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This is to certify that the

thesis entitled

The Eurasian-North American Plate
Boundary Through the
Area of the Laptev Sea

presented by

Daniel Richard Olson

has been accepted towards fulfillment
of the requirements for

Masters degree in Geology

 

 

Major p fessor

Date 6/l‘f/90

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

 

 

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MSU Is An Afllrmative Action/Equal Opportunity Institution
l __ __ __ Cimm, pita-9.1

 

 

 

 

 

 

THE EURASIAN - NORTH AMERICAN PLATE BOUNDARY
THROUGH THE AREA OF THE LAPTEV SEA
By
Daniel Richard Olson

A THESIS

Submitted to
Michi an State Universi
in partial fu llment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Geological Sciences
1990

ABSTRACT
The Eurasian -North American Plate Boundary
Through the Area of the Laptev Sea
By
Daniel Richard Olson

24 teleseismic events occurring between 1962 - 1988 in the Laptev Sea
region were examined to determine focal depths and focal mechanisms using
short-period and long-period synthetic waveform modelling and first-motion
analysis. Focal depths are calculated to be 11 _+_ 1 km for earthquakes on a
trend along the 130°E meridian, between 77° - 710 North latitude. Focal
depths increase by 7 to 14 km to the east and west of 130°E, respectively.
This distribution is consistent with a proposed rift margin extending north-south
through this region, with focal depths limited by high heat flow along the rift
axis. Focal mechanisms are predominately extensional to the north and have
greater strike-slip components to the south, with compression being dominant at
71°N. This is consistent with placement of the Eurasian - North American pole
of rotation near the Lena River Delta by Cook et a1. (1986). The data are

insufficient to further resolve a definitive plate margin.

ACKNOWLEDGEMENTS

My thanks go to Kazuya Fujita, Michael Velbel, Hugh Bennett and F.
William Cambray, and all others within Geological Sciences for making this
work possible. Special thanks go to Glenn Kroeger for use of his synthetic
seismogram package. This research was supported in part by NSF grant DPP
87-21119, and the Department of Geological Sciences and is gratefully
acknowledged.

TABLE OF CONTENTS

LIST OF TABLES
LIST OF FIGURES
INTRODUCTION
THIS STUDY
METHODOLOGY
INDIVIDUAL SEISMIC EVENTS
NOTATION ON FIGURES
April 19, 1962 '
November 22, 1984
January 21, 1976
February 22, 1987
August 25, 1964
July 20, 1976
March 21, 1988
April 7, 1969
September 22, 1987
August 5, 1986
April 23, 1977
June 10, 1983
January 1, 1988
' May 20, 1963

July 21, 1964
iv

12
12
13
16
20
23
23
29
32
35
39
39
42
46
46'

. 50.

50

February 1, 1980
DISCUSSION
GEOLOGIC INTERPRETATIONS
TECTONIC IMPLICATIONS
CONCLUSIONS
REFERENCES CITED

53
59 .
60
61
62
63

LIST OF TABLES

Table 1. Compiled focal information for the present study area.

vi

Figure 1.

Figure 2.
Figure 3.

Figure 4.

Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.

Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.

LIST OF FIGURES
Focal mechanisms , focal depths, and inferred grabens
(Fujita et al., 1990) in the Laptev Sea area.
Epicentral locations and dates for events studied.

Poles of rotation for the Eurasian-North American
plates within northeastern Asia

Short Period waveform fits for July 21, 1964 event
(Fujita et al., 1990a)

Solutions for the April 19, 1962 event.
Synthetics for the April 19, 1962 event.
Solutions for'the November 22, 1984 event.
Synthetics for the November 22, 1984 event.

Synthetics for the Dziewonski et a1. (1985)
mechanism for the November 22, 1984 event.

Solutions for the January 21, 1976 event.
Synthetics for the January 21, 1976 event.
Solutions for the February 22, 1987 event.
Synthetics for the February 22, 1987 event.
Solutions for the August 25, 1964 event.
Synthetics for the August 25, 1964 event.
Synthetics for the August 25, 1964 event.
Solution for the July 20, 1976 event.
Synthetics for the July 20, 1976 event.

