Pleistocene age have been identified in central Norton sound (Plaflcer et al., 1993) and young (1.5-0.2 Ma) cinder cones on St. Lawrence Island also form an east trend (Moll-Stalcup, 1994) indicating north-south tension. Northeast-southwest extension is also in agreement with the predicted orientation of the T-axis as obtained from geologic indicators in the region (Nakamura et al., 1980; Estabrook and Jacob, 1991). North of the Seward Peninsula, sedimentary basins underlie Kotzebue Sound and the eastern Chukchi Sea where late Cenozoic normal faults abound (Tolson, 1987). Refraction profiles conducted in 1994 through the Bering strait by the RfV Ewing show a slightly elevated Moho, at a depth of 30- 35 km, through the Bering Strait (Allen etal., 1995). This elevated Moho is also consistent with crustal gravity modeling of the Bering Strait (McCaleb et al., 1998). Thus, throughout the Seward Peninsula region, there is widespread evidence for young (<6 Ma) crustal extension The extension in the Bering Strait-Seward Peninsula region may have originated with the post-orogenic collapse of the crust in the mid-Cretaceous and the formation of gneiss domes in the Seward Peninsula (Miller and Hudson, 1991; Dumitru et al., 1995) and reactivated at ca 6-10 Ma. To the east, the presently active right-lateral Kobuk fault (Estabrook et al., 1988) may link with the Seward rift system. 234 Chukchi Peninsula From the Seward Peninsula, seismicity continues further west across the Bering Strait and into and off the north coast of the Chukchi Peninsula. Four large earthquakes with magnitudes of 6.2 - 6.9 occurred near Kolyuchin Gulf (Figure 5-2) in 1928. The most recent large event in this region was a magnitude 6.0 in October, 1996. The Harvard centroid moment tensor (CMT) for this event indicates normal faulting with northeast to southwest extension (Figure 5-3), assuming the preferred fault plane is parallel to the edge of HOpe basin A magnitude 5 .5 event occurred in this region in 1971, for which Fujita and Koz’min (1994), and Biswas et al. (1986) proposed a focal mechanism with fault plane solutions nearly perpendicular to those of the 1990 event. These mechanisms were constrained by Alaskan network first motion data, which are emergent and could be erroneous. The event is poorly recorded, however, short—period synthetic seismo grams calculated using the method of Kroeger ( 1978) for several stations are consistent with a mechanism nearly identical to the 1996 event (R. McCaleb, pers. comm; Figure 5-4; Table 5-1). An additional event occurred in the region in 1962. Because the teleseismic P-wave first motion data and waveform characteristics are similar between the 1971 and 1962 events (Fujita and Koz’min, 1994) it is likely that the 1962 event has a similar transtensional mechanism The geology of Chukotka is less well known. Russian geophysical data suggest a fault bounded trough just offshore of Kolyuchin Gulf (Aksenov et al., 1987), which may be a portion of a much larger Cenozoic rift system that covered the southern Chukchi Sea (Shipilov et a1, 1989). The rift system terminates roughly at the meridian of Kolyuchin Gulf and there is no indication in the published Russian literature that these grabens extend further to the west. Since there is also no seismicity west of the gulf, it appears either that the rift 235 KBSspz 1971 10 05 MBC spz Mechanism: FSJ spz Strike 267 Dip 55 Rake -145 TUG SPZ Depth: 20 km Source Time: 1.0 0.6 1.0 Crust: 1 2.0 1.16 2.0 0.5 4.5 2.35 2.4 0.5 6.1 3.53 2.9 10.0 M 6.7 3.87 3.0 20.0 8.0 4.62 3.3 ‘ WWW— oc B p T Figure 5-4. Focal mechanism and synthetic seismograms from the October 10, 1971 Chukchi earthquake (R. McCaleb, pers. comm). Top traces show actual digitized records, while bottom traces show synthetics. All digitized records are short-period vertical components from the stations indicated. 236 system ends at the gulf or that any segment west of the gulf is now inactive. A fault-bounded topographic lowland termed the Kolyuchin-Mechigmen graben (Pol’kin, 1984) extends from the southern end of Kolyuchin Gulf, east through the Chukchi Peninsula, to Mechigmen Bay. The graben is asymmetric with a steeper and higher northern side, and the faults are expressed in geophysical fields (Pol’kin, 1984). The southeastern end may be offset further to the south but there is no clear topographic basin. It is possible that the upper Ioniveem valley and the lowlands north of Yanrakinot represent an incipient rift. Russian geophysical investigations using both seismic reflection and potential fields have identified additional faults off the north coast of Chukotka and in the Bering Strait region (Aksenov et al. , 1987; Shipilov et al., 1989). Those of northwest and northeast strike, i.e., parallel to the northern and Bering Strait coasts of the Chukchi Peninsula, are thought to be normal faults (Aksenov et al., 1987). Re-leveling studies (Zolotarskaya et al., 1987) suggest that most of the north coast of the Chukchi Peninsula is undergoing weak uplift (0.6 - 2.3 min/yr), Diomede Island, in the Bering Strait, is subsiding (-2.1 mm/yr), and the entire south coast of Chukchi Peninsula is subsiding (-2 to -4 min/yr), which may indicate post- rifting-related subsidence on the southern side of the Chukchi Peninsula. The region of the most intense seismicity lies near the mouth of Kolyuchin Gulf where northwest-striking normal-fault—bounded basins filled with Neogene to Quaternary sediments are also found. Northeast to southwest linearnents in the seismicity extend parallel to the direction of extension of the northern coast of the Chukchi and Seward Peninsulas, and may represent transform faults offsetting rift segments (Figure 5-2). Natal’in et al. (1996) mapped a northeast-southwest trending fault with possible right lateral strike-slip offsets along the Chegitun river on the Chukchi Peninsula which corresponds with one of the linear trends of 237 seismicity (Figure 5-2). This linear trend is parallel to another seismicity trend just to the north Both linearnents also parallel the regional seismicity trends extending from the Bering strait region towards the Koryak Highlands (Figure 5-2). Late Cenozoic basaltic volcanism (K-Ar dated at 10.7-3.9 Ma) has also been identified along the southern coast of the Chukchi Peninsula (Belyi, 1970; Akinin and Apt, 1994). Imaev et al. (1998) cite the existence of multiple hot springs in the eastern portion of the Chukchi Peninsula, also consistent with an extensional regime. Based on the evidence, it is suggested that the Seward Peninsula rift extends into the northern Chukchi Peninsula. Perhaps the rift was once to the south, along the southern edge of the Chukchi Peninsula, which is now cooling and subsiding. Koryak Highlands The Koryak Highlands are located to the southwest of the Chukchi Peninsula. Seismicity trends enter the Koryak Highlands from the Chukchi Peninsula along two somewhat linear trends. The more southerly of the two trends, which crosses the Gulf of Anadyr’, is the better defined (Figure 5-2). The largest recent event along this trend was the nmgnitude 5.3 Gulf of Anadyr’ event of 1986. The Harvard CMT for this event indicates right-lateral transpressional motion with one nodal plane parallel to the seismic trend, although first motion data are more consistent with a greater thrust component (Figure 5-3; Table 5-1). The northern trend is weakly defined, primarily by teleseisms, the largest of which was a magnitude 5.1 event in 1995. The Harvard CMT for this event indicates right-lateral strike-slip motion with a nodal plane parallel to the trend (Figure 5-3; Table 5-1). The exact southwestern extent of the northern trend is difficult to define, although it probably connects with the southern trend in the northern Koryak Highlands. 238 \ bin. .i‘i w‘ In the Koryak Highlands there are abundant northeast- southwest trending linearnents visible on Landsat images and l:200,000 topographic maps, many of which have been mapped as Cenozoic thrust faults (Kovaleva et al., 1983; Figure 5-2). Present day seismicity crosses the Gulf of Anadyr’ and enters the northern Koryak Highlands where it follows a series of faults between 63° N and 63.3° N. These faults are superposed on, and cut across, a curved series of faults which follow the regional large scale structural trend. The seismicity and faults enter a large Quaternary sedimentary basin near 175° E. From this area, the seismicity trends to the southwest, where thrust faults and linearnents are also coincident with the present day seismicity, thus the thrust faults may be undergoing reactivation. In addition, recent tomo graphic studies have imaged imaged a deep northward dipping high velocity planar structure under the southern Koryak Highlands, which is suggested to represent localized subduction (Bijwaard et al., 1998). The location and focal mechanisms of recent seismicity are consistent with the suggested localized subduction, although there is a clear absence of deep events. The largest event recorded in the Koryaks was a magnitude 6.7 in 1991, which was followed by an extensive aftershock sequence. CMTs from the 1991 event and its aftershocks, as well as other events in 1985 and 1988, indicate thrusting with southeast to northwest compression (Figure 5-3; Table 5-1). As most of the Koryak Highlands are more than 600 km from at least four seismic stations, the mapped level of microseismicity in the Koryaks is artificially low when compared to that of western Alaska. The neotectonics of the Koryak Highlands