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This is to certify that the

thesis entitled

INTRAPLATE SEISMICITY IN
CENTRAL ALASKA AND CHUKOTKA

presented by

Michael J. Coley

has been accepted towards fulfillment
of the requirements for

 

 

M.S. degree in Geological Sciences

 

Date 8 Auqust, 1983

 

0-7639 MS U is an Affirmative Action/Equal Opportunity Institution

 

 

RETURNING MATERIALS:
MSU Place in book drop to
LIBRARIES remove this checkout from
w your record. FINES will
be charged if book is
returned after the date
stamped below.

 

 

 

 

 

 

 

 

INTRAPLATE SEISMICITY IN
CENTRAL ALASKA AND CHUKOTKA

by
Michael J. Coley

A THESIS

Submitted to
Michigan State University
in partial fullfillment of the requirements

for the degree of
MASTER OF SCIENCE

Department of Geological Sciences

1983

4 PP

 

ABSTRACT

INTRAPLATE SEISMICITY IN
CENTRAL ALASKA AND CHUKOTKA

BY
MICHAEL J. COLEY

Central Alaska and Chukotka (northeasternmost Siberia) exhibit high
levels of seismicity. The area is relatively far from present day plate
boundaries and lacks an easily identifiable source for the earthquakes.
A study of the areas largest magnitude events was undertaken to define a
regional model which would explain the anomalous seismic activity. Focal
mechanisms were constrained for twelve earthquakes and an additional four
mechanisms were obtained from: previously published data. Normal and
strike-slip faulting mechanisms were found to be more predominant in
western Alaska and the Bering Sea while reverse and strike-slip mechanisms
were prevalent in central Alaska. This suggests that the stress regime
changes from primarily extensional in the Bering_Sea to compressional in
central Alaska. In addition, the majority of the earthquakes occurred on

pre-existing faults.

ii

ACKNOWLEDGEMENTS

I must thank my wife for the vast amount of moral support and under-
standing she freely gave. She forgave me for the many times when I was
not there and for my all too common bad moods and anxiety attacks during
the few times we were together. She also put many hours into the prepar-
ation of this thesis. If Terri was the moral support than Kaz handled
the technical end of things. I can remember about six months of late
nights and early mornings he spent helping me with the never ending
computer problems, program bugs, technical questions, etc... which con-
stantly plagued me. His comments on the thesis were concise and to the
point. I know that his thoroughness took him many extra hours and saved
me valuable time. I must also thank Martha for her corrections on my
marginal spelling and English. A pair of people who also bear mention
are my parents who put up with me during some difficult years in high
school. Their faith and financial support saw me through times when a
degree seemed unobtainable. Four others who bear mentioning are Newberry,
Dave L., Dave P. and Bruce for the many hours of disucssion and friendship.
It made the hard times more easy to bear. Lastly, I must thank Michigan
State University and the Department of Geological Sciences for the use of
their equipment and financial assistance. This research was supported in

part by NSF grant EAR-8025267.

iii

TABLE OF CONTENTS

IntPOdUCtionoooooooooooo000000000000000000000000.0000...0000000000000.1

MethOdOIOQyoooooooooooon...00.000000000000000.oooooooooooooooooooooooo

6

First motion plots............................................... 6
Rayleigh wave radiation patterns................................. 7
Fault plane determination........................................10
Geology...............................................................14
Seward Peninsula.................................................14
Western Brooks Range.............................................l7
Yukon-Koyukuk Province...........................................17

Ruby Geanticline.................................................18
Central Alaska...................................................18
Chukotka.........................................................19
Southern Hope Basin..............................................19
Seismicity............................................................20
Seward Peninsula......... ..... ....................... ......... ...20
Yukon-Koyukuk Province...........................................21
Western Brooks Range.............................................25
Central Alaska.................. .............. .... ..... . ..... ....25
Chukotka and Southern Hope Basin.. ................ ...............26
Areas of No Seismicity...........................................26
Focal Mechanisms........................ ......... . .............. ......29
Rampart Series (Event 1).........................................29

iv

TABLE OF CONTENTS
(continued)

Chukotka Earthquake (Event 6)....................................45
Seward Peninsula (Event 7).......................................51
Event 8......... ...... ...........................................55
Events 9 and 10..... ..... .. ....... ...............................57
Events 11 and 12.................................................61
Huslia Series (Events 13 and 14) and Event 15....................64
Caro (Event 16)..................................................74
Discussion............................................................80
Conclusions... ............ ...... ...... ................................92

BibliographyOOOOOOOO0.0....00...O0.00.00......OOOOOOOOOOOOOOO000......93

18.

19.
20.

 

LIST OF FIGURES

Teleseismic earthquake epicenters in the study area............... 2
Theoretical Love and Rayleigh wave radiation patterns.............11
Index map of the study area.......................................16
Seismicity of Western Alaska: Epicentral locations.............. 23

Seismicity of Western Alaska: Relation of earthquakes to
StrUCtura] features......OOOOOOOOOO0......0.0...0.0.0.00000000000024

Chukotka epicenters...............................................27
First motion data of event 1......................................31
Rayleigh wave radiation pattern of event 1........................33
Gedney (1970) data for event 1....................................34
Aftershock relocation for event 1.................................35
Final mechanism for event 1.......................................37
First motion data of event 2......................................38
First motion data of event 3......................................39
First motion data of event 4......................................41
First motion data of event 5......................................42
Structural features near the Rampart series.......................44
Model of Weaver and Hill (1978/79)................................46

Rayleigh wave radiation pattern of event 6 before
Station de1etion000000OOOOOOOOOOOOOOOOOOOOOOOOOOOOO.0.0.0.0000000047

Rayleigh wave radiation pattern of event 6 after
Station deEetionOOOObOOOOOOOOOOOOOOOOOOOOOOOOOO...0.0000000000000048

First motion data for event 6.....................................50

First motion data for event 7.0.0.00000000000000.0.0.000000000000052
vi

21.

22.

23.
24A.
24B.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.

40.

LIST OF FIGURES
(continued)

Rayleigh wave radiation pattern of event 7
before Station de‘etion......OOOOOOOOOOOOOOOOOOO0.00.00.000.00000053

Rayleigh wave radiation pattern of event 7
after Station deletionOOOOOOOOOOOOOOO0......0.00.00.00.0000000000054

Final mechanism for event 7.......................................56
Focal mechanism solution for event 8:
after Sykes and Sbar (1974)......................................58
after Liu and Kanamori (1980)....................................58
First motion data of event 9.....................................59
Aftershock relocation of event 9.................................60
First motion data of event 10....................................62
Composite first motion data of events 11.........................63
First motion data of event 12....................................65
First motion data of event 14 from the ISS.......................66
First motion data of event 13 from the ISS.......................67
Proposed mechanisms for event 13.................................70
First motion and S-wave polarization angle data of event 13......71
First motion data of event 15....................................73
Rayleigh wave radiation pattern for event 16.....................75
First motion data of event 16....................................76
Seismic record at station "COL“ for event 16.....................78
Focal mechanism distribution.....................................81

Maximum horizontal compression (MHC) from
Nakamura and UyEda (1980)....OOOOOI.0.....00.00.00.00.0000000000086

MHC orientations from focal mechanisms...... ........ . ...... ......89
vii

LIST OF TABLES

List Of EventSOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO...00.0.00000000000000004-5

LiSt Of Focal MeChaniSMSOOOOOO0.0.0.0.0.......OOOOOOOOOOOOOOOOOO000082-83

viii

INTRODUCTION

In general, plate tectonic theory states that earthquakes occur as a
result of interactions between rigid pieces of the lithosphere as they
collide, pull apart, and slide past each other. This explains the high
level of seismic activity which is observed along the boundaries of the
plates. However, there are many areas far removed from plate boundaries
which also exhibit high levels of seismicity; for example, eastern and
central North America (Zoback and Zoback, 1980), the central Indian Ocean
(Stein and Okal, 1978), Australia, and Africa (Sykes, 1978). These
seismically active intraplate regions are not explained by general plate
tectonic theory. An increasing amount of research has been directed
toward such areas (Sykes and Sbar, 1974; Richardson et al., 1976; Wang et
al., 1979; Liu and Kanamori, 1980). Such studies have helped to define
the intraplate stress regime, zones of weakness within the plates, and
present day tectonic activity.

Central Alaska and Chukotka (northeast corner of Siberia) is another
intraplate area which exhibits anomalous high levels of seismic activity.
Amap of epicentral locations is shown in Figure I. Data for this
were obtained from the International Seismological Center (ISC) Bulletin
(1964-1981), the International Seismological Summany (ISS) from 1904 to
1963, Seismic Zoning of the USSR (Gonyachev et al., 1968), Novii Katalog
Sil'nikh Zemletryasenii. na Territorii SSSR (Kondorskaya and Shebalin,

1977) and the National Oceanic and Atmospheric, Administration (NOAA)

ENE >935 e: E mmmhzmugm quaorpmfi uEmEmmdh
H $5.9:

 

 

 

 

 

 

 

 

 

O .
O p . .
.4. S: K4 an ._.. .... .... a.