-. Solution. for the March 21, 1988 event. '

Synthetics for the March 21, 1988 event.
vii

11
14
15
17
18
19

21
22
24
25
26
27
28
30
3 1

'33

34

Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.

Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Figure 34.
Figure 35.
Figure 36.
Figure 37. .
Figure 38.

Solutions for the April 7, 1969 event.
Synthetics for the April 7, 1969 event.
Synthetics for the April 7, 1969 event.
Solution for the September 22, 1987 event.
Synthetics for the September 22, 1987 event.

Observed records for the Au t5, 1986 event
with suspected small P and p phases visible.

Solutions for the April 23, 1977 event.
Synthetics for the April 23, 1977 event.
Solutions for the June 10, 1983 event.
Synthetics for the June 10, 1983 event.
Synthetics for the June 10, 1983 event.
Solutions for the May 20, 1963 event.
Synthetics for the May 20, 1963 event.
Solutions for the July 21, 1964 event.
Synthetics for the July 21, 1964 event.
Synthetics for the July 21, 1964 event.
Solutions for the February 1, 1980 event.
Synthetics for the February 1, 1980 event.

viii

36
37
38
40
41
43

45
47
48
49
51
52
54
55
56
57
58

INTRODUCTION

The Arctic Mid-Ocean ridge defines one segment of the boundary between
the North American and Eurasian plates. This segment terminates at the slope
of the Eurasian continent north of the Laptev Sea, northeastern Siberia. South
of the ridge terminus, seismicity continues but occurs over a more diffuse zone
which does not define a sharp continuation of the plate boundary.

The Laptev Sea covers a section of the extensive continental shelf of
Eurasia, north of northeastern Siberia (Figure 1). It is bounded to the east by
the New Siberian Islands, and to the west by Taimyr Peninsula. Prominent
features on the south include the Lena River delta and Bour Khaya Bay, and to
the north is the Arctic abyssal plain. Tectonically, this area is of interest
because an oceanic ridge enters a continental margin at almost a right angle,
rather than at a low angle, as in the Gulf of California. Also a change in
stress field on the Eurasian-North American plate boundary occurs in the area,
extension to the north, compression to the south (Cook et al., 1986; Cook, 1988;
Parfenov et al., 1988), based on focal mechanism solutions for events along the
boundary.

Several grabens (Figure 1) are inferred to be below the Laptev Sea based
on magnetic and gravity surveys (Gaponenko et al., 1968; Genin et al., 1977;
Kim, 1986) and refraction lines (Kogan, 1974; Vinogradov, 1984). To the
southeast is the Moma rift system of northwest-southeast trending grabens.
The grabens are presumed 'tO be of Cenozoic age, are‘ sediment filled, ‘with

infered time of opening being earlier (Eocene earliest) for those grabens lying
1

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to the north (Savostin and Karasik, 1981). The Laptev Sea grabens are roughly

aligned north-south, sub-parallel to the trend of the adjacent Arctic Mid-ocean
ridge (Cook, 1988). They are commonly considered the result of continued
extension of the ridge regime into the continental margin.

To the south and southeast of the Laptev Sea, the Siberian topography is
a combination of rugged mountain ranges (Moma, Cherskii) and grabens or
fault bounded depressions (Savostin and Karasik, 1981). These grabens form a
trend to the southeast which has been suggested to be the plate margin
continuing through the Eurasian continent (Chapman and Solomon, 1976;
Savostin and Karasik, 1981). This entire margin has been called an extensional
margin based on high heat flow and subsidence of the area (Savostin and
Karasik, 1981). However, focal mechanism determinations for earthquakes in
this area (Cook et al., 1986; Parfenov et al., 1988) suggest that the Moma rift
system is currently undergoing compression. A few events from the northern
Cherskii mountains are examined in this study.