have not been extensively studied. Examination of presumed peneplanation surfaces and river terraces have been used by Russian authors to suggest recent uplift of 500 to 1000 m in the seismically active region of 239 the Koryak Highlands (Srnimov, 1995), although such surfaces may simply represent equilibrium surfaces (Keller and Pinter, 1996). Based on re-leveling surveys, the Anadyr’ lowlands to the north of the active seismogenic zone are subsiding at a rate of around 5-7 min/yr, while the coastline south is subsiding at about 2 mm/yr (Zolotarskaya et al., 1987). Portions of the Koryak range itself appear to be undergoing uplift between 2 and 5 mm/yr (Smimov, pers. comm). Terraces along river valleys show uplift rates of about 1 mm/yr (Glushkova et al. , 1987). These values are consistent with thrusting of the southern side over the Anadyr’ lowlands, as indicated by the region’s focal mechanisms (Figure 5-3; Table 5-1). The seismic trend continues southwest through the Koryak Highlands and into northern Kamchatka (Figures 5-1 and 5-2) where it borders a small region of the Okhotsk plate to the west (Riegel et al., 1993), and connects to the western end of the Aleutian arc. Aleutian Are The Aleutian arc, under which the Pacific plate is subducting, is located along the southern and southwestern boundary of the Bering plate. Right-lateral strike- slip faulting occurs at and immediately behind the volcanoes of the Aleutian Arc (Figure 5-3; Ekstrom and Engdahl, 1989; Taber et al., 1991; Ave Lallemant, 1996). The strike-slip faulting here is a result of transpressional fault motion partitioning along the subduction zone. Individual portions of the western Aleutian Arc are found to invrease velocity and rotate clockwise as they move westward (Geist et al., 1988). The increase in velocity and rotation of the Aleutian Islands occurs as faulting along the Aleutian Islands changes from subduction in the east to strike-slip in the west where a greater degree of coupling may exist. North of the Aleutian Arc, the Bering Sea forms a rigid core and is generally aseismic, with the exception 240 of the magnitude 6.6 1991 Zemchug Canyon normal-faulting event with north-south extension. DISCUSSION Many authors (e. g., Dumitru et al. , 1995 ; Whitney and Wallace, 1995) have proposed that southwestern Alaska is being extruded westward along a series of right-lateral strike-slip faults, including the Kaltag and Denali (Figure 5-3), resulting from north-south compression in south-central Alaska. Present day compression results from the ongoing accretion of the greater Yakutat (Brocher et al. , 1991) and other terranes in south-central Alaska (Estabrook et al., 1988) and subduction of the Pacific plate, driving the Wrangell block to the northwest (Perez and Jacob, 1980; Nakamura et al., 1980; Lahr and Plaflcer, 1980). The overall stress pattern through central Alaska is consistent with the Wrangell block acting as an indenter in southern Alaska (Estabrook and Jacob, 1991; Nakamura et al., 1980). This, in conjunction with Pacific plate subduction, is the driving force for the motion of the Bering plate. Activity along the Kaltag, Denali, and associated faults drastically decreases once they extend onto the Bering shelf. Dumitru et al. (1995) predicts north-south extension in the shelf, perpendicular to the direction of extrusion, along the western extent of the faults, resulting in basin formation on the Bering Shelf, as documented by Worrall (1991). This is also consistent with the CMT for the 1991 Zemchug Canyon event (Figure 5-2). Several of the young (<6 Ma) basaltic volcanic fields and basins in western Alaska and the Bering Sea coincide with normal or strike-slip faults (Moll-Stalcup, 1994; Worrall, 1991). The volcanic field near St. Michael Island likely falls along a splay of the Kaltag fault (Moll-Stalcup, 1994). In the Yukon delta, many of the volcanic fields follow the trace of the Anvik fault 241 (Moll-Stalcup, 1994). A focal mechanism from the April 1973 event which may be associated with the Kaltag fault indicates northwest-southeast normal faulting (Cook, 1988; Table 5-1). On St. Lawrence Island, Nunivak Island, and the Pribilof Islands, young volcanic cones are aligned approximately east-west along apparent faults or fractures (Moll-Stalcup, 1994). The westward motion and rotation of the western Aleutian islands implies that motion of the Pacific directly assists the motion and rotation of the Bering plate. The right lateral strike-slip faulting behind the Aleutian arc is also supportive of the extrusion along right-lateral strike slip faults in southern Alaska and the clockwise rotation of the Bering Plate. The strike-slip faulting here has two implications for the Bering plate. First, it defines the southern boundary of the Bering plate prOper (Figure 5-3). Second, given the westward motion and clockwise rotation of the Aleutian Arc, it is not unreasonable that some degree of coupling exists with the Bering plate, which assists the motion of the plate as a whole. Experimental work also supports development of an extensional regime oriented perpendicular to the direction of extrusion as one travels away from the region of compression driving the system (Peltzer and Tapponnier, 1988). The same work also indicates that very complex motions and fault patterns can develop in regions under extrusion, which may explain the seemingly random distribution of earthquakes (Figure 5- 1) and historic difficulty in explaining the tectonics of central and western Alaska. The exact position of the Bering plate boundary in southern Alaska is probably diffuse, with motion taken up along many faults, thus it is not easy to locate the boundary. However, as southwestern Alaska is extruding westward with respect to North America, it is most consistent to assign portions of Alaska west of the Wrangell block to the Bering plate, with 242 the boundary on the western side of the Wrangell block following Lahr and P1afl- «at ... ..I ” ‘ufi "e .5.‘ CONCLUSIONS A seismicity catalog, including phase data, was assembled for northeastern Russia. This catalog was found to be heavily contaminated with anthr0pogenic sources in some areas. However, this problem can be overcome by plotting only nighttime events, which is representative of the region’s tectonic activity. The resulting plot shows several trends of seismicity that are found to correlate with known faults (Figure C-l). In general, it is still difficult to trace individual active faults throughout the study area and precisely locate the regions plate boundaries. In addition, a large percentage of seismicity occurs in poorly defined, diffuse regions not presently associated with specific faults. This indicates in a broad sense the diffuse and complex nature of continental plate boundaries. Although focal mechanism studies have given us the bro ad tectonic plate motions for northeastern Russia, the microseismicity indicate that the boundary regions between the major plates are comprised of many small blocks and slivers that interact in a complex fashion. Considerable additional work remains to be done to fully understand these small scale plate interactions. Careful analysis of the assembled database resulted in the development of a new tectonic model for the Bering Sea region. This model proposes an independent Bering Plate which is driven by westward extrusion of south—central Alaska as a result of terrane accretion. The similarity of this model to the extrusion of southeast Asia from the India- Eurasia collision illustrates the occurrence and importance of lateral extrusion as a means of continental deformation. The assembled seismic phase data allowed the relocation of over 1,100 earthquakes While simultaneously determining the first crustal velocity model for the region. Although 252 ‘ P w-va Ix)! Queen‘s ll Ii, iffrtllt' ‘- ; :“rw’r g: &..'.Lv . 7m» n (I - . 1.13:? r st: the method employed to develop the velocity model was simple, it was clearly able to discriminate velocity differences among different geologic/tectonic settings. Because of the method’s simplicity and positive results, it may be useful for other regions where regional crustal velocities are poorly known. This study has greatly improved our understanding of the tectonics and crustal structure of northeastern Russia, a region of interaction between several major and minor lithospheric plates and blocks. However, considerable work remains bevore we understand the activity and behavior of plate boundaries in this region. The combined data set of hypocenters and earthquake phase data for northeastern Russia assembled in this study will provide a base for future studies. Future work in northeast Russia will include the deployment of digital acquisition seismic stations, which will be useful for seismic wave attenuation studies, better velocity models, explosion discrimination, etc. Future work in the region will also hopefully include comprehensive GPS studies to map the complexities of the regions small-scale tectonic interactions, which will reflect on the fundamental character of continental deformation. 253 8553:: 58528 mo 5258925 :0 women 8302 2a 333 850 use .6222 one 338 5.32 .Ewmsm E28236: ho SSE 3:8 98 365.33 25.232 .TO 053m 254 APPENDICES 255 APPENDIX A Alphabetized list of northeastern Russia seismic stations. 