‘3

 

 

United States Earthquake publication (1928-1963). The area has long
been recognized as a zone of intraplate seismicity. However, most
authors have either studied the effects of individual events (Bramhall,
1938; St. Amand, 1948; Davis, 1960) or have focused on the southern
portion of Alaska (Gedney and Berg, 1969a,b; Gedney, 1970; VanWormer et
al., 1974; Bhattachanya and Biswas, 1979).

The purpose of this research was to determine the cause of the
intraplate earthquakes in central Alaska and Chukotka. Focal mechanisms
were constrained for twelve of the area's largest magnitude events. An
additional four mechanisms were obtained from previously published data.
A list of the earthquakes used is shown in Table 1. Each event was
studied individually to determine whether it occurred on existing faults
or structural features. A regional study of all events was done in order
to explain the areal distribution of the observed focal mechanisms. The
findings of this research support the studies of Nakamura et al. (1977,
1980) that compressional stresses are dominant in central and southern
Alaska, while primarily tensional stresses exist in western Alaska and

the Bering Sea.

 

 

 

 

 

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H u4m<h

METHODOLOGY

This research is based upon the focal mechanisms of 16 earthquakes
which occurred in central Alaska and the southern Chukchi Sea (Table 1).
Of the 16 mechanisms used, 12 were constrained within this study. These
12 events were chosen from the ISC Bulletin and the 155 on the basis of
magnitude. Larger magnitude events create larger amplitude seismic waves,
thus they are recorded by more seismic observatories and are more easily
studied. Focal mechanisms were constrained using first motion data and
Rayleigh wave amplitude vs. azimuth plots. Once a mechanism was con-
strained, the fault plane was determined if possible by geological
criteria and, for those events exhibiting foreshock and/or aftershock

sequences, alignment of earthquake epicenters.

First Motion Plots

 

The polarity of the first motions were plotted for all of the events
included in this study. Often, the first motion plots constrained the
nodal planes well enough to give a good focal mechanism. The plotting
method used in this research is described by Herrmann (1975) and is
outlined below.

The short and long period vertical component seismograms were studied
to obtain the “first motion", whether it is up (compressional) or down
(dilitational), of the P-wave. This data set was supplemented with first

motions reported in the 15$ for earthquakes occurring between 1933

and 1964 and from the ISC Bulletin for events occurring since 1964. The
P-wave takeoff angle at the hypocenter, which is a function of focal
depth and epicentral distance, is obtained from Pho and Behe (1972). The
first motion polarity is then plotted on an equal-area stereonet as a
function of station azimuth, marked on the perimeter of the stereonet,
and takeoff angle, which is measured as degrees from the downward vertical.
The first motions will ideally divide the focal sphere (an imaginary
Sphere surrounding the earthquake hypocenter) into four quadrants. The
quadrants are separated by two mutually perpendicular nodal planes.
These are planes along which no P-wave energy is radiated.

In practice, the nodal planes will not divide the focal sphere into
quadrants which contain only compressions or> dilitations. There are
almost always a few compressions in with the dilitations and vice-a-versa
due to noise and operator error. However, in many cases (particularly
for strike-slip mechanisms), the nodal planes can be constrained well

enough to enable a good focal mechanism determination.

Rayleigh Wave Radiation Patterns

 

Further constraints were made on the focal mechanisms of four of the
events by calculating their Rayleigh wave radiation patterns. Records
from the World Wide Standardized Seismograph Network (WWSSN) are used in
the computations. The WWSSN is a network of seismic observatories which
usea standardized system of recording instruments and has been in
operation since 1964. This enables a standardized set of earthquake
recordings for events occurring since 1964 to be easily obtained. Records
from earthquakes which occurred prior to 1964 are difficult to find and
were recorded by instruments set to variable specifications. Thus,

7

Rayleigh wave radiation patterns were not attempted on any pre-1964

earthquakes. In addition, only larger magnitude events (Mb5.0) can be

used, as smaller magnitude earthquakes do not release enough energy to

enable propagation of surface waves over large distances. Thus, a

sufficient number of earthquake recordings are not available to calculate

a reliable radiation pattern. This method is also applicable to Love

wave data. However, recordings of Rayleigh waves are usually better than

those for Love waves and should give more accurate results. In addition,

Rayleigh waves do not have to be corrected for polarization angle as do 1

Love waves. '
The Frequency spectrum, (w) for a seismogram, where w is frequency,

is given by Kanamori and Stewart (1976) as:

_ in iwaB
4 C - wBa
1 1
XM = 1 e e {PLPL( ) * qL0L( )} e 50”

 

Jsine

where angular distance from the earthquake
earth's radius

phase velocity (km/sec)

group velocity (km/sec)

quality factor (attenuation)

O
a
C
U
Q

The decaying exponential term,

_ wea

e ZQU

shows that the signal is attenuated. The complex exponential,

e- TwaB/C

gives the phase as a function of position. The square root tenn,

I/Jsine

takes account for geometrical spreading. The term in brackets contains
the azimuthal variation due to the radiation pattern and the amplitude.

The terms are as follows:

PL

c056 sink sin6 sin2(¢f-¢) + cosA sin6 cos(¢f-¢)

qL cosA cosé sin(¢f-¢) + sink c0526 cos(¢f-¢)

The PL(1) and QL(1) terms account for the frequency and the elastic
constants. The last four terms are radiation terms and allow the
computation of the amplitude radiation pattern at a given frequency
(Kanamori and Stewart, 1976) by use of the equation:

Ah») = flux“)? + maid”)?
The pattern will depend upon the fault geometry: strike 6f, dip 6 ,

 

and slip A.
An equalization term is used in the calculation to account for the

difference in source-station distances (Kanamori, 1970) and is:

‘ , mam-8:) 1M} wa(6-0D)
New“ -'5—‘."—9 X(e,w) e ”C 2 e 20”
SineO

The M term accounts for higher order surface wave trains. These computations
were performed using a program written by Seth Stein of Northwestern University.

Rayleigh waves from HWSSN station recordings were digitized. These
data were converted to the frequency domain using Fourier transforms.
The average spectral amplitude can then be calculated over a specific
frequency range. This study uses frequency values from .033 Hz (30 sec.
period) to .0143 Hz (70 sec. period). The spectral amplitudes for each
station's recordings are corrected to a common station magnification
(1500) and distance from the source (90 degrees). The final output is a

plot for each station's wave amplitude vs. the station azimuth from the

earthquake epicenter for a specific frequency band. The theoretical

9

radiation pattern depends on the fault geometry; that is, the fault
strike, dip, and slip angle. Ideal radiation patterns, assuming the
earthquake energy is propagated uniformly in all directions, are shown in
Figure 2 for four different fault geomertries. In practice, the observed
pattern is more jagged. This is due primarily to irregularities in the
earth‘s crustal structure. The actual fault parameters are constrained
by allowing the program to calculate a theoretical pattern for a given
set of fault parameters; theoretical and observed patterns are then
superimposed. The correct parameters are those of the theoretical pattern
which most closely agree with the observed pattern.

The program also calculates a record's amplitude Spectrum which can
be used as a quality check on recordings from individual stations. Since
WWSSN instruments have peak periods of 20 seconds, many records which

show numerous spurious amplitude peaks are questionable.

Fault Plane Determination

 

Once a focal mechanism is constrained for a given earthquake, it
must be determined which of the nodal planes is the fault plane. This
will be the one on which movement has occurred. If the earthquake occurs
as either a single event or as an earthquake pair, the fault plane is
most easily obtained by a study of the nearby geology and structure. If
fault trends or the structural grain near an earthquake epicenter parallel
one of the nodal planes, then that plane is chosen to be the fault plane.