The objectives of this study is to determine the current locus of
spreading (the plate margin) through the Laptev Sea and to constrain the
location of the Eurasian-North American pole of rotation. In order to attain
these objectives focal mechanisms, focal depths and near-source crustal
structure are determined for seismic events (Figure 2) occurring in the vicinity
of the Laptev Sea.

The determination of the current locus of spreading relies on the
assumption that the spreading axis is associated with the highest heat flow in
the region. This assumption appears to be valid whether the style of rifting is
pure or simple, as thermal models for both calculated by Buck et a1. (1988) are
similar. The shallower brittle-dutile transition on the spreading axis would '

restrict the focal depth for events occurring there (Chen and Molnar, 1983),

ARCTIC OCEAN

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Large dot - Mb 5.0 or greater
Small dot - Mb less than 5.0

5
while off axis events may be deeper. Focal depths are calculated using: 1) the

difference in time between P and pP phase arrivals, using known or assumed
velocity structure in the near source area (Stein and Wiens, 1986) and by
synthetic waveform modelling. The better method is preferentially used.

The second objective of this study is to constrain the location of the
Euler pole of rotation between the Eurasian-North American plates. Cook et
a1. (1986) studied seismicity and focal mechanisms of northeast Asia, and placed
the current pole of rotation for the Eurasian - North American plates near the
Lena River delta (Figure 3). Other studies, involving global or regional plate
inversions (e.g., Chapman and Solomon, 1976; Chase, 1978; Savostin and Karasik,
1981; Demets et al., 1990; Minster and Jordan, 1978) place this pole further
south within northeastern Asia (Figure 1), and generally out of the present
study area. Since the pole of rotation is located near the plate boundary as
proposed by Cook et a1. (1986), then focal mechanisms north of Bour Khaya Bay
should have an extensional component, and those to the south a compressional

component.
THIS STUDY
24 teleseismic events of mb > 4.5 occurring between 1962 and 1988 were
choosen for study (Figure 2; Table 1) which occurred within 65° and 79°N.
latitude and 118° and 147°E. longitude.

METHODOLOGY

Waveform modelling of station records is used to determine various focal

parameters associated with an earthquake. There are six such parameters that

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are of primary interest. These are 1) the strike (azimuth) of the fault plane, 2)

the dip of the fault plane, 3) the angle of motion (slip) of the hanging wall
block with respect to the foot wall block, 4) the focal depth of the event, 5)
the source-time function (STF, rise time, flat time, fall time), which is basically
a record of the faulting process (which for simplicity is approximated as a
trapizoidal or pyramidal function). STF is also a measurement of event duration
and 6) crustal structure in the near source region.

Synthetic seismograms are computed for stations in a distance range of
30° - 90° (Helmberger and Burdick, 1979). Stations at less that 30° are
generally not used due to the possibility that un-modellable triplication phases
may appear in the records, stations at greater than 90° are not used because of
effects of the core.

First-motions can constrain the focal mechanism of the event, and
waveform modelling can be used to estimate all six parameters.

Some crustal structures for areas under study have been calculated based
on two refraction lines, one shot across Bour Khaya Bay (Kogan, 1974) and
another shot to the west of the Lena River delta (Vinogradov, 1984). The
interpreted results of gravity and magnetic surveys are presented in Gaponenko
et a1. (1968). From this information sediment-filled grabens below the Laptev
Sea, are inferred (Vinogradov, 1984).

When no estimates of the crustal structure are known, a reasonable
simplified crustal model is assumed.

For some of the largest (mb 6.0-6.2) well-studied events, a reliable
source-time function exists. Jemsek et al., (1986) is notable for producing
source-time functions for the August 25, 1964, April 7, 1969, and June 10,
1983, Laptev Sea events. These source-time functions are used as initial

estimates for STF for these events.

1 0
To create a model waveform, an initial estimate for all focal parameters

for the event is needed. Very often the results of a first motion analysis and
the geology/structure of the area (if known) will be the basis for the first
estimate in waveform modelling. Waveforms are created through forward
modelling using the algorithm of Kroeger (1987).