256 ‘1 guy he... E F. FHL E tin FIG /1 APPENDIX A Alphabetized list of northeastern Russia seismic stations. Station Name Engl. Code Aku Alla ALL Alygdzher ALY Arncdichi ACHS Amguerna AMG Amrmnl'naya MMS Anadyr AN YS Anadyr-1 AN SS Angarakan AGK Anyuisk AN C Apacha APC Apakhonchich APN Arshan ARS Artyk AYKS Avacha - old AVH Avacha - new AVH Babushkin BAU Baikal’sk BKK Balygychan BLG Barluk BRUS Batagai BTGS BaZOVSkii Berezovaya BER Bering BKI (Nikol'skoe) Bilibino GSN BILL Bilibino BILS Bilibino-1 BLl BOdaibo BOD Bodon BDN Bogachevka BGC Bomnak BMKS Bykov gda CGD Chara CRS Russian Code All AMII AMT AHlI AHII AHC AI'IX APIII AP ABH ABH Bill" BTF BP3 BPH BPI' BILL BJIB BJIH BIIB BHB BOII BIIH BMH ‘ITII ‘IP Lat. PM 56.46 54.688 53.633 57.03 67.05 64.55 64.734 64.77 56.348 68.34 52.925 56.00 51.908 64.18 53.07 53.265 51.717 51.522 63.91 54.533 67.653 56.53 52.27 55.195 68.065 68.059 68.04 57.807 53.713 54.850 54.705 47.317 58.75 56.9 Long. (°E) 120.91 110.82 98.218 122.85 178.88W 143.18 177.496 177.57 113.67 161.56 157.131 160.84 102.433 145.13 158.5 158.738 105.867 104.133 154.09 101.717 134.630 123.42 158.433 165.99 166.452 166.449 166.44 114.03 110.1 160.900 128.847 142.567 130.60 118.267 257 Elev. (m) 700 550 920 930 150 540 55 40 1430 10 700 840 700 900 470 460 139 525 127 1080 299 283 260 245 540 325 185 710 Date Open --/68 10/63 1/66 05/89 1 1/65 --/61 4/89 9/96 1 1/80 1 U7 6 6/64 2/90 —/64 -/5 8 06/7 1 —/88 —/63 7.76 6/66 1/64 7/63 1 1.60 --/7 5 --/70 —/81 -/62 8/95 8/81 8/64 11/60 11/69 —/64 3/7 4 6/68 --/68 1 1/61 Date Qual. Close --/68 5/70 1/67 08/89 4/66 --/62 7/93 OPEN 1/89 8/81 3/65 OPEN -/80 --f7 1 10/7 6 OPEN 9/66 2/66 6/64 OPEN "/70 —/94 OPEN OPEN 4/92 1/65 --/83 p—aa—aN—da—d—a—a Ie—Iflfl—ap—ia—aa—Ip—Ilp—IH—AHa—Ap—l—au a—ep—dla—ly—a a ?- =r. Lea-don; l' ‘ '7'.) L‘s :11 .1. u‘fbk limo Cherskii CES LIPC Chil'chi Chita CIT LIT Chochurdakh Chokchoi CKHS ‘IK‘I Chulman-1 CLlS l{JIM Chulman-2 CLNS ‘UIM Davsha DAS Debin DBI JIBH Dimnoe Dovochan DOV Dunai DUYS Ill-l Dyrynmakit Egvekinot EGVS 3FB Ekimchan EKI EKM Emegachi EMG Esso ESO 3CC Esutoru ESU (Uglegorsk) Evensk EVES 3BH Firsovo Ganali GNL FHJI Garmanda GRM PPM Gorely GRL FPJI Gomotaezhnoe GRZ F PT GRT Gomovodnoe GRD FPB GRV Gornozavodsk Gomyi GNY I'PH Gusinoozersk GOO FCH Ilimei ILR l/UIP Irmngra-l Irmngra-Z IMNS I/IMT lnstitut INS Vulkanologii lrkana IKN Irkutsk IRK I/IPK lul'tin ILT I/UTT Kabaktan KBKS KBK KBT Kabansk KAB KB Kalgannakh Kamenistyi ULZS KMH Kamenistaya KMN Karnenskoe KAM Karam KRMS Karyrmki - old KH KPM? KAP? 68.75 56.06 52.033 72.83 57.65 56.85 56.84 54.538 62.339 73.233 56.462 73.92 56.60 66.323 53.067 56.567 55.925 49.083 61.92 47.65 53.942 62.18 52.552 43.70 43.70 46.567 50.762 51.283 67.26 56.75 56.62 53.066 55.867 52.272 67.87 56.68 52.05 71.83 65.41 55.76 62.456 55.133 54.030 161.33 122.33 113.55 116.25 121.72 124.90 124.90 109.503 150.751 142.400 117.533 124.49 121.13 179.127W 132.945 118.158 158.700 142.033 159.23 142.567 157.620 159.08 158.080 132.15 134.733 141.85 136.455 106,517 167.96 121.24 120.71 158.605 111.253 104.31 178.74W 122.42 106.658 114.33 144.83 160.240 166.210 107.583 159.480 258 10 500 790 240 650 760 460 332 1 094 460 1 8 485 960 490 100 22 20 1200 140 1250 220 270 50 500 600 350 395 540 175 480 467 235 1010 465 670 1 100 790 --/7 9 --/70 070 08/13/75 --/89 --/62 —-/86 1/64 —/7 4 3/7 4 7/62 1 1/89 --/67 —/90 1 1/7 9 9/62 —/65 12/39 -/80 SH 9 1/88 12/66 7/80 7/90 7/88 12f71 12/78 llf71 10/64 07/67 --/75 11/81 1/64 12/01 3/66 05/89 US] 07/21/75 --/88 10/90 -/94 1/66 7f7 4 --/ 88 --/70 09/2 6/7 5 --/89 «I86 6/65 —/92 M4 9/63 --/67 —/94 4/63 OPEN -445 7/93 1 1/7 9 OPEN 5/67 OPEN 6/7 2 2/7 2 10/65 08/67 --/7 9 OPEN 3/66 OPEN 7/93 08/89 08/10/75 “/88 OPEN OPEN 3/66 -/86 la—da—J—aa—lp-dp—apalop—aa—ap—a—alp—Ia—ay—a pap—aHp—a p—tg—aHp—aHNI what—IH'I—I He—aa—nu—a—ala—l 5115332112 .. _ .. 12.13 E 111' . n ‘ L1 |~"', -* ‘ ‘I-Q ’Nm‘l. ‘. l-n .ut'ilfli re“ J , 5.1m the FPS.“ l ‘1 {11.110151 1.5.110 M30101 tingl- L- ‘ .k-J“|'} KarymSki - newKRY Khaim Khandyga Khani Khapcheranga Khatystyr Khingansk Kigilyakh Kirovskii Klyuchi Kobdi Kolokol'chik Korito Korsakov Koryak Kotelnyi Kotikovo Kovokta Kozelskaya Kozyrevsk Kozyr Krestovaya Krestovskii Kronoki Krutoberegovo Kul‘dur Kulu Kumora Kurbulik Kurul'ta Kyakhta Kyubyme Kyusyur Lamutskoe Lapri Lazarev Lcsogorsk LopatinO Lurbun Magadan KAIS KHG KHNS KPC KHY KNN KNG KIRS KVO KZL KOZ KZY KTB KRS KRI KBG KLDS KU-S KMO KBK KYA KYUS LMT LZR LRB MAG Magadan-GSN MA2 XM XH XTC KPC KBK K3JI K3P KPH KBF KB KPB KJI KHY KMP KCP JIMT H3P MFII 54.036 52.602 62.65 57.04 49.707 55.71 49.122 73.367 54.428 56.313 64.20 55 .966 53.292 75.767 49.133 56.133 53.201 56.05 56.070 52.665 56.214 54.596 56.255 49.205 61.889 55.883 53.708 56.90 50.35 63.38 70.68 65.54 55.69 52.2 49.442 46.6 56.63 59.560 59.575 159.449 108.085 135.56 121.01 112.392 121.57 131.192 139.867 126.983 160.852 145.51 160.222 158.636 137.60 144.25 113.05 158.894 159.87 159.900 106.395 160.558 161.134 162.705 131.642 147.431 111.208 109.038 121.11 106.45 140.95 127.37 168.85 124.91 141.493 142.2 141.825 117.883 150.805 150.768 259 900 480 125 390 950 475 520 440 80 800 1000 1050 10 1180 950 450 560 1200 10 425 655 475 460 495 760 950 20 178 640 120 780 78 339 9/89 10/69 --/69 --/67 --/75? 12/68 --/68 “/75 7/80 6/7 3 4/7 4 —/48 2/ 89 06/7 1 -/67 10/97 --/51 7.75 8/7 2 7/69 --/81 -/7 6 -/62 1 1/89 7/7 1 7/87 8/66 —/68 1/80 9/66 1/64 --/68 3.52 --f7 4 --/85 4/65 --/7 2 12/80 7/69 4/69 8/62 5/67 1/52 9/93 OPEN 5/70 --/94 --/67 --/7 6? --/68 --/82? 4/84 0...; H~H~H~H~0.H'H 'Np—lp—d—eNp—da—Aa—la—A Ha—lg—ep—a' ,It ,1- Maiskii MKI MAI/1 Mal. Ipelka MIP Maritui MRU Markovo MKVS MPK MKN Mednyi MED MHH Milkovo MLK MJIK Moma-Khonuu MKUS MOMA Moneron Mondy MOY MHII Murino MUO Myakit MYAS MKT Mys Khvoinova Mys Nerpichii Mys Diring-Ayan Mys Shmidta SMT IIIMT Nagornyi Sta HTP Nagornyi Naiba NAYS HE Nalychevo NLC HJIH Naminga NMG Narninga-l NMGl Nel'koba NKBS HJIB Nel'koba NKl H1113 Nelyaty NLY HJIT Nesterikha NSR HCT Neryungri NYGS HPF Nezhdaninsk NZDS l-DKJI Nikola NKO Nikolaevsk- NKL HKJI Armr Nizhnii NIZ N-A Angarsk Nizhnii Armudan Nogliki HKJI Novaya Sibir Nyvrovo NW HBP O. Vrangelya VRN BPH Obo OBO Ogon'ki Oimur OIM Okha (New) Okha OKH OXA 68.97 52.276 51.783 64.68 54.786 54.70 66.47 46.258 51.673 51.475 61.407 74.267 75.833 75.950 68.88 55.92 55.95 70.85 53.171 56.60 56.70 61.34 61.34 61.338 56.492 53.647 56.68 62.50 51.893 53.142 55.766 50.817 51.817 75.050 54.317 70.94 61.80 46.775 53.333 53.55 173.71 156.758 104.217 170.41 167.556 158.63 143.22 141.25 100.993 104.408 152.093 140.883 143.333 139.917 179.38W 124.97 124.92 130.73 159.345 118.517 118.583 148.81 148.81 148.813 115.7 109.708 124.66 139.06 104.827 140.783 109.55 142.533 143.15 147.000 142.617 179.62W 149.77 142.383 106.833 142.933 260 261 370 520 25 155 192 1300 470 670 920 840 1380 1160 531 531 531 470 480 760 603 460 25 487 150 25 10 440 70 460 24 8/ 82 8/97 -/08 10/86 —/7 3 —/62 10/ 89 --/ 83 9/7 1 —/58 8/66 --/83 4f7 6 4/7 6 5/7 6 4/65 --/7 7 --/69 --/85 —/67 3/69 3/84 5/67 --/63 9/ 83 6/63 6/97 1/61 7/70 «f7 7 --/80 --/80 7.71 9/70 10/61 9/66 10/64 --/88 4/75 1 1/81 2/66 --/7 7 6/68 10/59 --/65 12/5 8 6/94 OPEN —/1 8 4/92 4/69 12/64 6/75 4/66 —/7 7 9/68 —/60 -—/65 a—la—Ia—Ia—d p—la—a "QI—lr-l'r—i Ip—aa—aa—ay—I' ada—a—IOp—ahdg—ep—aa-d. a—lt—AH p—r IHIflWI Ol’khon OLK Omchak OMCH OMLI Omolon OMOS OM11 Onnlon-l OLl OM11 Omsukchan OS 1 OMC Omsukchan OMS OMC Onguren ONR OHF Ootomari OOT (Korsakov) Oran ORA OPH Orlik ORA OPJI Orotukan ORT Ossora OSS OCC Ostrov Mednyi MED MJIH Otiai (Bykov?) OTI Ozernaya OZE 03H Ozernaya AYZS O3P Ozero OZR O3P Ozhidaevo Pakhach PCH Palana PAL Pauzhetka PAU I'DKT PetrOpavlovsk PET ITTP Pevek PVK ITBK Podkova PDK I'IIIK Polovinka PLK Pravda Provideniya PVD IIPB Provideniya-1 PVl IIPB Romny RMN S PMH Russkaya RUS PYC Saidy SAYS C11 Sasyr SSYS C C P Savino SOV Sedlovina SDL Seimchan SEY CMH Semlyachik SEL CMJI Severo BaikalskSVBS C-B Severo Muisk SVK C-M Shamanka SHMS Shara-Tagot SRTS Shebunino Shikka SKK (Shikuka; Poronaisk) Shimki SMKS Shipumki SPN IllITl-I Sinegor'e SNES CHI“ Slyuda Solontsovaya SOL CHLI 53.20 61.67 65.23 65.25 62.52 62.52 53.233 46.65 55.933 56.295 62.26 59.25 54.786 47.325 56.295 63.75 54.692 47.033 60.558 59.093 51.467 53.024 69.70 56.140 51.798 46.942 64.424 64.45 50.855 52.432 68.70 65.16 52.543 53.278 62.93 54. 12 55.64 56.183 53.125 53.005 46.433 49.233 51.675 53.107 62.09 56.33 54.17 107.342 147.87 160.54 160.52 155.77 155.77 107.592 142.767 113.667 113.983 151.34 163.065 167.566 142.783 113.983 146.11 160.392 142.392 169.125 159.963 156.810 158.650 170.27 160.780 104.35 142.008 173.226W 173.18W 129.4 158.507 134.45 147.08 102.15 158.884 152.38 159.98 109.35 113.533 105.6 106.717 141.858 143.117 102.012 160.011 150.52 124.12 108.35 261 490 820 260 260 527 527 500 36 705 620 470 10 20 620 875 220 1 10 150 20 800 470 25 20 210 75 88 580 720 1235 21 1 505 850 700 500 765 170 400 1080 45 8 7/69 9/99 6/82 12/63 1/63 12/67 -/88 --/09 9/7 9 9/7 8 --/7 7 —/7 3 —/7 3 2/34 9/7 8 06f7 1 10/66 --/7 6 —/92 —/94 1 1/61 —/51 5/65 —/83? 8/66 9/7 1 9/80 1/65 NH 8 12/87 --/80 --/86 9/68 9/91 4/69 1 1/61 —/7 8 1 1/7 6 8/59 10/59 9n 1 --/28 1 1/66 —/62 «f7 6 «f7 3 —/7 9 9.69 OPEN 7/93 1/65 1 I64 OPEN -445 --/7 7 OPEN —f75 --/45 «m l—au—ae—ag—a ly—ea—eN—ea—tp—aa—aa—da—ea—ly—INNHIp—Ip—Ay—a—Ia—aa—dp—alg—aN—ai Earthy ( Sirloin 3a.: 1131: Star 511’. Szioi'ryi $210131 .2 k" v a ..