However, two of the earthquakes studied were large magnitude events
which were either preceded and/or followed by many smaller magnitude
events (see Table 2). 'These series of smaller magnitude earthquakes are
referred to as foreshock or aftershock sequences. If the epicenters

10

SURFACE WAVE
RADIATION PATTERN

VERTICAL
STRIKE SLIP

45°
STRIKE SLIP

VERTICAL
DIP SLIP

45°
DIP SLIP

 

LOVE RAYLEIGH

FIGURE 2
THEORETICAL LOVE AND RAYLEIGH WAVE RADIATION PATTERNS

11

of a foreshock and/or aftershock series tend to align in a well defined
trend, and if the trend parallels one of the nodal planes, then that
plane is the likely fault plane. It is possible ‘to use epicentral
locations published in the ISC Bulletin for the determination of such a
trend. The problem lies in the 150's epicentral location nethod. The
method uses reported arrival times to determine earthquake hypocentral
locations. It also assumes a spherically symmetric earth model in it's
location prognmn which does not take into account lateral crustal and
mantle inhomogeneities. The inhomogeneous nature of the earth will cause
the actual travel time for a ray traveling between a specific source-
receiver pair to be different than the ideal travel time of a ray in the
symmetrical earth model for the same source-receiver pair. The difference
between the actual and ideal travel time is called the residual and is
a major cause of error in the ISC location method. In certain cases,
however, the residual can be used to more accurately locate an earthquake
as it is, in effect, an estimation of the earth's lateral inhomogeneities
along the path between a specific source-receiver pair (Everenden, 1969).

The approach used is called the master event relocation method. In
this study, it is used to relocate the events of the foreshock/aftershock
sequence. The largest magnitude event of a sequence (referred to as the
main shock) is recorded by more seismic stations than other events in the
series. It should be the most accurately located event and thus should
have the most accurately calculated travel-time residuals. Since the
events of a single series occur within a relatively small area, the travel
path for a seismic ray between any of the events and a specific receiver
is assumed to be the same. The residuals calculated from the main event

for specific source-receiver pairs are then removed from the travel time

12

for the same source-receiver pair of the smaller events. This in effect
removes the lateral inhomogeneities of the earth and gives better relative
location. Mapping of relocated epicenters in both cases in this study

improved the accuracy in choosing the fault plane.

13

GEOLOGY

Most of Alaska and Chukotka are geologically complex. Much of the
area is a collection of different tectonstratigraphic terrains (Jones et
al., 1977, 1981; Jones and Silberling, 1979; Berg et al., 1978; Churkin
et al., 1979, 1980; Fujita and Newberny, 1982, 1983). Alaskan studies
are further complicated by areas of large-scale thrust faulting (Sains-
bury, 1969) and strike-slip faults (Grantz, 1966). These areas and their
structural features are located on Figure 3. The following is a geolog-

ical and structural summary of the study area:

Seward Peninsula

The Seward Peninsula is dominated both by Precambrian gneiss and
sediments, with the remainder being composed primarily of Mississippian
carbonates and limestones (Sainsbuny et al., 1970; Hudson, 1977). The
geologic map of Seward Peninsula compiled by Hudson (1977) shows that the
southern and western portions are heavily faulted. Large-scale thrust
faulting was first inferred by Collier (1902). It has since been shown
that thrust sheets occur over most of the peninsula (Sainsbuny, 1969;
Sainsbury et al., 1970). Sainsbury et al. (1970) show that thrusting
probably occurred in the Early Cretaceous. Two large normal faults with
signs of recent activity are observed in the southern part of the peninsula
(Nakamura et al., 1980). The faults both trend east-west and lie along

the Bendeleben and Kigluaik Mountains.

IGURE 3

INDEX MAP OF THE STUDY AREA

The areas marked by vertical lines are ophiolites described by
Patton et al., (1977). The dotted line is the boundary between uplifts
(south) and depressions (north). The intraplate earthquakes studied are

located on the map. The number by each earthquake refers to TABLE 1.

15

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532.29.
. «<33 «m.
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16

Western Brooks Range

The Western Brooks Range is primarily composed of Paleozoic carbon-
ates, sandstones, and conglomerates which are intruded by Devonian grani-
toid plutons (Beikman, 1980). A series of rootless ophiolites emplaced
during the Middle Jurassic to Early Cretaceous have been mapped by
Patton et al. (1977). Dating by the K-Ar method has yielded Jurassic
ages for the ultramafic layered gabbro complex (Patton et al., 1977).
The compressional forces which emplaced the ophiolites also caused
thrusting within the Western Brooks Range. The area was shortened by at
least 100 km (Martin, 1970) with a maximum possibly in excess of 250 km

(Snelson and Tailleur, 1968).

Yukon-Koyukuk Province

 

The Yukon-Koyukuk province is a wedge-shaped region in the center of
the study area. The province is composed of a central volcanic pile of
Cretaceous age with a huge thickness of volcanically derived sediments
flanking the central pile on all sides (Patton, 1973). The volcanic
sediments may be as thick as 25,000 feet (Patton, 1973). Jurassic
ophiolites border the province on the north and southwest margins
(Patton, et al., 1977). These slab-like bodies dip beneath the volcanic
sediments at angles between 10 and 80 degrees and have been interpreted
as the root zone for ophiolites in the Western Brooks Range and Ruby
Geanticline (Patton et al., 1977). The province is offset on the south
end by the Kaltag fault, a right-lateral strike-slip fault (Patton, 1973;
Patton et al., 1977). Displacement as determined by offsets of alluvium,
is from 65 to 130 km (Patton and Hoare, 1968). Three smaller faults of

unknown type with general northeast-southwest trends have been mapped in

17

the central and northeast portions of the province (Patton and Hoare,

1968).

Ruby Geanticline

The Ruby geanticline is a northeast-southwest striking Cretaceous
uplift approximately 350 km long and 100 km wide. The central portion of
the uplift is composed of Precambrian and Paleozoic schist and quartzite
and is intruded by granites of Lower Cretaceous age (Miller et al., 1959).
A discontinuous belt of ophiolites of Jurassic to 'Triassic age occur
along the southeastern side of the uplift. Patton et al. (1977) believe
this belt to be the remnant of an Ophiolite sheet, once continuous, that
covered the Ruby geanticline. The root zone of that ophiolite sheet
became the present, narrow discontinuous belt of ophiolotes observed
along the southeastern border of the Yukon-Koyukok province. The Kaltag
fault has offset the southeastern portions of both the metasedimentary

rock and the ophiolote belt from 80 to 130 km (Patton et al., 1977).

Central Alaska

The Yukon and Tanana rivers border a geologically and structurally
complex area in central Alaska. The region is composed of six tec-
tonostratigraphic terrains which all have general northeast-southwest
trends (Jones et al., 1981). The terrains are primarily fault bounded
(Beikman, 1980). From northwest to southeast they are the Innoko,
Wickersham, Livengood, White Mountain, Minchumina and Yukon-Tanana (Jones
et al., 1981). For descriptions of the individual terrains see Jones et

al. (1981).

18

Chukotka

Chukotka is the northeastern most portion of Siberia. Structurally
it is composed of a region of slight (500m to 700m) Pliocene to Quaternary
uplifts in the central and southern portions and a band of Pliocene to
Quaternary depressions on the northern margin (Goryachev et al., 1968).
The boundary between the two regions has a northwest-southeast trend and
is shown in Figure 3. Two small exposures of ophiolites with probable
Jurassic emplacement ages, have been identified in the region (Fujita and
Newberry, 1982). It has been suggested that the same tectonic event
emplaced both the Chukotka and central Alaska ophiolites (Fujita and
Newberry, 1982). Precambrian rocks outcrop on the eastern end of the

peninsula while Carboniferous rocks are predominant on the northern shore.

Southern Hope Basin

 

North of Chukotka is a sedimentary prism deposited in the Hope Basin.
Using velocity studies gathered from seismic reflection data, Grantz et
al. (1975) conclude that deformed Paleozoic metamorphics underlie the
southern portion of the basin. The basin's structural axis parallels the
coastlaine of northern Chukotka (Grantz et al., 1975). It appears to
have formed by crustal extension due to the east-northeast movement of
northern Alaska along the Kaltag fault in early Tertiary times (Patton

and Hoare, 1968; Grantz et al., 1975).

19

20

SEISMICITY

Figure 1 is a map of epicentral locations for all earthquakes that
have occurred in the study area since 1900. Most of the events are
confined to one of the numerous seismic regions, defined here as areas
exhibiting higher levels of seismicity than surrounding areas. Most of
the seismicity is of low magnitude with only a few earthquakes which have
body wave magnitudes greater than 5.0. The following is a description of
the seismic regions in the study area. All magnitudes stated within the
text are body wave magnitudes unless otherwise noted. Specific earth-
quakes will be referenced as to the year-month-day and (where need be)

hour.minute.second of occurrence.

Seward Peninsula

The Seward Peninsula is an area of apparently low seismicity. Events
with sufficient magnitude to be recorded teleseismically are confined to
the southern half of the peninsula. The largest of these events, a
magnitude 5.3 shock, occurred on 64-12—13 in the Kigluaik Mountains and
had epicentral coordinates of 64.96N, 165.4W. It appears that much of
the seismic activity on Seward Peninsula is undetected by seismic stations
around the world due to the low magnitudes of most events. A local
seismograph network installed by the NOAA recorded about 300 earthquakes
with magnitudes (ML) between 1.0 and 4.5 over a two year period (1977-

1978). A plot of these epicentral locations by Biswas et al. (1980) is

shown in Figure 4A. Biswas et al. (1980) suggest that most of the
epicenters occur in linear trends which can be related to the structural
features and fault zones of Seward Peninsula and the surrounding area

such as the Kigluaik Mountains and the Kaltag fault (Figure 4B).