The algorithm uses a spike with an amplitude determined by the radiation
pattern of the assumed focal mechanism. This spike is then reflected and
transmitted within the near-source crustal layers. The resulting spikes, with
various amplitudes and time delays, are convolved with the source-time function
for the event, the Futterrnan Q operator for attenuation and the instrument
response, to create the synthetic waveform. The remainder of the waveform
modelling technique involves changing the six parameters until an acceptable fit
for all observed station records for that event is obtained. A good fit requires
that all phases in the observed records (discounting high frequency and
incoherent noise) are matched with phases of similar amplitude, polarity and
time delay in the synthetic waveform. An example from Fujita et al. (1990a)
(Figure 4) shows four waveform fits for the July 21, 1964 event. The upper
three would be considered "good" fits, the fourth is not good.

This process requires that synthetic phase amplitudes approach those of
the observed records, and that polarities and time delays are correct. This is a
trail and error procedure.

Both long-period and short-period station records are generally available
for study. Short-period records are useful for small events, long-period records
are most useful for studying larger events. For the largest events (111}, _2_ 6.0),
short-period records tend to record the motion of the initial break of the
event, long-period records record the overall motion. This can allow - two

different solutions to be created from data for one event. When long and short

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period solutions exist and are in conflict, the long-period record solution is

accepted as a better record of overall tectonic stress release.

Errors can arise in determining all of the above parameters. Error
sources include: noise in the observed record, over-simplified near-source
crustal model, or an invalid crustal model, the source-time function was
incorrect, or the focal mechanism or focal depth were wrong. These
parameters are discussed in Stein and Kroeger (1980). Attenuation and
magnification factors have relatively little effect on waveform appearance, and
are generally held constant during modelling.

Confidence is highest for a solution which produces good waveform fits
(similar phase amplitudes, polarities and time delays in the observed and
synthetic waveforms) for several stations with wide azimuthal coverage. The
solution may not be unique. Always a survey of possible values is made on
the chance of detecting another solution. It is possible to quantitatively
evaluate the error in waveform matching as is done for long period records, it
is not practical at present for short period synthetics. Error margins are

discussed event-by-event.
INDIVIDUAL SEISMIC EVENTS

NOTATION ON FIGURES

Stations plotted on a focal mechanism may appear as one of three
symbols: open circle, denoting a dilatational first motion; filled circle,
denoting a compression; open triangle, denoting no first motion pick was
'made. , Larger symbols with station abbreviations are first mofion picks of this

study. Smaller symbols with no station abbreviations indicate first motion picks

1 3
compiled from the 188 or ISC bulletins.

Source-time functions in this study are comprised of the rise time, flat
time, and fall time of the event. The units of the source-time function are
seconds.

Crustal structures are represented by four columns of numbers. Each
column is labelled below with: column 1: Alpha, indicating P wave velocity
(km/sec), column 2: Beta, indicating S wave velocity (km/sec), column 3: Rho,
indicating density (g/cm3), column 4: T, the thickness of that layer in
kilometers.

Focal mechanisms are given as angles of azimuth, dip and slip,
respectively. Focal depths are in kilometers.

In presenting comparisions of observed and synthetic station records,
these records are placed .in pairs, with the observed station record above and
the synthetic record below. sz and lpz refer to short period vertical and long

period vertical records respectively.
April 19, 1962

The April 19, 1962 event (mb 6.2) is located near the northern terminus of
the Cherskii mountains, in the south-east section of the study area (Figure 1).
First-motions for this event are all compressions. A previous solution by Cook,
(1988) (Figure 5) based on 188 reported first-motions finds a poorly constrained
mechanism which is purely thrust on a fault striking nearly north-south (strike
170 dip 40 slip 90).