- 1’. (J . Her I \Mfi‘lih -&.a. ‘11" . ".JV N‘ llLI T‘ . '1'- ‘udi ll. . i ‘. _I‘a .' “.J.lrl Sovetskaya Gavan Srednekolynuk S-K C-K Srednii Kalar SRK KIIP Srednii Sakutan SDK Stekol'nyi STK CTK Stekol'nyi MGD MAI MTIl—l CTK Stolb SOTS CTB Susuman SUU CMH CCM Sutam Suvo SUVS CYB Syllakh SYLS CJ'IX Syul’ban SYB Tabalakh TBKS T1511 TB Taimylyr TMLS TMJI TMP Takhtoyamsk TTYS TXT Talaya TL- THA T11 Talaya TAL TAJl TasYuryakh-l TasYuryakh-2 Tenkeli TLIS THK Terne' TEI TPH Tilichiki TIL Tikhrnenevo Tiksi TIK TKC TIXI Tiksi-GSN Tokarikan Tokhoi TKH TX Tonnel'nyi TNL TH” Topolovo TOP TOTI T1111 Toyohara (Y uzhnO) Tsipikan ZIP LIPK Tsiveluch SVL IIIBH Tungurcha-l TUGl Tungurcha-Z TUG THT Tungusskii AY3S Turan TRNS Turikan TNKS THK Tyrmvskoe TYV TMC TMSS Tynda TYD THE 48.967 67.46 55.86 56.898 60.046 60.046 (60.046) (150.730) 72.40 62.78 140.283 158.71 117.38 118.095 150.730 150.730 126.82 148.15 (62.78) (148.15) 62.779 55 .96 63.655 57.12 56.065 67.54 72.61 60.20 61.134 51.681 56.64 56.62 70.18 45.067 60.433 49.2 71.632 71.64 56.10 51.361 56.283 53.230 54.917 56.583 57.33 57.27 64.20 54.425 51.633 56.383 50.85 55.133 148.163 127.59 110008 121.86 117.222 136.52 121.92 154.68 152.398 103.644 121.33 121.41 140.78 136.6 166.075 142.9 128.863 128.87 126.42 106.608 113.35 158.041 113.35 161.225 121.48 121.48 146.38 119.933 101.666 113.108 142.65 123.717 262 50 30 716 750 221 221 60 11 730 579 395 415 110 30 150 38 30 800 820 155 1110 900 315 1080 630 870 695 100 610 6/69 4/64 -/61 2/63 7/64 3/7 1 -—/94 --/85 8/69 --/95 --/98 «I69 -/84 --/89 6/62 --/80 --/86 9/87 1/89 1 1/82 02/67 07/67 --/84 7/82 -/94 6/69 3/56 --/95 --/7 2 1 1/7 1 1 1/7 6 1 1/61 --/43 —/7 5 10/80 --/70 --/7 8 --/7 1 1 1/61 12/66 8/8 1 4/69 7f70 11/70 12/64 10/63 5/66 OPEN a—Ia—dI—l—IHOHr—ah—I n—iI—It-‘H—‘H' ...-n Qt—d -HWa—IU—I'HN~H- |._gp—aa—ay—-II—ll—dI—"—" ljrrgrr label l'rl’ll ldoiu: [3.11: 1:132 1'71”: 11.111; ljcgrl ["3120 fun if: N: 13 .\'j is“ ['1 iii Bl Est B: lit Sr 13‘ O '7' D UtillC hilt; 13:111.: K2: lladl2 1310;: Vim 11:10:; 13ng 111.311. 1113.1: Yam; 1352:; t- . .‘ 1E5”: £3 ‘4‘ L14 ,2.“ 3 ‘1 £7.52: 2? pg? 1“ (if .f_. .4. fr? v iv A (43;! av {EpJ Nu—I' H'HNWn—a Tyrgan TRG TPF 52.758 106.342 600 1/60 Tyubelyakh ULlS TEB 65.37 143.15 380 --/88 --/88 Uakit UKT YKT 55.495 113.62 1140 12/62 —f75 Udokan UDK 56.75 118.305 810 4/67 4/69 Udzha 71.25 117.17 08/27/75 09/19/75 Uelen 66.16 169.84W 5 --/81 —/82 Uglegorsk UGL YFJI 49.083 142.083 20 -/51 --- Ulyukchikan UCK YJIK 53.87 109.598 490 7/70 9/70 Ulyunkhan ULNS OJUI 54.867 111.07 560 7/89 --- Unknown UNYS YHIO onan YOA YH 56.13 111.77 520 -f79 --- Ust'Nera-l UNRl Y-HP 64.566 143.230 485 --/62 --/92 Ust' Nora-2 UNR Y-H 64.565 143.242 485 --/92 OPEN Ust'Nyukzha USZ Y-H 56.56 121.59 415 --/64 --- Ust'Urkima UURS VPK 56.31 123.16 540 --/81 --- Ust'Bolsheretsk UBL 52.842 156.308 20 11/61 -/64 Ust'Belaya U-B Y-B 65.51 173.28 20 11/66 5/67 Ust'Srednikan SRD CPII 62.44 152.32 580 12/62 11/63 Ust'Omchug USO Y-OM 61.13 149.63 580 --/68 --/83 Utesnoe 46.6 143.075 20 W73 9/79 Vankarem VNK BHK 67.84 175.85W 10 3/66 6/66 Verkhene VKM 54.627 158.473 170 10/66 —/75?I'I Kamchatsk Vladivostok VLA 31111 43.12 131.893 75 -/29 —/31 Vodopadnii VDP BLIP 55.770 160.220 1060 —/77 -/91 Vzmor'e 48.85 142.517 20 7/82 12/82 Yablochnyi 47.167 142.067 20 6/68 9/68 Yagodnoe YAG 62.53 149.62 480 --/77 --/77 Yakutsk-1 YAKl 62.015 129.722 90 10/57 --/62 Yakutsk-2 YAK 51K 62.030 129.677 91 --/62 OPEN Yaruga YRGS HPF 57.49 123.07 780 --/89 --/89 Yasnyi YASS HCH 53.29 127.983 310 1/75 -- Yubilcniya YUBS 10511 70.74 136.10 10 --/86 --/93 IOBT Yuzhno YSS IOCX 46.958 142.762 100 --/57 OPEN Sakhalinsk Yuzhno YSSl 47.02 142.717 40 10/47 --/57 Sakhalinsk (Novo Aleks-androvsk) Zakarmnsk ZAK 3KM 50.383 103.292 1125 12/60 «- Zapadnyi ZAP 56.613 118.433 1600 4/67 8/67 Zarech’e ZARS 52.550 107.15 460 7/59 -/60 7/69 9/69 Zarya ZRY 57.24 118.917 655 10/59 9/68 Zemlya Bunge 74.833 142.583 4/75 6/75 Zeya ZEA 3E51 53.755 127.293 270 6.76 -- Zhigalovo ZGL 54.808 105.15 625 12/67 2/67 Zhuravlikha ZRV )KPB 53.517 109.375 475 7f70 9f70 Zimmki ZMN 3MH 45.475 134.258 150 7/88 -- 263 HHIHp—aa-da—IdeOHH a—ap—aN—Ip—Ia—II a—d Nr—tt—lr—I'N Zyryanka ZYRS 3PH 65.72 149.82 120 --/82 --/90 1 Zyryanka-l ZYl 3PH 65.74 150.89 37 1/64 10/64 1 *q “ll '51: “4 9.x APPENDIX B Annual plots of seismicity in northeast Russia. Original Russian epicenters are used. 265 .83: 23 «ESE E38055: .5 92523. 252: ._..m oSmE ambush 26590 o§w~ 266 dmaémfi E £35m Eogwaostoc ..o EEEmfim .m-m 953m 267 .83 E £93m £22855: ...o bEEflom .m-m 853m 59qu o§- 268 .GE E «and 8882—20: he £2828 41m 2&5 853E Q :3» Q \ 269 .N00— 5 Emmsm scanner—to: no b._o_Em_om .m- m USN—m 270 .39 E «a . . 3% E 8805:2— mo 58:: . . 20m . . c-m 95m. .m embosm 2680 271 .33 E Samsm 5000:0520: mo 38E§0m Hi 0::wE Umbofinm we . :6 0 $- 272 :J' \S .m60— a: «Em—4M EoumwOr—COC MO ~AumUwEmm0m .wum Ouswmnm 273 =3 NUQ Q» musk .p I ~ 50m 52% . RR 5 5000 0:02 nuv .80 M E a . Em: m E 0:00 055: 5 a :05: . £0 . m . 9m 0 5w. .m 274 “‘ 853 5 «vii 5060055: 5 555.5% .o_-m 05wE 275 503 E Emmsm 6050055: 05 bEEmBm 4 Tm 053m 276 doe :_ E . .mmsmE 03:05.5: .5 55:: . . 20m . . 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N -m oSmE 302 .33 E «63% 523055: no >238£om .mm-m "5&5 303 .OQQ~ G.— mmmmzm Panama—tho: ho hflmomEmmom .mMnm Dhnmml \ DNEUQQ . mu . =3 0 \ I x u *HNQQVQ 1 o r 1 c 86 . Em .. . de ”85 o a .fi. s . QQO ofiofifi :6 . a. . 304 .83 E Em . . 3% E Bmaocto: mo >225 . . 20m . . 21m 2:? .m 8.:th 2630 305 .33 E «63% Eofiaocto: mo axes—flow .3 ambush @5qu -m Emmi c§- 306 APPENDIX C 1999 Digital station deployments and observations 307 APPENDIX C 1999 Digital station deployments and observations Overview In the summer of 1999, the author traveled to Magadan to upgrade existing photo paper seismic stations to digital acquisition. This work was performed in conjunction with the Magadan Experimental Methodological Seismological Division (MEMSD), from Magadan, Russia. A total of six digital acquisition systems were purchased and imported into Russia for deployment. The digital acquisition systems were manufactured by PC System Design, Palo Alto CA, and use 8 channel, 24 bit A/D cards with GPS t'nning. Data are recorded on PC computers, which were purchased in Russia. At all stations, except as noted, the seismometers (3 components) recorded are Russian (Soviet) SM3-KV short period instruments, with the free period set at 1.5 seconds. Seismometer output is amplified 1000 times, and a thz cutoff low pass filter is used. The amplifier/filter was designed and manufactured by MEMSD. Digitization of time and all seismometer components is 30 s.p.s. During the travel period, four stations (Susuman, Talaya, Nelkoba, and Ust’Nera) were deployed in permanent locations, upgrading existing photo paper stations, and two stations were deployed temporarily (Stokolviya and Matrosova) during fieldwork. Coordinates for all stations were determined with a Magellan handheld GPS unit. Location of stations deployed are shown in Figure CL A brief description of each station follows. 308 $283 :03me 228 So: 28 Ame—mgr: some 2223 b80922 A3355 @823 323% “cocagom .32 E 333% 2.236 2838 mo 30:82 wfiaofi :2on smegma—Z 05 Co ENE .70 Esmfi 4 958.552 « «bis—Sm 52.2—5m :uE=m=m a32%: 309 Permanent Stations SUSUMAN (62.7792° N, 148.1633o E) Susuman is a town of approximately 5,000 people and is the center of placer gold production in the Magadan region. The seismic station is operated by MEMSD and is located at the meteorological station just west of the main town. The station is sufficiently far from the town that cultural noise should be minimal. The seismic vault in Susuman consists of a large concrete block set approximately 1.5 111 into a permafrost foundation. TALAYA (61. 1337° N, 152.39 80° E) Talaya is a resort town of approximately 500 people; the main attraction is a large resort with natural hot springs. The seismic station is operated by MEMSD. The seismic vault in Talaya consists of a concrete pad set into volcanic bedrock on the side of a hill. The seismic station is located on the east side of town next to the now abandoned cinema theater. NELKOBA (61.3383° N, 148.8128° E) Nelkoba is a small town whose primary function was a regional supply and repair center for support of the gold mining industry in the region. The seismic vault in Nelkoba consists of a concrete pad measuring approximately 1 m x 1 In set into permafrost. The concrete pad is housed in a small wooden shed on the grounds of the Nelkoba kindergarten, on the west side of town. The current station site was constructed in the summer of 1997 and is approximately 200 in north of the previous site. At some time in the past, SKM instruments were Operated at the old station site. The town of Nelkoba was permanently abandoned in late September, 1999. The station was moved into an abandoned mine near the Matro sova temporary station deployment site (see below). 