Yukon-Koyukuk Province

 

The Yukon-Koyukuk province exhibits three zones of seismicity. The
center of the smaller, less seismically active zone is located at 64.6N,
160.2W in thick Cretaceous sediments. A magnitude 5.8 event occurred
within this zone on 65-4-16 and has been studied by Sykes and Sbar (1974)
and Liu and Kanamori (1980). It is interesting to note that this event,
like the Kigluaik event on Seward Peninsula, had no foreshocks or
aftershocks associated with it as do many of the larger intraplate
earthquakes of Alaska.

In the large Cretaceous volcanic pile located in the northcentral
portion of the Yukon-Koyukuk province at 66N, 156.6W is a seismic zone of
much greater extent with larger-magnitude earthquakes, and more numerous,
smaller events. The Huslia series occurred within this zone. This series
consisted of a main shock (on 58-4-7 at 15.30.40) of Gutenberg-Richter (G-
R) magnitude 7.3 and a one week long aftershock series (Davis, 1960).
Two of the aftershocks had G-R magnitudes of 6.5 and were recorded
teleseismically.

The third zone is a north-south trending band of epicenters located
between 66.0N and 66.5N at 157.5W. All of the events in this area have

low magnitudes (less than 4.0) and many occur as earthquake pairs.

21

FIGURE 4.

SEISMICITY OF WESTERN ALASKA
The data was gathered by a local seismograph network operated by the
National Oceanic and AtmOSpheric Administration during 1977-78 and were

mapped by Biswas et al. (1980).

4A - Epicentral locations

4B - Relation of earthquakes to structural features

22

173°

 

 

 

us" we
a, j 1 e,
+ erucswienor “amount
"9‘ IL<451 +-
+
+
‘I’ +
+ + + +
+ + +
+ + ‘ +
l +
+ +
+ +0. 4- + + +
+ 41+ "’ .h + *0,
+ \ . +
T + #‘* 4.
+ + . “it
”0-: ‘ I + ‘+ 3 ha
. + +++ + + + + +
- + +4‘ t: +++# + +
+
+.‘ + + +
+ + ' ‘6 t. + H
+ +
+ ++ 4+ 'IZ + a.“ + +
4 ‘+ + + + +
+ +
+ + + +
+ + + + " +
+
+ +
H L
"° 1 1 1
173° 105° 156°
FIGURE 4A

 

69°

 

ANTICLINE
SYNCLINE
RIDGE AXIS

—*— FAULT TRACE

—-'-—4— QIOMML nun

H THIUST nun

:_;_*—

4.

106°

 

113°

m w
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00'
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FIGURE 4B
24

173°

Western Brooks Range

The western Brooks Range has many widely scattered, lower magnitude
earthquakes. The only area of high epicentral density occurs at 67.8N,
161.5W. Almost all of these events are part of the foreshock and aftershock
sequence of the 81-7-12 event which had a magnitude of 5.2. The sequence

occurred between 81-7-4 and 81-8-3 and consisted of 17 events.

Central Alaska

 

The density of earthquakes in the central Alaska area is similar to
the Huslia seismic zone of the Yukon-Koyukuk province except for the
prominent north-south alignment of epicenters stretching from 65.0N to
66.0N at longitude 150.0W. Most of the events within this cluster were
part of the Rampart sequence of 1968. A magnitude 6.0 earthquake occurred
on 68-10-29 at 22.16.16 and was followed by at least 140 aftershocks
which occurred over a three month period. Other events and smaller
sequences have occurred along the same fault zone but none with as great
a magnitude or aftershock duration as the 1968 Rampart sequence.

Two other areas in central Alaska have experienced earthquakes of
larger magnitudes than most of the area's events. Two earthquakes, both
of G-R magnitudes 6.3 with epicenters at 65.2N, 152.0W occurred on 58-5-
10 and 58-5-11. No related foreshock or aftershock sequence was recorded,
possibly due to the poor seismograph station coverage in the 1950's. It
should also be noted that these events occurred about one month after the
main shock of the Huslia sequence.

In another series, two earthquakes with epicenters located at 66.8N,
148.2W occurred on 66-12-20. The magnitudes were 4.8 and 4.9. No

foreshock or aftershock sequence was recorded.

25

Chukotka and Southern Hope Basin

 

Seismic activity in this area is centered at 68.0N, 173.0W. Three
of the events occurred as a series in February of 1928, the largest being
of G-R magnitude 6.9. Recent seismicity in the area includes two magnitude
5.0 earthquakes, one in 1962 and the other in 1971. Other than this
seismic region, there are very few teleseismically located events.

A map including very low magnitude earthquakes (as low as 1.0 unified
USSR magnitude) compiled from the Akademiya Nauk SSSR "Novi Katalog Sil'
nikh Zemletryasenii SSSR" (Kondorskaya and Shebalin, 1977) is shown in
Figure 5. This area, like the Seward Peninsula, exhibits a high level of
low magnitude earthquakes. A north-south trending zone of epicenters
occurs at 178.5W but is probably a bias due to the location technique.
The seismicity is more active in the northern portions of this region
than in the southern. It is interesting to note that the border between
the northern and southern zones falls on what Goryachev et al. (1968)
have mapped as the border between "considerable Pliocene-Quaternary
uplift" to the south and “Pliocene-Quaternary depressions" to the north.
Earthquakes which are of such low magnitudes that they can only be recorded
by local seismograph networks may occur with more frequency than is
presently believed. The magnitude 5.0 event of 71-10-5 is reported in
the ISC Bulletin as a single event. However, Lazareva (1970) reports an
associated aftershock sequence of 50 events which were recorded by a

local station.

Areas of No Seismicity

 

As the previous sections have pointed out (particularly the Seward

Peninsula section and Chukotka and Southern Hope Basin portion), many

26

 

 

 

 

I; J. J. "r 9..
I— W
O
O
o. ’
O
a d
0
~00 o 'I»
fi
0 ’ 1. . 014:1
O O .
O
o
O
O . “I
47
no
' o
I. ‘9“
in 111. 11‘: u ’
FIGURE 5

CHUKOTKA EPICENTERS

27

of the areas which appear as aseismic regions in Figure 1 actually exhibit
high levels of lower magnitude seismicity. Examples are northwestern
Chukotka, Norton Sound, northern Seward Peninsula and the western Yukon-
Koyukuk province. This still leaves large areas covered by oceans and
much of northern Alaska as being aseismic with respect to teleseismic
events. Whether local seismograph networks in these regions would reveal
an abundance of low magnitude seismicity as it did on Seward Peninsula or

that they may be truly aseismic areas remains to be seen.

28

29

FOCAL MECHANISMS

Focal mechanisms for twelve earthquakes are constrained in ‘this
study. Four additional mechanisms were obtained from previously pub-
lished data. The following is a discussion of the methods used to con-
strain each focal mechanism. The event number listed in the title of

each section and referenced within the text refer to Table 1.

Rampart Series (Event 1)

 

The main event of the Rampart series occurred on 68-10-29 at 22.16.16
and has an epicentral location of 65.46N, 150.07W. It was a magnitude
6.0 earthquake and is the most widely recorded event in the study area
with 312 reported arrival times in the ISC Bulletin. Accompanying the
main event was a long and active aftershock sequence which lasted for
approximately three months and consisted of 140 earthquakes, the largest
of which occurred on 68-10-31 at 00.25.46 and had a magnitude of 4.6.

First motion data was read from WWSSN and Canadian Network records
when possible and was supplemented with first motions reported in the ISC
Bulletin. Figure 6 is a plot of the first motion data and shows the
nodal planes which best fit the data.

For the main Rampart event, the Rayleigh wave amplitude from 14
seismic stations were calculated for four different frequency intervals
centered at 40 sec., 50 sec., 60 sec., and 70 sec. All of the plots

exhibit similar four-lobed patterns with the pattern orientation varying

FIGURE 6

FIRST MOTION DATA 0F EVENT 1.
First motion data for all earthquate focal mechanisms in this study

are plotted on a lower hemisphere stereographic projection and are

reported as follows:

I. - Compression read from short and long period seismic record

C) - Dilitation read from short and long period seismic record

. - Compression read from short period seismic record

C) - Dilitation read from short period seismic record

a. - Compression reported in the International Seismological Summary

(ISS) or the International Seismological Center (ISC) Bulletin

0
I

Dilitation reported in the ISS or ISC Bulletin

The fault plane, when known, is marked by a double slash (//).
Fault planes marked with dashed lines are poorly constrained. Shaded or

cross-hatched quadrants are compressional.