First-motions were read and synthetic waveforms calculated for six
long-period records - (FigUre 6) and good matches were obtained for the
mechanism (strike 80 dip 39 slip 58). This mechanism is rotated by roughly 90

14

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16
degrees from the previous solution and includes a slight strike-slip component.

The crustal model used is a mantle half-space overlain by a thick (30 km)
crustal layer. This area is recognized as structurally complex (Cook, 1988), and
no attempt is made to detail crustal reflectors. This event occurs beneath the
north-south trending Yglan graben. It is suggested that the north-south
trending nodal plane of this mechanism is the fault plane, which is a graben
bounding fault now under compression.

Error margins, within which the waveforms fit well are i 20 degrees
azimuth and i 3 degrees dip. The stations plot on the focal sphere nearer the

auxiliary plane, so that the waveform fits best constrain the slip angle.
November 22, 1984

This event (mb 5.4) occurred beneath the northern Cherskii mountains. It
has been studied previously by Dziewonski et al. (1985), who calculate a
centroid-moment tensor solution, and by Cook (1988) whose solution is based on
first-motion information (Figure 7). The Cook (1988) mechanism (strike 58 dip
90 slip 5) involves nearly pure strike-slip motion, the Dziewonski et al. (1985)
mechanism (strike 87 dip 75 slip 46) has slightly more thrust component. Both
of these solutions can be reconciled with fault trends in the area as reported
by Zonenshain et al. (1978) and Savostin and Karasik (1981). The crustal
structure created in this study is an half space with two overlying layers. In
this study first-motions are picked and five short-period station records are
modelled (Figure 8) to produce the solution (strike 58 dip 90 slip 6), which is
very similar to that of Cook (1988). The waveform fits are only fair. The
difference which exists between the solution 'of this Study and that of

Dziewonski et al. (1985) may be explained by the loose constraint afforded by

17

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20
these fairly low quality of waveform matches, or by noting that the centroid-

moment tensor technique used by Dziewonski et al. (1985) becomes more
unreliable for events at shallower depth, longer wavelengths being less reliable
(Michael and Geller, 1984). The depth for the event is calculated at 11 km. It is
possible that both of the above possiblities contribute to the difference
observed. Synthetics were also created for the Dziewonski mechanism (Figure

9), showing relatively poor short-period waveform fits.
January 21, 1976

The January 21, 1976 event occured in the northern Cherskii mountains
(Figure 2). Previous solutions were created by Savostin and Karasik (1981),
Koz’min (1984) and Cook (1988), based on first-motions from WWSSN station
records (Figure 10). The mechanism of Koz’min (1984), strike 88 dip 58 slip
354, has a dilatational component and a large strike-slip component. That of
Savostin and Karasik (1981), strike 106 dip 34 slip 32, and Cook (1988), strike
90 dip 45 slip 29, both have a slight thrust component and a large strike-slip
component. In this study first motions from WWSSN stations were picked and
three short-period station records were modelled (Figure 11). Waveform fits of
good quality were produced using the mechanism (strike 56 dip 70 slip 11), this
mechanism being very similar to the thrust mechamisms of Savostin and Karasik
(1981) and Cook (1988). The crustal model used was a half-space of typical
continent material, overlain by a layer of lighter material with an arbitrary
thickness of one kilometer. The calculated fault plane can be reconciled with
faults in the area reported by Zonenshain et al. (1978), Savostin and Karasik
(1981), and with fault traces observed on LANDSAT imagery by CDDk (1988).

Poor constraint of this mechanism exists due to very few stations used for

21

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23
waveform modelling.

In the northern extreme of the study area a group of events occur on
the Arctic Mid-Ocean ridge proper and extend south over the slope of the
Eurasian continent. The most northern of these occured on February 22, 1987,

on the Arctic Mid-Ocean ridge.
February 22, 1987

A centroid-moment tensor solution exists for this event from Dziewonski
(1988b) (Figure 12): strike 136 dip 51 slip 238, which has a large dilatation
component. This event occurs on the Arctic Mid-Ocean ridge.