310 UST’ NERA (64.565° N, 143.242° E) The seismic station in Ust’Nera is operated by the Yakutsk seismic network. The station in Ust’Nera has occupied its present site since 1992. The seismic vault consists of a large concrete block set into a permafrost foundation. The vault is located between two large apartment buildings, thus cultural noise can be high at times due to passing cars and children playing on top of the vault. Seismometers recorded here are Russian (Soviet) SKM short period instruments. Seisrnometer output is amplified 60 db and a 30hz cutoff low pass filter is applied. The amplifier used in Ust’ Nera is a USGS Prototype Series Seismic Amplifier. The amplifier was originally part of an IASPEI system installed by the author in Ust’Nera in 1997. Temporary Stations MATROSOVA (61.6432° N, 147.8205" E) Matrosova is a small town with an operating gold mine. Seismometers were located on the concrete foundation of a building used for ore transfer. The foundation of the building was set into bedrock consisting of black shale. In Matrosova, digitization of time and all seismometer components was 120 s.p.s. The station was operated for only a few hours, to record blasting from the mine. Two large blasts of 1,300 and 1,500 kg of Amrnonite were recorded, among several much smaller explosions. Several abandoned mine adits are also in and near Matrosova, which may be good sites for future permanent stations. In late September, 1999, equipment from the closed station in Nelkoba was moved to a site a few kilometers northeast of Matrosova and a new permanent station was constructed (Omchak, 61.67° N, 147.87° E, h = 820 m). The station was constructed in an abandoned mine adit. 311 STOKOLVIYA (61.8475° N, l47.6598° E) Stokolviya is a hydrological research station in a remote, unpopulated region. The seismic vault consisted of a one meter pit dug into the side of a hill. Ground material consisted primarily of angular cobble sized rocks of volcanic origin. The material was consistent with the bedrock surface being close. When installing the station, it was intended that Stokolviya would be a permanent site. All permissions were obtained, and the hydrologic research station agreed to operate it prior to installation. However, the individual workers at the station were unfamiliar with computers and refused to consider Operating the station. This was unfortunate, as the site is quiet. Also at Stokolviya are several boreholes, some of which exceed 200 m depth. These boreholes are not is use, thus may be ideal for borehole instrument installation. Results of Station Deployments and Future Research One objective of the station deployment was to try to record regional mine blasts to get ground truth seismic velocities and seismic waveform signatures. Two blasts (1,300 and 1,500 kg) from the Matro so va mine were recorded at the mine and at the station in Stokolviya (Figure C-2). The explosions were not large enough to be recorded at the Other deployed digital stations. Equipment from the temporary stations deployed in Matrosova and Stokolviya is being redeployed. One station has been installed in Seimchan. The GEOSCOPE station in Seimchan has not operated for several years due to a failed tape drive used to log data Based on conversations between MEMSD, who operate the station, and the French who installed the station, there is little hope of repair in the near future. Therefore, the output of the existing Streckeisen seismometer is being recorded at 30 Sp. 5. on one of the ViSeis PC based 312 1099.07.02 0041:30 772m: 0.0031115 0.0001105 i: 0.001110: 5 cars L 1 I 1999.07.02 1:041:30 772m: 1 01mins 108 205 ”s o. ows _ «i- W 5 0001098 1 . . . : 0.001 999 07 02 0041 30 772m 305 0.0011113 0.0005003 F; E 00005003 § 41.001 0.001515 1909.07.02 00:41:30 772m; 0.002003 105 205 305 0.001% g " 5 00010415 , 1099.07.02 004130 772113 15009418 . -. 105 206 305 55m 1': ows . E 415-000w: "Em 1000.01.02 09:41:36 772111: ° " 1 16009413 05 20s 303 52000015 011113 B W 1m.07.02 004130 1721114 ' " 105 20s 1am ”5 se-ooaus g “”3 ‘ a 0) 55000103 “'5 20$ ans Figure 02. Recording of 1,500 kg blast from the Matrosova gold mine. Trace l - time, traces 2,3,4 - from Matrosova mine (250 m from blast), traces 5,6,7 - station at Stokolviya (25 km north of Matrosova). 313 systems. Data acquisition began in early October, 1999. The remaining station and a Guralp CMG—40T seismometer will be moved to a permanent location in Anadyr in late 1999, travel permitting. Figures C-3 through OS are sample seismograms recorded at the deployed stations. All seismograms are raw data. It is hoped that additional larger explosions can be recorded in the future to use as ground truth events to develop seismic discriminants between explosions and tectonic events and to improve location capabilities. This deployment of digital stations is the first step in developing a digital seismic network in the region. The digital stations will allow much easier analysis of the data, as well as greatly increase the portability of the data The next proposed station deployment will be Okhotsk, in the summer of 2000. Okhotsk is on the north shore of the Sea of Okhotsk, about 400 km west of Magadan. A station here would provide data in the “hole” of seismicity north of Sakhalin Island that occurs near the juncture of several of the northeastern Russia seismic network boundaries. Microseismicity levels have never been studied in this region, although it is speculated that the boundary between the Eurasian plate and Okhotsk plate is in this vicinity. Other Stations Visited In the course of fieldwork associated with this dissertation, several other seismic station sites have been visited in the Magadan and Yakutsk region. A brief description of station sites is given below. Coordinates listed below were obtained on-site with a GPS unit. 314 1900.00.24 0525:“ 14am 0W G.M1WS 0.0001ws 1 000020115 L 1999.” 05:25:14 143!!! 105 208 ans 406 508 605 QM D.M‘l W3 mums «QWHAISJ QM! QWISI 19g0024 053314 143m 10$ 205 ”S ‘08 508 608 QM 0.0001ws 0.0001ws: 0000211051 0W8 Figure C-3. Local event recorded at Susuman on June 24, 1999. Epicenter unknown. 315 1999.09.18 21 23315 795111: o.ooosws '] l fin .3 :— 6:: M E g P24!) 11ka 0.000305% J I j «0.001ws 1999.09.18 21:”:45 795m: P36 208 405 605 NS 1005 G.MJ ’ ...— 0000291131 j {IW‘ 1 1999.09.10 213045 795111: [0T6 208 405 cos cos 1005 J 3 l ‘1 l g @— 32>, P24!) 000041031 Figure 04. Regional distance recording from Nelkoba of Mb 6.2 Kurile Island event of September 18, 1999. 316 1990.00.01 16:15:56 1451113 G O.W1 HIS! -:—- . ll se-ooswsI {\ P24!) IU unZ _—= ”WWWW l” l éE-ooswsr U 1999.00.01 16:15:56 1401113 ZEM G 205 405 605 cos TEN, « Hill!“ W“ I “INN M 3 45005111131 i 2500501131 1999.00.01 16:15:56148111: ORG 206 405 605 005 ‘E-(XJSWSJ ... 11 1 “ZEM! 1 ~11; W011: 002 Figure C-S. Teleseisrnic vertical component recordings of the Mb 5.8 California-Nevada border region event of August 1, 1999. Stations are, from top to bottom, Ust’Nera, Nelkoba, and Susuman. 317 KULU. (61.889° N, 147.431° E) The town of Kulu is an agricultural town of about 200 people. The station was located in a house approximately 1 km north of the center of town, which is outside of town. The vault, located in a wooden outbuilding, consists of a large concrete pad set into permafrost. The town of Kulu is presently about 50% abandoned. DEBIN. (62.339° N, 150.751o E) The town of Debin is located along the Kolyma River. The seismic station was located in the southwest portion of the town, about 100 mnorth of the Kolyma highway. The station was in a house which is now destroyed. Based on station location, it is assumed that the vault consisted of a concrete pad set into permafrost. MYAKIT. (61.407° N, 152.093° E) The station in Myakit was located near the center of town. Within the town, the station was in a house just in front of the of the blue wooden schoolhouse, and about 30 m off the east side of the Kolyma Highway. The vault in Myakit consisted of a small concrete block (about 0.7 m on a side) set into permafrost ground. The town of Myakit is now entirely abandoned, with the schoolhouse one of only a few buildings remaining intact. The house containing the station has been burned. PROVIDENIYA. (64.424° N, 173.226° W) The station in Provideniya was located in a large apartment building just west of the center of town. The site is located at the base of a mountain consisting of volcanics. Although the exact vault was not inspected, ground material around the site consists of rock rubble and dirt matrix. 318 MAGADAN. Two station sites have been visited in Magadan. The original Magadan station, and headquarters of the Magadan network, is located in a residential part of the city about 1 km south and a little west from the center of town. The station consists of a single story moderately sized wooden office building. The seismic vault is located under the station in a cellar about 5 m below the ground surface. The vault contains a large concrete pad measuring about 2 m per side mounted in a rocky soil. This old station site in Magadan had considerable noise due to vault conditions and cultural noise. The vault is no longer used except for instrument testing purposes. The new GSN station Magadan (MA2) is recorded at the old Magadan station site. He vault of the new Magadan GSN station is located on top of a mountain about 2.5 km to the northwest of the old station. The vault consists of a bunker set into bedrock of granitic CO nmosition. YA KUTSK. The seismic station at Yakutsk is the headquarters of the Yakutsk network, and is the site of the GSN station Yakutsk (YAK). Located about 2 km southwest of the center 0 f t own in a residential/light industrial area, the station property contains two buildings. The mail-‘1 station building consists of a single story moderately sized wooden office building. The Second building is a two story concrete building used for storage and engineering work. The seismic vault contains a large concrete pad and is located at the bottom of a 15 m Vertical shaft below the concrete building. The entire shaft is in permafrost. Coordinates for ‘he station in Yakutsk (see Table 1-1 or A-l) were taken from 1:200,000 scale Russian I“illitary topographic maps. 319 BATAGAI. The town of Batagai, in northern Yakutia, is a regional headquarters for geological expeditions. The seismic station here is located about 1 km west of the center of town in a wooden house. The vault is in a room attached to the side of the house and consists of a dirt pad on permafrost ground. A PC based IASPEI digital acquisition system recording a three component set of Kinemetrics Ranger seismometers was installed in Batagai in 1993 by S. Crumley of the GeOphysical Institute, University of Alaska The seismometers failed within weeks due to leveling problems with the dirt pad. The Rangers were removed in 1995 , with the IASPEI system remaining in place and recording the Russian seismometers. The station was rebuilt in 1996 or 1997 and a concrete pad may have been installed. I RKUTSK. The station in Irkutsk is at the Institute of the Earth’s Crust. The institute is lo cated approximately 3.5 kilometers south and a bit west of downtown Irkutsk, across the 1‘th gara River. The vault is located in the basement at the rear of the main building on the institute grounds. It is unclear whether the concrete pad in the vault is isolated from the building foundation. Judging from the surrounding area, the vault is located in soil as 0 ppo sed to permafrost or bedrock. 