3O

 

 

FIGURE 6

31

less than 10 degrees. Figure 7 shows the pattern calculated from the
data (solid line) for a 60 sec. band. The best fit theoretical pattern
is given as the dotted line. Gedney (1970) used first motion data read
from Canadian, Alaskan, and WWSS network records and obtained nodal planes
with similar strikes and dips to those determined in the Rayleigh wave
study. Gedney's data are shown in Figure 8.

The fault was constrained by a study of the aftershock sequence.
Epicenters reported in the ISC Bulletin for the sequence are shown in
Figure 9A and exhibit a general trend of N12E. The master event relocation
program of K. Fujita was used to obtain a more accurate epicentral trend.
Relocated epicenters are plotted in Figure 9B. The relocated epicentral
trend is now NBE. The larger dots in Figure 9B correspond to earthquakes
in which more than 20 stations were used in relocation, which should
yield more accurate locations than events using fewer stations. These
also exhibit the N8E trend. Gedney (1970) noted that the Minook Creek
Valley, a linear topographic feature with a N8E trend, lies within the
epicentral zone and is probably the surface expression of the fault plane
for the Rampart series (see Figure 9A). Although the relocation places
the aftershock epicenters about five km to the west of the Minook Creek
Valley, the simlarities of the trend still suggest that the valley is
the fault's surface expression. It is also possible that a slight westerly
bias exists in the relocated epicenters.

The first motion and Rayleigh wave focal mechanisms are similar in
that both are consistent with strike-slip faults and steeply dipping
nodal planes. However, the nodal plane orientations differ by about 20
degrees. A study by ChUng and Kanamori (1978) found this same discrepancy
in the focal mechanism determination of the New Hebrides earthquake of

32

 

 

 

 

 

STRIKE=200 D|P=80 SLIP=4 PERIOD=60 SEC.

FIGURE 7
RAYLEIGH WAVE RADIATION PATTERN OF EVENT 1

33

 

 

FIGURE 8

GEDNEY (1970) DATA FOR EVENT 1

34

BEFORE RE LOCATION

 

AFTER RELOCATION

 

r
30.2

1

1 W T
150.0 149.8

 

 

1 j _I T 1 I

1502 150.0 140.0

O
P . cl

$'

0“ 0

*1

>053 0 O -

 

 

'- 05.! ""

 

 

FIGURE 9A

 

 

FIGURE 9B

AFTERSHOCK RELOCATION FOR EVENT 1

January 19, 1969. They suggested that since the two mechanisms are
constructed from different portions of the seismic signal, that it is not
unusual for the focal mechanisms to differ slightly. They went on to
suggest that in the case of a complex rupture along the fault, the seismic
signature of the Rayleigh wave may be complicated by seismic energies
from other ruptures. It is likely that the main Rampart event, which was
associated with a very active aftershock sequence, was not a simple,
single rupture event. The final mechanism, shown in Figure 10, chooses
nodal plane orientations which agree with the observed fault trend and
lie between the nodal planes of the two focal mechanisms.

Event 2 is one of the larger aftershocks in the Rampart series. It
occurred on 68-11-3 at 7.37.39 at location 65.47N, 149.8611 and had a
magnitude of 4.4,. First motion data read from WWSSN records and those
reported in the ISC Bulletin were used in an attempt to determine the
focal mechanism (Figure 11). Although there are not enough data for
accurate determination of the planes, the data that are available show a
mechanism similar to that of the main Rampart event is not possible.
There is a much larger component of either normal or reverse faulting and
the nodal plane orientaiton must be different from the main event. Other
earthquakes which have occurred within the area of the Rampart aftershock
zone (though not within the 1968 Rampart series itself) also have focal
mechanisms different than the 1968 event. Bhattacharya and Biswas (1973)
used local seismic network records to determine the mechanism for event 3
which occurred less than 10 km from the main Rampart event. The mechanism
is shown in Figure 12. It is a very well constrained mechanism and is
that of a normal fault with a horizontal B-axis. Event 4 is an earth-
quake which occurred on 61-01-30 near the southern end of the aftershock

36

. 1

IIIIIIII

 

FIGURE 11

FIRST MOTION DATA FOR EVENT 2

38

 

 

FIGURE 12

FIRST MOTION DATA FOR EVENT 3
Data are from Bhattacharya and Biswas (1979).

39

zone at 65.2N, 150.2W. The first motion data from the 153 are plotted in
Figure 13. Though only eleven first motions were reported, they are
sufficient to show that the mechanism has a much larger component of
reverse faulting than the main Rampart event. Gedney (1970) used local
seismic network data to determine the mechanism for event 5. It is a
magnitude 5.4 earthquake with an epicentral location to the south of the
1968 Rampart series at 64.8N, 147.4W. A strike-slip mechanism similar to
the main Rampart event was obtained and is shown in Figure 14.

In order to obtain such a diverse group of focal mechanisms, a
complex tectonic scheme is necessary. There are many factors which could
affect the stress regime in the Rampart zone as shown in Figure 15.
First, the Kaltag-Tintina and McKinley-Denali fault systems, both long
strike-slip faults, are part of a system of large strike-slip faults
which bisect the southern portion of Alaska (Grantz, 1966). The Kaltag-
Tintina system passes through the Rampart area. Second, there is a bend
in both of these fault systems which lies about 130 km to the east of the
Rampart area. It has been suggested that the bend may have formed when
Arctic Alaska rotated from the Canadian Arctic to it‘s present position
(Freeland and Dietz, 1977) and could have an effect on the local stress
orientation. Finally, there is the stress applied by the subducting
lithoSphere to the overriding continental plate. Even though the subduc-
tion zone is located 400 km to the south, VanWormer et al. (1974) have
mapped the top of the lithosphere beneath south central Alaska and found
that it is at a depth of 50 km at a point 175 km south of the Rampart
zone. At such a shallow depth it is probably coupled to the continental
crust and thus would effect the stress regime.

Another suggestion for' the~ diverse focal mechanisms seen in ‘the

40

 

FIGURE 13

FIRST MOTION DATA FOR EVENT 4

41

 

FIGURE 14

FIRST MOTION DATA FOR EVENT 5
Data are from Gedney (1970).

42

FIGURE 15

STRUCTURAL FEATURES NEAR THE RAMPART SERIES
The faults are dashed where inferred. The lines in the lower left
are depths to the top of the subducting oceanic crust as determined by

VanWormer et al. (1974). The box surrounds the Rampart aftershock zone.

43

 

 

~06

 

T
161

 

 

MAIN

 

     

RAMPART EVENT

66"

65‘

 

 

 

 

FIGURE 15

44

Rampart zone is based on a model by Weaver and Hill (1978/79) for earth-
quake swarms in California. In .their study they note that earthquake
swarms with normal fault mechanisms which may be associated with small
crustal Spreading centers tend to be located between pairs of strike-slip
faults. Their model is shown in Figure 16. Gedney et al. (1980) suggest
that a seismic zone near Fairbanks, Alaska, about 150 km to the southeast
of the Rampart zone, may be similar to the Weaver and Hill (1978/79)
zone. Although this model cannot explain all of the anomalous events,
striking similarities exist between the Rampart and California zones such
as the large strike-slip faults, the normal and strike-slip focal mechan-

isms and the fault orientations.

Chukotka Earthquake (Event 6)

 

The Chukotka earthquake occurred on 71-10-5 and has epicentral
coordinates of 67.38N, 172.57W. It was a magnitude 5.2 event and was
recorded by 133 seismic stations. There was no associated foreshock or
aftershock sequence reported in the ISC Bullein. It is the only event in
Chukotka or southern Hope Basin of sufficient magnitude to enable a focal
mechanism solution.

The Rayleigh wave radiation pattern was calculated for seismic waves
with mean periods of 30 sec., 40 sec., 50 sec., and 60 sec. Data for the
50 sec. period wave are shown in Figure 17. In all, records from 23
stations were used. The spectral amplitudes for each station's record
were examined. Records which exhibited many Spurious amplitude peaks
were not used in the final Rayleigh wave radiation pattern. This technique
allowed for the elimination of seven stations. The pattern after station

deletion is shown in Figure 18. Comparison with Figure 17 reveals little

45

 

FIGURE 16

MODEL OF WEAVER AND HILL (1978/79)

46

 

 

 

 

 

 

 

 

PERIOD = 50 SEC.

FIGURE 17
RAYLEIGH WAVE RADIATION PATTERN OF EVENT 6 BEFORE STATION DELETION

47

 

 

 

 

 

 

 

PERIOD= 50 sec.

FIGURE 18
RAYLEIGH WAVE RADIATION PATTERN OF EVENT 6 AFTER STATION DELETION

48

 

 

change in the pattern before and after station deletion.

The Rayleigh wave radiation pattern offers few constraints, if any,
on the focal mechanism. If data from stations with azimuths between 110
and 270 degrees was available, better definition of the pattern lobes may
have been possible. Another problem may be that the earthquake was not
of sufficient magnitude to enable the recording of the surface waves at
large distances. The seismic signal was low in magnitude, often just
above background noise. In any event, the Rayleigh wave data is incon-
clusive.

First motion data read from WWSSN records and Canadian Network
records and reported in the ISC Bulletin are plotted in Figure 19. The
strike-slip mechanism with the nodal planes shown in the figure is the
most probable solution and results in only two inconsistent data points.
If the c0mpressional pick from station QUE near the B-axis is ignored, a
mechanism similar to that in Figure 19 would be very poorly constrained.
Such a data set would be better explained by a normal fault mechanism
with nodal planes striking approximately east-west. Fujita et al. (1983)
suggest this as a possible mechanism for the event. The QUE pick is
questionable at best, making the strike-slip mechanism of Figure 19 poorly
constrained.

Much of the structural grain in northern Chukotka parallels the N44W
trending nodal plane. Voyevodin and Sukhov (1977) note that the trend of
the Keukvun'-Iul'tin and Maynypontavaam faults, both “deep seated“ faults
in northern Chukotka, are N45W. Goryachev et al. (1977) have mapped the
boundary of the Pliocene-Quaternary downwarps to the north and the slight
Quaternary uplifts to the south as having a trend very close to the N44W
trending nodal plane of the focal mechanism. This nodal plane is the

49

 

FIGURE 19

FIRST MOTION DATA FOR EVENT 6

50

probable fault plane.

It should also be noted that the epicenter occurs near the epicenters
of four events in 1928 (three of which occurred within one series) of G-R
magnitude between 6.2 and 6.9 and a magnitude 5.0 event in 1962. These
have been the only large magnitude events in Chukotka and the Hope Basin.
All are curiously located within a relatively small area. Fujita et al.

(1983) have associated these events with the uplift of Kotzebue arch.

Seward Peninsula (Event 7)

 

The Seward Peninsula event occurred on 64-12-13 in the southeast
portion of the peninsula at 64.88N, 165.75W. It was a magnitude 5.3
event and was recorded by 131 seismic stations. A smaller magnitude 4.5
event preceded the main shock by 1 min. 19 sec. and was the only other
event associated with the main shock that was reported in the ISC Bulletin.

First motions were read from WWSSN records and were supplemented by
first motion data reported in the ISC Bulletin. The first motions are
shown in Figure 20. Although the dilitational first motions outnumber
compressions by about 4 to 1, they are distributed such that a focal
mechanism cannot be constructed with a high degree of certainty. However,
the data in Figure 20 strongly suggest the normal mechanism shown.

The Rayleigh wave radiation pattern was determined for this event
and is shown in Figure 21. Twenty nine earthquake records were digitized
and used in the original amplitude vs. azimuth plot. To determine the
reliability of individual station records, the amplitude spectra were
plotted for each of the 29 stations. Using this check, 15 of the 29
stations were discarded. The final plot is seen in Figure 22. The

pattern after station deletion is not significantly different than the

51

 

FIGURE 20

FIRST MOTION DATA FOR EVENT 7

 

 

 

 

 

STRIKE=85 D|P=26 SL|P=90 PERIOD=5O SEC.

FIGURE 21
RAYLEIGH WAVE RADIATION PATTERN OF EVENT 7 BEFORE STATION DELETION

53

 

 

 

 

 

 

 

 

 

 

 

 

STRIKE=85 D|P=26 SLIP=90 PERIOD=50 SEC.

FIGURE 22
RAYLEIGH WAVE RADIATION PATTERN OF EVENT 7 AFTER STATION DELETION

54

previous pattern. A two lobed pattern is the best fit to the observed
data and is not inconsistent with the normal faulting mechanism which the
first motions suggest. The computer generated radiation pattern is the
dotted line in Figure 22. It is difficult to determine the exact
inclination of the B-axis from this radiation pattern though it appears
to be within 20 degrees of the horizontal. Thus, the amount of the strike-
slip component of motion is not accurately known. If a horizontal B-axis
is assumed, the mechanism shown in Figure 20 is obtained.

The focal mechanisms from the first motion data and the Rayleigh
wave data both indicate normal faulting mechanisms. However, the strikes
of the nodal planes are different. This was also the case with the main
event of the Rampart sequence. This discrepancy was discussed by Chung
and Kanamori (1978). Their argument was outlined earlier in this paper
and will not be discussed here. The final mechanism is shown in Figure
23.

The earthquake occurred in the north-central Kigluaik Mountains of
Seward Peninsula, an east-west trending arch of Precambrian gneiss. The
area is heavily faulted. Nakamura et al. (1980) discusses the Bendelben
and Kigluaik faults in the southern portion of the peninsula (see Figure
3). They define a normal fault system 175 km long which shows large
amounts of Cenozoic dip-slip displacement (Nakamura et al., 1980). The
Seward Peninsula event occurred on the Kigluaik fault. The mechanism
determined in this study agrees well with the observed normal faulting in

the area.

Event 8

One of the more thoroughly studied earthquakes in central Alaska is

55

a magnitude 5.8 event with epicentral coordinates of 64.7N, 160.2W.
Sykes and Sbar (1974) used first motion data and S-wave polarization
angles for their study. They concluded the earthquake had a normal
faulting mechanism with the B-axis trending N55W and lying nearly
horizontal. Their data are shown in Figure 24A. Liu and Kanamori (1980)
arrived at the same solution after calculating synthetic seismograms for
20 different stations. Their solution is shown in Figure 24B. Two
similarities exist between this event and the Seward Peninsula earthquake.
First, the B-axis in both events is nearly horizontal. Secondly, neither
event has an aftershock sequence, which is contrary to what many of the

other larger intraplate earthquakes in Alaska exhibit.

Events 9 and 10

Events 9 and 10 occurred to the north of the Seward Peninsula, one
near the tip of Baldwin Peninsula in Kotzebue Sound and the other in the
western Brooks Range. Both have nearly identical focal mechanisms.
Event 9 was a magnitude 5.2 event which occurred on 81-7-12. It was
recorded at 161 seismic stations and has an epicentral location of 67.71N,
161.20W. A small foreshock and aftershock sequence consisting of 16
events is associated with the main shock. First motion data from the ISC
Bulletin for the main event are plotted in Figure 25. A strike-slip
mechanism with nearly vertical focal planes fits the data well.

The foreshock and aftershock sequences were relocated using the
master event technique. Figure 26A and B show epicentral locations before
and after relocation, respectively. The trend of epicenters after

relocation is N70W which suggests that the N74W striking nodal plane is

57

 

FIGURE 24A
FOCAL MECHANISM SOLUTION FOR EVENT 8: AFTER SYKES AND SBAR (1974)

 

 

 

 

 

FIGURE 24B
FOCAL MECHANISM SOLUTION FOR EVENT 8: AFTER LIU AND KANAMORI (1980)

58

 

 

FIGURE 25

FIRST MOTION DATA OF EVENT 9
The dilitation at 285° azimuth is a short and

long period dilitation reported in the ISC Bulletin.

59

,4.-

 

BEFORE RELOCATION
I I 1 T

 

 

 

 

 

 

 

1 l i 1
- . 07.0-
.— . .0 . 01.04
. O
- T . 07.7-
. 0
I— 07.0 -I
I- 07.5 -
102.3 1 "EL" 1 101.5 1 1311 1010.7
AFTER RELOCATION
1 I I I I 1 1 I
)— 619-
O
- w 70 w\ \ . 07.0 -
O
’ \..\ O . .
.. \ \ 07.7 -
\
\
— 676 -'
' 0
- . 07.5 -
11:23 1 1011.0 1 1 011.5 1 131,1 1010.7
FIGURE 26

AFTERSHOCK RELOCATION OF EVENT 9

60

 

has exhibited signs of activity in recent geologic time (Brogan et al.,
1975) and which has about the same N74W strike as the fault plane.

Event 10 occurred on 66-8-26 and was a moderate magnitude 5.0 event.
It occurred at 66.71N, 162.7W and was recorded by 82 seismic stations.
First motion data read from WWSSN records, the Canadian Seismograph
Network and reported in the ISC Bulletin are plotted in Figure 27. The
nodal planes are fairly well constrained and show a strike-slip mechanism
nearly identical to the previously described event. No data is available
which would definitely determine the fault plane. However, the geologic
structures in southwest Hope Basin have east-west orientations (Grantz et
al., 1975). There are also two east-west seismicity trends within
Kotzebue Sound (Biswas et al., 1980). This evidence, along with the
focal mechanism similarity with event 8, suggests that the east-west

plane is the fault plane.