320 APPENDIX D Output of event relocations for comparison with lul’tin and Western Alaska network hypocenter determinations APPENDIX D Output of event relocations for comparison with lul’tin and Western Alaska network hypocenter determinations EVEINT OF 1981 4 6 MAGNITUDE = 2.1 STATIONS USED = 5 DEPTH FREE, ORIGIN TIME PREE, HYPOCENTER FREE, NO CORRS USED NO CORRS WRITTEN N ORIGIN TIME DOT LAT. DLAT(KM) LONG. DLON(Iotuc>octhOan H N I I I I I ...; N N I I I I I I I b‘DNONNmmmdhmw Ht» mrau) I I I n) rawpa w “QWQQOOthOQUWQOO-‘WQGQHQ I I I I I I I I I I I I I I I I I I I I I I qwummowqmwoqwuthwhoqwq H PM NNH I I I I I I wwwwwwwwwwwwwwwmwmmwmmmmwmmmmmmmmmmm gggggggoooooooo0000000000000000000000000000 00m mmwwwwwwwwmwwwwwwwwwwmwwmmmmmmmmmm mmmw wwww mcv wwwwww U)M\O\Jwtbd\m~40\mcnO\N\oa\><3cr4~Jo<301NIHCJH MFOOOOOOwNNNNPPOONNNNHooooooowmwwwpwwwoouUH NHNmQhfiF—‘HOQI’ANONH MM” w330444400000mmmm0000mmmmmmmmmmmmpppgppbt800000 COMP (fl (9 9m 915 07w 000 \0 N ...I www W S HHOO 928 001 005 1022 31025 1029 851030 851106 851108 851109 851110 851122 851122 851123 851128 851128 851129 851204 851208 851210 851217 851221 860105 860106 860108 860110 860118 MM wwwmw (fl 19 04 06 15 22 11 13 01 20 09 22 06 03 20 04 06 12 12 15 19 12 02 06 21 19 19 04 15 10 04 12 15 03 00 13 11 18 05 09 12 16 16 22 15 03 19 21 10 03 19 01 05 18 22 02 07 10 03 07 03 04 16 00 02 23 08 20 16 05 08 12 16 05 00 05 07 02 13 26 51 13 50 01 27 50 27 37 22 29 11 19 36 41 02 17 48 20 47 57 52 22 32 54 04 08 16 15 04 10 38 54 37 30 07 35 12 42 15 03 34 19 41 03 03 52 44 28 09 12 59 32 26 06 54 34 17 53 09 47 53 37 00 38 13 57 42 28 27 43 42 35 27 25 30 13 55. 39. 01. 16. 56. 44. 51. 19. 20. 31. 16. 40. 53. 31. 22. 57. 30. 42. 35. 09. 00. 04. 43. 05. 59. 60. 09. 15. 24. 33. 07. 04. 34. 55. 05. 44. 16. 40. 08. 49. 39. 53. 18. 56. 38. 47. 05. 56. 10. 20. 29. 59. 11. 35 11. 08. 06. 09. 51. 47. 19. 51. 39. 32. 00 22. 49. 09. 03. 32. 14. 11. 49. 22. 24. 18. 38. 22. U1 qmowomwwmmmomww \lm hanra thU‘O OWWO‘CDO‘O 500‘ N qummoo W0 0 63 51 53 50 53 70 54 54 65 50 59 53 59 62 64 52 65 65 54 60 54 60. 54 54 55 55 64 55 65 52 53 64 65 54 54 67 69 53 69 55 54 55 48 49 62 58 53 62 53 55 54 49 53 59 54 53 55 54 63 65 53 50 53 60 64 64 57 53 55 54 55 52 51 70 53 52 64 60 .59N .18N .40N .98N .62N .43N .48N .51N .37N .86N .22N .62N .79N .81N .38N .04N .51N .52N .86N .20N .33N 03N .19N .42N .09N .58N .85N .14N .23N .50N .62N .85N .25N .04N .19N .41N .80N .68N .84N .91N .85N .61N .97N .80N .23N .92N .71N .22N .43N .95N .25N .71N .75N .43N .11N .58N .41N .74N .28N .67N .98N .30N .73N .63N .30N .89N .29N .62N .86N .35N .16N .91N .26N .20N .08N .35N .70N .21N 148 133 132 132 125 141 125 135 142 141 124 152 146 175 122 171 126 143 122 151 125 123 131 133 144 123 144 132 135 144 144 127 125 171 133 125 176 126 126 130 126 131 145 149 135 143 125 123 127 125 150 128 125 132 134 172 136 122 125 149 175 144 127 125 125 135 128 128 133 128 134 132 153 .488 .048 .918 .088 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05.48 31.9 59.41 58.46 02.6 06.28 08.6 26.1 38.20 06.89 60.56N 54.78N 63.56N 48.69N 54.72N 68.86N 63.98N 48.31N 48.61N 64.81N 64.11N 64.47N 54.00N 60.26N 59.82N 63.66N 49.87N 52.86N 53.96N 59.12N 70.72N 64.07N 49.40N 59.75N 54.97N 57.40N 55.30N 51.81N 64.86N 55.45N 55.89N 53.55N 64.08N 66.96N 72.81N 51.56N 59.89N 54.80N 49.03N 50.08N 72.87N 54.71N 49.58N 65.11N 52.15N 53.61N 56.50N 64.60N 53.87N 53.91N 55.07N 63.55N 63.40N 48.70N 48.75N 52.98N 56.01N 52.34N 69.75N 52.41N 57.63N 51.96N 55.60N 52.17N 53.60N 54.01N 65.47N 55.65N 55.65N 53.69N 65.29N 138.728 0. 139.278 22.8 179.798 0. 126.418 26.1 131.678 17.3 128.238 9.2 153.108 3.5 126.258 33.0 126.398 19.1 144.028 16.2 153.468 15.4 172.58W 5.7 136.798 19.1 161.048 10.5 144.988 17.9 152.668 11.1 131.678 16.1 134.748 17.1 128.268 15.8 154.488 15.4 130.298 10.5 145.258 6.5 131.778 6.0 145.798 14.1 135.388 5.2 126.388 30.3 123.558 30.9 131.938 5.2 144.058 16.0 124.448 23.8 124.308 24.1 124.748 4.6 145.258 9.8 130.348 23.4 126.318 25.3 138.098 5.3 156.868 7.9 131.088 14.7 131.638 24.0 132.368 15.7 121.698 16.8 126.498 11.2 123.458 33.0 176.28W 2.7 126.328 20.0 125.478 6.7 121.038 9.0 147.108 17.0 123.798 27.0 123.518 33.0 123.428 33.0 147.808 5.6 143.988 22.8 126.618 29.3 126.368 15.9 132.588 17.9 125.998 20.1 138.558 13.9 140.408 5.2 131.718 17.8 125.528 12.4 122.378 5.5 120.768 25.8 136.178 4.6 132.078 13.4 134.378 8.5 173.10W 7.3 126.508 21.7 126.498 11.1 125.518 11.4 170.33W 33.0 5.1 5.2 63.88N 178.48W 16.6 511 51.62N 65.37N 53.73N 55.77N 54.21N 63.78N 130.118 12.5 173.06W 24.3 125.538 13.8 130.588 5.7 126.068 19.5 145.728 12.2 861129 861130 861202 861203 861209 861218 861218 861218 861221 861225 861226 861229 861231 870109 870109 870110 870110 870116 870120 870122 870131 870203 870204 870204 870205 870208 870209 870210 870211 870211 870211 870211 870211 870211 870211 870211 870211 870211 870211 870211 870212 870212 870214 870218 870221 870222 870225 870303 870304 870308 870308 870310 870316 870322 870324 870326 870329 870403 870405 870405 870412 870415 870421 870421 870421 870422 870430 870430 870430 870505 870510 870512 870516 870524 870525 870526 870528 870602 20 03 11 19 22 09 12 18 04 19 03 17 07 04 13 19 19 23 07 19 03 17 01 05 09 19 23 12 00 01 01 01 05 06 07 07 07 08 11 17 02 02 07 21 19 16 03 04 00 06 13 06 20 00 02 00 20 13 10 11 02 23 12 13 18 03 03 05 20 22 12 09 09 16 00 13 14 16 04 17 06 32 54 26 11 04 28 12 21 16 21 29 14 33 34 53 42 01 55 16 43 58 30 57 00 34 58 03 09 22 52 19 28 28 47 42 36 32 10 12 16 53 33 12 03 52 09 47 52 00 58 19 39 45 16 06 00 36 31 09 39 21 20 38 57 41 04 29 11 41 29 15 58 59 46 40 59.9 12.70 20.00 37.5 21.44 40.43 14.4 12.03 23.9 29.45 04.97 49.6 58.3 45.00 17.46 25.18 39.53 18. 18. 23. 08. 45. 46. 31. 41 21. 01.30 42.73 20.93 09.55 51.56 23.15 49.59 16.38 18.00 59.72 11.44 45.18 16.41 23.70 47.0 21.01 29.76 15.3 14.6 04.33 38.29 02.51 28.42 26.71 12.05 02.0 53.45 50.43 54.16 57.33 36.8 12.1 15.49 36.46 18.5 21.1 48.43 00.30 46.72 20.69 45.88 35.5 20.34 03.2 04.73 25.8 41.1 47.7 03.0 12.68 55.68 20.06 I mHquotsH 54.26N 64.94N 54.32N 53.66N 64.13N 67.49N 51.85N 61.20N 49.19N 62.36N 61.37N 55.04N 56.59N 60.04N 67.18N 65.44N 65.35N 58.57N 50.28N 55.71N 56.67N 55.54N 52.90N 64.53N 57.52N 49.15N 65.44N 65.47N 62.87N 62.77N 62.84N 62.89N 62.88N 62.85N 62.84N 62.89N 62.87N 62.86N 62.86N 62.88N 49.62N 62.81N 59.44N 50.35N 53.49N 62.90N 63.02N 65.40N 61.15N 62.06N 59.77N 50.41N 64.08N 65.40N 65.41N 65.43N 52.46N 52.88N 63.11N 63.18N 53.61N 58.56N 64.92N 64.82N 64.90N 72.18N 69.34N 54.74N 62.87N 56.80N 64.46N 51.85N 48.44N 54.38N 53.26N 65.38N 66.96N 62.74N 131 154 126 125 148 171 133 143 130 156 149 122 121 153 172 173 125 134 130 124 132 144 120 131 172 172 156 156 156 156 156 156 156 156 156 156 156 134 156 148 132 132 156 179 136 148 143 145 143 167 172 173 173 142 134 174 174 125 121 170 170 170 129 178 123 156 122 172 132 130 121 133 173 172 156 .378 .548 .128 .508 .428 .24W .698 .738 .838 .418 .448 .768 .118 .018 140. .92W .38W .118 .788 .748 .788 130. .728 .638 .858 .488 .78W .65W .848 .888 156. .878 .788 .778 .918 .858 .868 .868 .868 .858 .748 .808 .038 .288 .138 .838 .048 .298 .428 .578 .498 .548 .318 .98W .05W .00W .088 .638 .898 .818 .698 .598 .43W .86W .47W .738 .238 .288 .868 .958 .64W .608 .958 .758 .058 .27w .17W .738 088 638 788 04 H H hJHrunJHvu N F‘thhJH FJMFJ tAhJHrAF¢HrakaN +4030(DKDthkJfilbfimdemcath(DUJNm80C>mnaqcnav4u>m x) o a U'I 6.2 HNHH MommuMHhommuwomwwoch-uwwmmwmw N I I N N I I I I I I HPJF' QQHmHQHGWmeowmi-J\lOQUIWUinh-IQGDUI H H I I I I I I I I I 880728 15 30 47.34 57.12N 129.478 13.7 . 890423 02 45 40.1 57.08N 122.178 24.1 880804 06 07 15.39 56.01N 129.858 1.4 . 890423 03 44 27.9 57.06N 122.188 25.1 880818 08 00 43.08 59.69N 145.678 0. . 890423 17 58 28.7 57.09N 122.228 29.3 . 880825 05 03 27.58 56.87N 127.158 5.6 . 890424 01 33 59.8 57.11N 122.278 24.4 5.0 880825 07 17 24.40 56.86N 127.178 7.8 . 890424 04 45 38.19 58.95N 151.878 10.0 . 880830 12 58 55.7 54.00N 137.688 18.6 . 890424 08 18 37.1 57.09N 122.188 26.7 880902 09 35 32.4 54.16N 122.718 33.0 . 890428 15 20 53.0 57.07N 122.218 33.0 880922 11 25 41.9 57.41N 122.688 19.2 . 890429 00 05 07.9 57.18N 122.208 33.0 880922 22 58 11.30 61.89N 160.488 17.6 . 890429 01 27 13.8 57.09N 122.228 30.6 . 880923 08 32 14.25 55.45N 137.218 33.0 . 890429 06 25 38.9 57.15N 122.248 33.0 5.6 880926 08 59 49.57 67.53N 143.868 5.8 . 890429 07 05 24.2 57.14N 122.258 28.2 . 880930 03 09 09.3 54.15N 137.758 10.4 . 890429 08 40 01.6 57.08N 122.308 0.0 881007 00 56 29.24 57.57N 125.128 17.6 . 890429 10 33 56.1 57.05N 122.298 0.0 881013 00 32 09.10 61.71N 169.788 8.7 5.9 890429 17 35 30.1 57.05N 122.258 0.3 881013 00 49 26.91 61.81N 169.638 33.0 . 