Events 11 and 12

Focal mechanisms were determined for two sequences on the west end
of Seward Peninsula. Mechanism 11 is a composite mechanism of four events
which occurred near the northern coastline (see Table 1). Two of the
four events occur at the same location (65.9N, 167.0W) while the other's
are located 50 km to the north and 50 km to the south. Even though the
latter two events occur a considerable distance from the first two, they
are included in the composite for two reasons. First, each of the distant
events occur in conjunction with one of the central events. Secondly,
the first motion data reported in the ISS from all of the events are

consistent and result. in the focal mechanism shown in Figure 28.

61

 

 

FIGURE 27
FIRST MOTION DATA OF EVENT 10

 

 

 

FIGURE 28

COMPOSITE FIRST MOTION DATA FOR EVENTS 11

63

 

Event 12 occurred on 64-6-11 and had a magnitude of 5.0. It's
epicenter was on the most western point of Seward Peninsula at 65.35N,
168.3W and was recorded by 61 seismic stations. First motion data reported
in the ISC Bulletin are used for the focal mechanism shown in Figure 29.
This gives a mechanism similar to that of event 11 discussed in the
previous paragraph. Biswas et al. (1980) note that east-west fault
trends and seismicity trends have been mapped in the area which lends
support to the proposal that the east-west striking nodal plane is the
fault plane. If the fault plane is assumed to be correct, then the
proposed north-south nodal plane is reasonably well constrained by the

given data.

Huslia Series (Events 13 and 14) and Event 15

 

The earthquake which occurred on 58-4-7 at 15.30.40 (event 13, Table
I) was the largest event (magnitude=7.3) of a series of earthquakes which
are hereafter referred to as the Huslia series. An aftershock sequence
of greater than 50 events followed the main shock (Davis, 1960). Two of
the aftershocks, one of which was event 14 (58-4-13 at 9.7.24) were
reported in the ISS and were of magnitude 6.5. First motion data from
the 15$ for events 13 and 14 were used in an attempt to constrain the
focal mechanisms. A focal mechanism constraint for the third large
magnitude event of the series was not possible due to the lack and
inconsistency of the first motion data reported in the ISS. The data for
event 14 allows for two possible solutions, a normal fault and a reverse
fault. Both are shown in Figure 30. First motion data for event 13, the
main event of the series, are shown in Figure 31. The data are too

inconsistent to allow any constraints to be placed upon a focal mechanism.

64

 

FIGURE 29

FIRST MOTION DATA OF EVENT 12

 

 

FIGURE 30

FIRST MOTION DATA OF EVENT 14 FROM THE I38

66

 

 

FIGURE 31

FIRST MOTION DATA OF EVENT 13 FROM THE 155

67

Four mechanisms have been proposed by other authors for event 13. Three
of the mechanisms (Schaffner, 1961; Balakina, 1962; and Stevens, 1964)
are shown in Figure 32 and are predominantly strike-slip. The mechanism
proposed by Ritsema (1962) is for a normal fault and is shown in Figure
33. Ritsema used first motion data read from seismic records as well as S-
wave polarization angles. The southeasterly dipping plane is well
constrained by the first motion data. The S-wave polarization data fit a
plane trending N46E and dipping 36 degrees to the northwest. This focal
mechanism is very similar to the normal mechanism proposed for event 14.
It is preferred to the solutions of Schaffner (1961), Balakina (1962) and
Stevens (1964) since the nodal planes are well constrained by Ritsema's
(1962) data. In addition, the agreement with the proposed normal faulting
mechanism for event 14 and with field data gathered by Davis (1960),
which will be discussed momentarily, suggest it is the preferred mech-
anism.

It should be noted that the orientation of the northwest dipping
plane determined in this study is different from that which Ritsema (1962)
proposed. The plane used in the Ritsema study is consistent with the
first motion data but not the S-wave polarization angles. However,
Ritsema was trying to prove a single couple mechanism for the event and
did not attempt to fit nodal planes with differing orientations to the
data. Ritsema (1962) used the polarization angles as an argument against
a possible northwest dipping nodal plane. The northwest dipping nodal
plane suggested in this study is in good agreement with the Ritsema (1962)
data.

In a field report for the Huslia series (Davis, 1960), Davis took
measurements of the vertical displacement on 35 fissures parallel to the

68

FIGURE 32

PROPOSED MECHANISMS FOR EVENT 13
Schaffner (1961); solid lines with hachures
Balakina (1962); dot-dashed lines with hachures
Stevens (1964); dotted lines
The hachures marks on the mechanisms of Schaffner and Balakina point

into the compressional quandrants.

69

 

FIGURE 32

7O

 

FIGURE 33

FIRST MOTION AND S-NAVE POLARIZATION ANGLE DATA OF EVENT 13

71

epicentral zone and found a net downward movement on the northwest side
of the faults relative to the southeast side. This is in agreement with
the proposed normal focal mechanism and would require the northwest
dipping nodal plane to be the fault plane. However, two other pieces of
data suggest the southeast dipping nodal plane to be the fault plane.
The fissures on which the displacement measurements were taken had trends
averaging N56E, which is close to the N58E strike of the southeast dipping
nodal plane. In addition, Brogan et al. (1975) note that the Huslia
fault is observed near the epicentral zone of the Huslia series. The
N70E trend of the fault is closer to the N58E strike of the southeast
dipping plane than the N46E strike of the northwest dipping plane.
Determination of the fault plane is not attempted due to the contradictory
evidence available.

On 58-5-10 and on 58-5—11, about one month after the Huslia series,
two magnitude 6.3 earthquakes occurred with epicentral coordinates of
65.23N, 152.01W, and 65.1N, 151.9W respectively. The location is approx-
approximately 260 km to the southeast of the Huslia series. The first
motion data for event 15 (the 58-5-10 event) were obtained from the ISS
and are shown in Figure 34. The earthquake epicenter is located on the
Kaltag fault, a 440 km long strike-slip fault in west central Alaska.
The Kaltag fault has a N80E strike at the epicenter, which is close to
the N75E nodal plane. Thus, it is probable that the Kaltag fault is the
fault plane. The second nodal plane cannot be constrained with any degree
of accuracy. Regardless of its orientation, the first motion data will
not allow a pure strike-slip focal mechaniSm solution. A large component
must be either thrust or normal faulting.

It is curious that two large series of earthquakes separated by 250

72

 

 

FIGURE 34

FIRST MOTION DATA 0F EVENT 15

73

km should occur within a month of each other. It may be due to a large
scale reorientation of the stress field within Alaska. In 1957 an
extremely powerful earthquake (magnitude=8.0) occurred in the Aleutian
arc on 57-3-9. It was strong enough to be recorded at 365 stations. An
aftershock sequence of 135 earthquakes over the following week occurred
along a 1000 km section of the Aleutian Arc (Rothe. 1969). Such a large
magnitude main event and the extremely widespread aftershock sequence
would have some impact on the regional stress field. The 1958 events in

central Alaska may have been one outcome of this.

Caro (Event 16)

The Caro series consisted of two earthquakes which occurred on 66-12-
20 at 00.26.28 and at 00.57.53. The epicenters were located at 66.82N,
148.1W and 66.79N, 148.4W and were of magnitudes 4.8 and 4.9 respectively.
No aftershocks were reported. An attempt to calculate the Rayleigh wave
radiation pattern was made. However, the magnitude of the events were
not large enough to enable the recording of good long period records at
more than 9 stations. The Rayleigh wave data from 9 stations for the
first event of the pair are plotted in Figure 35. The pattern does not
offer any constraints on the nodal planes. First motions from the ISC
Bulletin and read from WWSSN and Canadian Network seismic records enable
a focal mechanism to be determined for the event at 00.57.53. It is
shown in Figure 36. The mechanism for the other event is assumed to be
the same due to the striking similarities of their long period seismic
wave forms. Figure 37 shows the record of both events recorded at College,
Alaska. The first 60 seconds of each signal are nearly identical. The

geologic map of Alaska (Beikman, 1980) shows the general fault pattern of

74

 

 

 

PERIOD=30 SEC.

FIGURE 35
RAYLEIGH WAVE RADIATION PATTERN FOR EVENT 16

75

 

 

FIGURE 36

FIRST MOTION DATA OF EVENT 16

76

FIGURE 37

SEISMIC RECORD AT STATION "COL" FOR EVENT 16
The left record is from the event 66-12-20 at 00.26.28; the right
record is from the event of 66-12-20 at 00.57.54.

77

 

 

 

 

 

 

MI .1

TWW [ VII/”W I

 

the area to trend N55E which is close to the N60E trending nodal plane.

As a best approximation then, the N6OE plane is picked as the fault plane.

 

79

80

DISCUSSION

The focal mechanisms from this study are shown in Figure 38 and are
summarized in Table 2. It was found that the majority of Alaskan and
Chukotka intraplate seismicity is associated with pre-existing faults.
The Rampart series (events 1 and 2) and the event on Seward Peninsula are
two examples of this. McKenzie (1969) suggested that shallow earthquakes
will occur along planes of weakness in the crust. The McKenzie study
found that the P-axis orientation determined by a focal mechanism is only
weakly controlled by the greatest principle stress direction ($2). The
$1 direction need only lie in the same quadrant as the P-axis and may be
nearly perpindicular to it (McKenzie, 1969). This suggests that pre-
existing faults will be the overriding factor in determining the location
of shallow earthquakes. It has also been shown that shear stresses
involved in shallow earthquakes are too small to produce new fractures
(Chinnery, 1964; Brune and Allen, 1967). Thus, the stresses that build
up in the shallow crust will be released on' pre-existing faults.

Sykes (1978) found that many intraplate regions which exhibit seismic
activity such as Britain and the Appalachians occur in areas of pre-
existing faults or fold belts which parallel the continental margins.
The geologic map of Alaska (Beikman, 1980) shows that Alaska exhibits
this same structural style. The model that Sykes (1978) proposes which
best explains the intraplate seismicity of Alaska is one in which a

relatively weak area, in this case central Alaska, is compressed between

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83

two stronger blocks, the stronger blocks being the area north of the Brooks
Range (the North Slope) and the oceanic lithosphere south of the Aleutian
trench. Sykes (1978) points out that while oceanic lithosphere is thinner
and therefore considered weaker than continental crust, the strength of
the continental crust may be greatly reduced if it is highly faulted.
Many regions of Alaska, particularly the area surrounding the Rampart
series, exhibit this characteristic.

The above model accounts for intraplate seismicity which occurs in
heavily faulted areas such as central Alaska and Seward Peninsula.
However, other areas in central Alaska which are seismically active do
not have such a heavily faulted character. An example of this is the
high level of seismicity observed in the central and southern Yukon-
Koyukuk province. Only a few minor faults are shown on the geologic map
of Alaska for this region and topographic maps show no linear features
typical of a fault's surface expression.

A possible explanation can be drawn from work done by Nakamura and
Uyeda (1980) and Nakamura et al. (1977, 1980). Nakamura mapped the
orientation of the maximum horizontal compression (MHC), and whether it
was $1 (axis of greatest stress) or $2 (axis of intermediate stress) in
southern Alaska and the eastern Bering Sea. The directions were deter-
mined by measuring dike swarm trends associated with volcanoes and stress
orientations needed to produce the observed movement on recently active
faults. The MHC orientations from Nakamura et al. (1980) are shown in
Figure 39. Along the Aleutian arc the MHC direction is parallel to the
direction of plate motion. In central Alaska this orientation begins to
vary and tends to form a fan shaped pattern with the line of symmetry
trending approximately N25-30W from the eastern end of the Aleutian trend.

84

 

FIGURE 39

MAXIMUM HORIZONTAL COMPRESSION (MHC) FROM NAKAMURA AND UYEDA (1980)

The wide spaced double line separates the primarily compressional
area in central Alaska from the primarily extensional area in western
Alaska and the Bering Sea. The thin double line with the hachures map
the MHC orientations. The large arrows are in the direction of motion of

the oceanic lith05phere subducting in the Aleutian trench.

85

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86

Stress orientations can also be estimated from focal mechanisms.
This is done by using the P and T axis to determine the S1 and S3 (axis
of minimum stress) directions respectively. As previously mentioned
(McKenzie, 1969), this assumption is not necesssarily valid. However,
Rayleigh et al. (1972) have shown that new faults will be created in
areas where existing faults are in unfavorable orientations. Their
experiments showed that only pre-existing planes of weakness lying between
approximately 10 and 50 degrees of S1 would slip. Thus if the P-axis is
assumed to lie at 45 degrees to the fault plane, the error in using the P-
axis as an estimation of the $1 direction is a maximum of 35 to 40
degrees. If the fault plane can be determined, the error can be reduced
to 20 degrees. The MHC orientations determined from this study are
plotted in Figure 40. They generally show good agreement with Nakamura's
data.

Another conclusion that Nakamura reached is that the MHC axis tends
to change from S1 in central and southern Alaska to $2 in western Alaska
and the Bering Sea. The two regions are separated by the double line in
Figure 39. Nakamura equates this as being a change from a predominantly
compressional environment in southern and central Alaska to an extensional
one in western Alaska and the eastern Bering Sea. The compressional
forces are probably caused by compression along the Aleutian arc and the
transmittance of these forces to the interior of Alaska. The extensional
forces may be caused by an upward movement of aesthenospheric material,
the same mechanism postulated for back-arc spreading (Nakamura et al.,
1980). It is also noted that the boundary between the differing stress
regimes corresponds to the boundary separating two different types of
volcanism. Polygenetic calc-alkaline volcanics are dominant in the

87

 

FIGURE 40

MHC ORIENTATIONS FROM FOCAL MECHANISMS
The wide spaced double line and thin, hachured double lines are from
Nakamura et al. (1980). The solid lines are MHC orientations where 81 is

the MHC. The dashed lines are MHC orientations where S3 is the MHC.

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89

southern, compressive zone whereas monogenitic alkali-basaltic volcanics
are more predominant in the northern, extensional region (Nakamura et
al., 1980). Even though extensional forces are predominant in the
northern zone, active spreading in the back-arc area is not observed. It
is possible that spreading will not occur unless there is a lessening of
compressional stresses at the arc or trench (Nakamura et al., 1980).
Regardless of the mechanism, the observed extensional forces are the
probable causes of the large magnitude normal mechanism earthquakes which
occurr in the Yukon-Koyukuk (events 8 and 13) and on Seward Peninsula
(event 7). Recent work by Fujita et al. (1983) has shown that the focal
mechanism for the Chukotka earthquake (event 6) may also be a normal
fault with east-west striking nodal planes.

Figure 40 is plotted slightly different than the Nakamura data
(Figure 39). For the normal fault mechanisms in which the S2 axis is the
MHC, the S3 axis is plotted. The S3 axis should be a better indicator of
an extensional stress regime than the S2 axis, which is what Nakamura et
al. (1980) mapped. The $1 and $3 axis determined from this study show
good agreement with the Nakamura et al. (1980) data near the boundary
between the compressional and extensional regimes. To the west of this
boundary the data is in poor agreement with Nakamura's model. Even if
the $2 axis for the normal mechanism events are plotted, the axis
orientations still do not agree with the Nakamura data. However, all of
the mechanisms in the area are either strike-slip or normal fault mechan-
isms which agree with the general conclusion that the stress regime in
Alaska and the Bering Sea is typified by extensional forces.

An observation should be noted. The larger earthquakes which occur-
red in central Alaska, such as the Rampart series and the Huslia series,

90

 

each had large aftershocks sequences associated with them. The large
magnitude events in other areas occurred either as earthquake pairs (the
Caro and Seward Peninsula events) or as single isolated events (event 8
and the Chukchi earthquake).

The two types of events have a distribution which roughly corresponds
to the compressional and extensional zones described by Nakamura et al.
(1980). The events with aftershock sequences fall within or near to the
compressional zone while the events with no aftershocks fall within the
extensional zone. The two events in the Huslia series occurred on the
border between the two zones and exhibit characteristics of both; the
normal fault mechanism corresponds to the northern extensional zone and
it has the aftershock sequence typical of southern zone earthquakes.

It should also be noted that event 9 does not fit this pattern. It
is a large event of magnitude 5.2 which occurred in the northern zone.
However, it is associated with a foreshock and aftershock sequence
consisting of 16 events. Event 6 had an aftershock series of 50 events
(Lazareva, 1970) and also occurred in the northern zone. However, all
were of very low magnitude and were recorded by a local station in
northeast Siberia while the aftershock series of the Alaskan events were

of great enough magnitude to be reported in the ISC Bulletin.

91

92

CONCLUSIONS

Central Alaska and Chukotka has been shown to be a very seismically
active intraplate region. This study placed constraints upon the focal
mechanisms of 12 of the area's earthquakes. An additional four mechanisms
from other authors supplemented this data set. The majority of the
earthquakes studied are associated with pro-existing faults. However,
some of the events occurred in areas where faulting is not known to exist.
A possible explanation for this lies in work done by Nakamura et al.,
(1977, 1980). Nakamura found that the maximum horizontal compression
(MHC) direction determined from dike swarm orientations and recent fault
movement form a fan shaped pattern across central Alaska. The occurrences
of earthquakes in the unfaulted areas of Alaska are generally related to

the two stress regimes of Nakamura et al., (1977, 1980).

 

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93

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97

 

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