890429 21 00 46.4 57.08N 122.258 24.4 881013 03 24 33.51 61.95N 169.628 24.0 . 890503 23 53 38.4 57.03N 122.178 23.2 881015 07 05 37.98 61.89N 169.578 25.4 . 890504 14 10 28.4 56.57N 121.158 14.8 881017 00 27 59.09 62.85N 148.808 6.7 890504 14 10 28.4 56.57N 121.158 14.8 . 881020 04 17 55.42 56.77N 127.118 6.1 890506 06 54 38.5 57.13N 122.028 26.8 . 881024 07 15 32.1 54.80N 133.028 17.2 890507 16 28 05.9 57.08N 122.228 33.0 5.1 881025 10 12 52.89 62.87N 148.858 8.5 890508 11 49 47.0 57.05N 122.218 27.8 . 881101 21 37 31.2 56.85N 123.938 14.0 890511 14 37 43.2 57.07N 122.208 29.4 . 881102 23 32 55.4 53.86N 126.398 6.0 890514 16 21 51.7 57.14N 122.198 27.0 . 881109 13 07 58.6 57.90N 120.578 22.5 890517 05 04 35.8 57.05N 122.248 32.0 5.8 881112 10 27 10.91 57.77N 126.458 15.2 890517 07 25 49.0 57.05N 122.258 30.7 . 881118 11 29 32.4 57.86N 121.198 26.0 890517 07 40 39.1 57.00N 122.138 23.5 . 881118 22 22 05.1 56.67N 121.688 7.6 890517 10 21 02.1 57.06N 122.148 19.9 . 881203 15 29 49.50 56.06N 126.358 11.5 890517 15 55 22.9 57.0€N 122.248 32.1 . 881207 16 50 30.55 65.23N 144.668 0. 890518 10 06 01.1 57.02N 122.218 29.8 . 881207 18 23 22.27 65.29N 144.718 0.4 890519 15 39 08.1 57.06N 122.228 26.3 . 881222 20 10 11.1 52.46N 134.438 28.5 890519 19 29 23.96 62.42N 155.218 3.3 . 881224 02 43 21.51 64.10N 148.748 8.9 890519 22 06 27.6 57.07N 122.208 27.9 . 881226 13 29 00.65 61.63N 168.938 10.9 890523 08 12 12.2 57.00N 122.188 29.2 . 881230 07 16 43.30 61.12N 153.708 1.7 890524 09 38 02.21 61.31N 144.728 11.2 . 890113 12 52 11.1 49.01N 131.598 22.1 890524 19 42 32.5 57.07N 122.238 30.5 . 890113 16 21 59.87 64.59N 147.178 10.4 890525 11 48 06.2 57.06N 122.238 33.0 . 890114 04 53 36.49 61.89N 143.748 9.5 890601 08 17 10.5 57.07N 122.208 29.9 . 890115 17 17 30.78 58.69N 151.208 6.1 890601 16 41 42.51 63.47N 139.828 0. . 890115 22 13 26.8 57.08N 121.618 32.5 . 890601 16 41 42.51 63.47N 139.828 0. . 890124 16 50 14.54 65.41N 144.438 19.0 . 890601 19 25 46.7 57.07N 122.298 28.4 . 890129 23 23 01.67 62.86N 144.888 12.7 . 890601 22 38 53.0 56.99N 122.168 28.5 . 890202 07 18 12.67 57.59N 128.308 13.1 . 890603 08 57 17.1 54.51N 123.238 23.5 . 890205 16 26 59.20 54.21N 126.668 25.7 . 890604 04 17 01.52 68.12N 132.548 30.9 . 890212 03 54 21.16 59.20N 147.668 5.7 . 890604 21 49 34.7 57.08N 122.148 27.6 . 890213 09 35 29.09 66.17N 172.588 11.8 . 890605 00 13 20.7 57.02N 122.228 30.3 . 890215 18 46 19.32 58.07N 129.428 17.3 . 890608 22 41 20.2 57.10N 122.258 24.4 . 890217 02 57 48.16 63.37N 146.198 6.9 . 890610 05 09 43.96 71.31N 129.138 2.9 . 890217 19 10 15.20 69.80N 129.148 20.4 890615 16 24 12.2 57.03N 122.238 30.2 . 890218 05 47 41.5 48.52N 131.478 2.5 890616 07 36 08.68 63.57N 142.748 9.0 . 890219 18 00 30.11 65.18N 146.228 5.7 . 890616 19 40 45.76 61.02N 145.448 14.6 . 890224 11 44 19.57 61.25N 162.918 9.8 . 890617 13 37 27.02 67.24N 143.758 8.5 . 890225 04 56 17.06 61.73N 157.718 8 6 890619 15 48 34.3 57.13N 122.278 30.3 . 890321 10 53 05.21 64.91N 145.198 9 3 890622 20 52 20.7 57.10N 122.288 23.5 . 890329 01 10 46.0 56.79N 123.528 15.5 . 890627 10 38 36.3 57.15N 122.308 27.3 . 890331 12 47 12.06 69.73N 128.968 17.0 . 890628 20 16 28.1 57.05N 122.148 26.5 . 890404 07 30 24.43 62.09N 157.498 20.3 . 890630 20 00 00.25 59.25N 152.738 2.2 . 890409 04 16 24.70 59.80N 145.148 33.0 5.0 890701 14 14 40.42 57.21N 137.808 33. . 890410 09 12 58.94 66.75N 174.60W 33.0 . 890702 22 58 49.62 59.61N 150.018 4.2 . 890412 00 45 33.7 56.72N 120.378 22.2 . 890706 04 08 09.4 57.09N 122.228 33.0 . 890420 22 56 00.2 57.13N 122.238 31.8 . 890707 10 51 43.57 64.99N 141.668 14.4 . 890420 22 59 52.3 57.34N 122.118 16.3 6.6 890709 18 11 49.32 59.91N 152.718 8.8 . 890421 00 04 44.9 57.04N 122.188 23.4 . 890709 20 07 46.3 57.09N 122.328 33.0 4.2 890421 00 16 00.7 57.08N 122.208 18.8 . 890709 21 04 27.2 57.06N 122.248 33.0 . 890421 00 52 30.7 57.04N 122.188 22.3 . 890710 07 19 33.9 57.05N 122.208 33.0 . 890421 00 57 54.6 57.01N 122.178 19.2 . 890713 19 13 54.00 62.57N 143.888 0. . 890421 01 04 20.1 57.1ON 122.068 31.5 . 890716 16 32 31.2 57.07N 122.218 30.4 . 890421 01 44 00.1 57.01N 122.198 17.7 . 890718 03 02 54.7 57.07N 122.228 33.0 . 890421 03 12 05.8 57.07N 122.148 23.9 . 890719 00 24 03.7 57.08N 122.178 26.8 . 890421 08 10 05.7 57.05N 122.208 22.7 . 890720 02 53 38.2 57.05N 122.178 30.1 . 890421 08 29 29.5 57.06N 122.168 27.7 . 890720 03 16 21.8 57.05N 122.188 31.5 . 890421 08 30 08.2 57.1ON 122.168 25.7 . 890720 23 26 43.7 57.09N 122.158 9.7 . 890421 08 51 39.0 57.04N 122.188 28.6 . 890721 01 40 08.2 57.10N 122.168 10.3 . 890421 15 22 01.0 57.04N 122.228 27.6 . 890721 01 46 35.0 57.04N 122.168 26.6 . 890421 15 49 35.7 57.05N 122.218 25.7 . 890721 07 43 50.5 57.08N 122.208 10.5 . 890421 19 08 37.4 57.07N 122.278 33.0 4.9 890722 02 36 12.6 57.09N 122.158 7.5 . 890421 19 29 28.5 57.06N 122.258 30.8 . 890722 10 01 40.8 57.07N 122.228 27.7 890421 20 09 42.2 57.07N 122.228 30.8 . 890723 12 01 31.2 54.54N 124.928 27.0 890421 22 30 36.0 57.05N 122.188 25.6 . 890723 14 33 00.4 54.55N 124.978 18.8 890723 890723 890724 890725 890731 890801 890801 890803 890804 890804 890804 890804 890806 890906 890910 890918 890919 890924 890924 890926 890927 891004 891017 891018 891027 891028 891030 891030 891106 891109 891109 891110 891113 891118 891202 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0. 960710 05 20 11.74 58.45N 157.708 0. 960710 06 22 05.38 58.52N 157.348 18. 960710 07 15 54.93 58.67N 157.458 15. 960710 14 09 33.80 58.57N 157.648 3. 960710 18 36 37.17 58.46N 157.308 28. 960727 07 28 34.95 60.49N 148.668 7. 960803 12 32 49.64 58.65N 157.218 29. 960803 13 09 13.64 58.68N 157.088 0. 960807 18 51 14.48 58.58N 157.218 7. 960808 17 09 40.38 58.75N 157.208 22. 960824 07 29 41.80 60.05N 153.058 17. 960902 23 37 54.6 56.60N 123.888 19. 960904 12 56 31.38 57.55N 128.038 15. 960913 15 45 07.54 58.70N 157.648 960914 05 29 59.84 58.65N 157.418 960914 05 41 45.02 58.73N 157.488 960914 09 57 57.82 58.74N 157.148 960916 03 05 07.56 58.56N 157.358 1 960925 01 48 26.12 63.38N 150.438 961024 19 31 50.91 67.04N 173.08W 961024 21 57 36.71 67.08N 173.20W 1 961126 20 18 18.31 58.66N 157.548 961207 10 38 20.98 62.12N 153.758 1 970129 15 51 38.11 58.81N 149.658 970304 21 49 26.30 62.07N 155.928 1 970607 11 59 41.19 64.19N 148.328 970614 13 31 08.39 63.87N 148.578 970616 09 54 07.72 64.07N 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APPENDIX F An alternate method of determining earthquake focal depths As discussed in Chapter 3, the depths determined for the relocated earthquakes depend primarily on the Pn arrivals, but problems may arise if the Pn velocity used is incorrect. The discussion here outlines a procedure for determining depth of earthquakes from apparent Pn arrival residuals calculated from a location and origin time determined using the best-fit Pg and Sg velocities. The depth determination dependence on the Pn arrivals can be illustrated by locating a set of earthquakes using only the best-fit Pg and Sg travel time curves, allowing the depth to range between 0 and 15 km. Because there are few close stations, the Pg and Sg arrivals cannot constrain the depth, which tends go either above the surface or below the 15 km boundary. In general, events constrained to the surface are too shallow, which results in a raypath for the Pn phase that is too long. Thus when we observe the residuals associated with the Pn arn'vals, they should be negative. Likewise, events constrained too deep shorten the Pn raypath, which will result in positive residuals. This is observed in a test area from the Magadan region (6063 N x 150-155 E); the same test area and events are used throughout the remainder of this discussion. Here, we see Pn residuals for events constrained at the surface generally being negative, at around -2.0 seconds, and Pn residuals for events constrained at 15 km generally being positive, at about 1.0 second (Figure F-l). If the Pn phase arrivals were used in these hypocenter determination, these residuals would be essentially nulled out in the depth determination. Therefore, we can use these residuals to 336 16 14~ 12- 10- Depth Pn Residual Figure F-l. Illustration showing that Pn residuals are low when depth is 0, and high when depth is 15 km. Events here were located by Pg and Sg arrivals with the best fitting velocities. The Pg and Sg arrivals are generally unable to constrain the depth which was allowed to vary from a minimum of 0.0 km to a maximum of 15 km. Events used here are from the Magadan test region. 337 calculate depths when the actual Pn velocity is unknown, or differs from the available travel time curve. First, an assumption is made that for continental seismicity, the average depth of earthquakes is 10 km Of course, this can be changed, but 10 km is probably reasonable. In this procedure, a set of earthquakes is located with the Pg and S g phases only, using the best- fit velocities determined by the trial method outlined in Chapter 3. However, the depth here is constrained to 10 km in the location procedure. For each event, the Pn residuals are summed and the average residual is determined. Because we make the assumption that the average depth is 10 km, we would expect that half of the events would have positive residuals, and half would have negative, and that the sum of all the average residuals would be zero. Of course, this would actually only be true if the Pn travel time curve were correct. If the Pn travel time curve were too fast, we would expect the sum of all residuals to be positive, and negative if the travel time curve were too slow. Correcting for this will be discussed below. A table can be constructed by using the Pythagorean Theorem to determine the differences in travel time for depths above or below the assumed 10 km average. An average crust with a Pg velocity of 6.0 km/sec and 35 km thickness overlying an 8.0 krn/sec mantle is used here. Although the actual thickness and velocities may vary somewhat, the effect of this on the travel times is negligible, as we are only worrying about changes in the initial short downgoing segment. Figure F—2 illustrates the simplified crustal model, with three different initial earthquake depths of O, 10, and 35 km. In the case of the 10 km depth, the assumed earthquake average, it will take the downgoing P wave 7.72 seconds to reach point A (the critical angle here is 41.4°, thus the wave travels 37.8 km in the crust and 11.4 km 338 (it 0 km V=6.0 kin/sec 10km | [35 km A V = 8.0 km/sec Figure F-Z. Simplified crustal model and diagram used to calculate differences in Pn travel time from hypocenters of varying depth to point A. Path length from point A to the seismic station is the same for all depths. along the Moho). For the event at 0 km depth, the P wave takes 8.82 seconds to reach point A, traveling 52.9 km through the crust. The path beginning at a 35 km depth travels 39.7 km along the Moho boundary, taking 4.96 seconds to reach point A. Thus, relative to the event occurring at 10 km, the difference in travel time for the 0 km event is 1.1 seconds, and the difference for the deeper event is -2.76 seconds. Column 2 in Table F—l lists the theoretical Pn travel time differences relative to a depth of 10 km for events up to a depth of 25 km. It is also necessary to adjust our table here to account for a shift in the origin time determined with the Pg - S g best fit travel time curves as the depth is increased or decreased from the 10 km average. It would be difficult to theoretically calculate the effect of changing the depth on origin time because this is related to the distances and distribution of stations relative to the epicenter, which are different for every earthquake. Therefore, an empirical relationship is deve10ped using the events in the Magadan region test area In doing this, the events are first located, constraining the depths to 10 km, and the calculated origin time is noted. The same events are then located again, constraining the depths at 0, 5, 15, 20, etc. km, and the shift in origin time from the 10 km depth determination is noted for each event at the various depths. It should be noted that changing the constraining depth of the earthquakes between 0 and 25 km has essentially no effect on epicenter coordinates, which generally vary less than 0003". Figure F—3 shows the resulting shift in origin times of the Magadan test area events from different constrained depths, with a second order regression fitting the data. Column 3 on Table F—l gives the result of this regression. At this point, you are probably wondering where the heck I am going with all this gibberish Because the shift in origin time is depth-dependent and not event-dependent, the origin time—depth correction can be added onto the Pn time difference in column two (Table F—l) to get column four 340 Table F- 1. Depth determination table based on Pn residuals. Depth Pn Time Dcpth- Pn h (km) Diff. from QT. Cor- Residual h of 10 km motion Shift (see) (sec) (sec) 0 1.103 0.078 1.181 1.124 1 0.9927 0.0752 1.068 1.011 2 0.8824 0.0716 0.9540 0.8966 3 0.7721 0.0671 0.8392 0.7813 4 0.6618 0.0615 0.7233 0.6648 5 0.5515 0.0548 0.6063 0.5474 6 0.4412 0.0472 0.4884 0.4289 7 0.3309 0.0385 0.3694 0.3094 8 0.2206 0.0289 0.2495 0.1890 9 0.1103 0.0182 0.1285 0.0642 10 0.0 0.0 0.0 -0.0585 11 -0.1103 -0.0063 -0.1169 -0.1788 12 -0.2206 -0.0201 -0.2407 03033 13 -0 3309 00349 -0.3658 -0.4289 14 04412 -0.0507 .049 19 -0.5555 15 -0.5515 -0.0675 -0.6l90 -0.6830 16 -0.6618 -0.0853 -0.7471 -0.8117 17 -0.7721 -0. 1042 -0.8763 -0.94 14 18 -0.8824 -0. 1241 -1.0065 -1.0721 19 -0.9927 -0. 1450 -l.1377 -l.2038 20 -l.103 -0.1670 - l .2700 -1.3366 21 -1.2133 -0.1899 -l.4032 -1.4704 22 4.3236 -0.2139 -l.5375 4.6052 23 -1.4339 —0.2389 -1.6728 -l.7410 24 4.5442 -0.2649 -1.8091 —1.8778 25 4.6545 -0.2920 -1.9465 341 Amos: @283 £982.: 85288 983 5:5 8:: 23$ 5188on 886 9508 a .23 E 8a «:5 .08: Ewto co £5 83688 25 Ex o_ :88 59% E 09:20 .8253 9:80:22 38:55 .mi ouswfi €22 £80 Egm om mm om m— _ _ _ or _ ed- r We. . so- - no- .. No- I. F.0I l C? O (S) ewu U!5!O U! mus 342 (Table F- 1), which is the expected Pn residual shift from a 10 km focal depth. This colurrm is now used to determine the depths of the earthquakes, matching the average Pn residual for an event, and reading off the depth. The origin time correction should also be used to adjust the origin time of the earthquake after the final depth is determined. As noted above, if the Pn velocity in the travel time curve used to compute the residuals is too high or low, the average of all the residuals will be nonzero when depth is held at 10 km In this case, we can assume the calculated average is simply an offset due to an incorrect Pn velocity (it could also be true that the offset is due to the average depth being other than 10 km, although it probably does not vary greatly). This correction will be termed the residual offset correction. The offset can simply be defined as the new residual zero point, and the Pn residual can be recalculated from this for each event. The new average Pn residual can then be used to determine hypocenter depths from Table FL The depth determination method is tested here with events from the Magadan test area. Table F-2 lists the Pn residuals for the events, which were constrained at 10 km, as well as the depths determined using Table FL The Pn arrivals here were not used in the locations, but the residuals were calculated using the Jeffreys-Bullin F wave travel time curve. The depths calculated in the normal location procedure, which used the Pn phase, are also given for comparison. However, in this case, the average Pn residual (sum of column four divided by 46 events) is 0.34 seconds, which indicates that the Pn velocity used to determine the residuals is a bit off, with a resulting average depth of about 7 km This is corrected by subtracting 0.34 from the average Pn residual and using the resulting value to determine the final Pn residual calculated depth (column 7, Table F-2). On average, this 343 Table F-2. Depth determinations from the Magadan test area. All depths (h) are in kilometers. EVENT SUM OF RES- NUMBER 08 AVERAGE Pn h FROM ORIG. Pn OFFSET NUMBER IDUALS (s) Pn PHASES RESIDUAL Pn RES. DEPTH CORRECTED h 1 3.375 2 1.688 0 0 0 2 5.303 3 1.768 0 0 0 3 1.940 3 0.647 5 5 7 4 -1.299 3 -0.433 14 15 16 5 -0.011 2 -0.005 10 10 13 6 3.213 3 1.071 1 2 4 7 -1.495 5 —0.299 12 12 15 8 -0.535 2 -O.269 12 12 15 9 —0.710 4 -0.177 11 11 14 10 —2.381 2 -1.191 19 15 22 11 -22.797 21 -l.086 19 20 21 12 -2.050 7 -0.293 12 12 15 13 -O.664 2 -0.332 13 13 15 14 -6.010 7 -0.859 17 15 19 15 1.815 2 0.908 2 5 5 16 —0.744 4 -0.186 12 15 15 17 1.919 3 0.640 5 5 7 18 -1.664 2 —0.832 17 17 19 19 4.749 4 1.187 0 2 3 20 1.821 3 0.607 5 6 a 21 —0.475 5 -0.095 11 11 13 22 2.305 5 0.461 6 8 9 23 0.597 6 0.099 9 9 12 24 4.676 8 0.584 5 6 a 25 4.983 6 0.831 3 4 6 26 2.198 2 1.099 1 2 3 27 2.284 6 0.381 7 7 9 28 2.330 2 1.165 0 1 3 29 3.556 3 1.185 0 1 3 30 1.693 3 0.564 5 6 a 31 0.760 2 0.380 7 a 9 32 6.353 3 2.118 0 0 0 33 1.399 4 0.350 7 7 1o 34 1.922 2 0.961 2 4 5 35 -2.097 6 -0.349 13 13 16 36 1.822 2 0.911 2 5 5 37 1.895 3 0.632 5 6 7 38 0.445 26 0.017 10 10 12 39 2.819 4 0.705 4 6 7 40 3.294 3 1.098 1 6 3 41 0.037 8 0.005 10 10 13 42 —1.700 2 -0.850 17 14 19 43 —2.614 2 -1.307 20 20 23 44 0.152 4 0.038 10 11 12 45 2.916 2 1.458 0 1 0 46 1.430 2 0.715 4 5 7 344. residual offset correction results in a 3 km increase in the depth of each earthquake for the test region, with the resulting average then being 10.0 km The depths determined by both the original location, which used Pn arrivals, and the method derived here correlate extremely well. Figure F4 compares the depth results determined from both methods. Open circles depict comparison of the original location depths with depths determined using this method, prior to inclusion of the residual offset correction (Table F-2, column 4). Here, there is a near 1:1 correlation, with differences in most cases being 1 km or less. The maximum observed difference is 5 km, in event number 40. Closed circles depict comparison of the original location depths with depths determined in this study using the residual offset correction. Although this method was not used for depth determination throughout northeast Russia, it illustrates an alternate method of depth determination using Pn residuals when the actual Pn velocity is unknown. It also illustrates the dependence of depth on PD arrivals when the crust is assumed to be one layer with one velocity for crustal Pg and Sg arrivals. 34S 30 '0 O :5 GE) 25- — o g 0 T5 20- 9 00 o O c 00 “- 15~ .. . cc» 0 O (3 .0 o 8 10‘ :0 c O E a O b 5_ O O % .0 CD. 3 0006 5 0‘ .3 Q Q) o 0 5 10 15 20 25 30 Depth determined in locations using the Pn arrivals Figure F-4. Comparison of depths determined in the normal location routine with depths determined by the Pn residual method described here. Note the near 1:1 correlation. Open circles without residual Offset correction. Closed circles with residual offset correction. "Illlllllllliilillli .-q—n-----------'