Three short-period stations records were modelled for this event (Figure
13), and first-motion information was considered, to produce the mechanism
strike 126 dip 75 slip 210. Waveform matches were of good quality, but because
only three stations could be modelled, occupying essentially two points on the
focal sphere, the mechanism is poorly constrained. Because this event occurs
on the Arctic Mid-Ocean ridge, it is reasonable to expect a large extensional

component for this mechanism.

August 25, 1964

The August 25, 1964, event (mb 6.2) occurred on the Arctic Mid-Ocean
ridge and just off the slope of the Eurasian continent (Figure 1). It has been
studied by several previous workers (J emsek et al., 1986; Sykes, 1967; Cook,
1988), who have consistently produced solutions for this event (Figure 14) with
'a - very large extensional component, acting on a fault plane striking roughly

north-northwest - south-southeast, with a range in azimuth covered by the

24

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29
three fault strikes of between 338 and 356 degrees. The dip of these fault

planes varies by 30 degrees, while the amount of strike-slip component, that is
the slip angle, varies from 270 degrees, (no strike slip motion, Jemsek et al.
(1986)), to 247 degrees (slight strike slip motion, Savostin and Karasik (1981)).
These mechanisms overall are fairly consistent with each other.

In this study first motions were picked and long-period waveform
modelling was performed for ten WWSSN stations (Figures 15 and 16). The
mechanism produced: strike 310 dip 36 slip 250. provides good waveform fits to
the long-period station records. This mechanism resembles most closely that of
J emsek et a1. (1986), but with a greater strike-slip component.

The crustal model and the source-time function used in studying this event
were calculated by Jemsek et al. (1986). This crustal model was somewhat
simplified for this study due to software limitations. This mechanism, like the
previous mechanisms, fulfills the general expectation that a ridge earthquake
have a large extensional component and act on faults aligned sub-parallel with

the ridge.
July 20, 1976

The July 20, 1976 (mb 4.8) event took place on the Arctic Mid-Ocean ridge
very close to where the August 1964 event occured (Figure 1). This event has
not been studied previously. First motions were picked and waveform modelling
was done for two short period station records (Figures 17 and 18). The
resulting fits were fair. From this information, a mechanism can be roughly
constrained: strike 320 dip 50 slip 210. This is an extensional mechanism with a

considerable strike-slip cOmponent. The error margins 'are wide, with so few

stations modelled, that large changes, for example ,1 30 degrees in azimuth

30

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cannot be disallowed. The crustal structure created for the study of this event

is very similar to that used by Jemsek et al. (1986) to study the nearby August
25, 1964, event.

March 21, 1988

The March 21, 1988 (mb 6.0) event is one of the four most northern
events under study (Figure 1). This event has not been previously studied.
First-motion picks were considered and five long-period and one short period
station records were modelled, providing the mechanism (strike 348 dip 68 slip
220) (Figures 19 and 20). Waveform fits were fair, including the short-period
station LUB. The fact that a short-period station record is modelled moderately
well using a source-time. function of twelve seconds duration is interesting,
since such a long and complex source-time function usually rules out the use
of short-period waveform modelling altogether. The adequate fit of the
short-period record from LUB may indicate that the crustal model, adopted
from Jemsek et al. (1986) is correct for this area of the continental slope. This
event occurs near the north end of the Severnyi graben. The generally
north-south strike of the fault plane (azimuth 348°) may be reconciled with the
western (north-south striking) fault of this graben (Figure 1). It must be noted
that this event occurs near the slope of the continent and that the slope of the
ocean floor in the near-source area may affect the appearance of the observed
records (Wiens, 1987). Because the short-period station record LUB is matched
reasonablely well, it is concluded that the effect of the slope of the ocean
floor in the epicentral region in the case of LUB is insignificant. Long-period

records are matched fairly well using this mechanism.

33

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35
April 7, 1969

The event on April 7, 1969 (mb 5.4) occurred on the Eurasian continental
shelf, near the northern end of the Omoloi graben, below the northern Laptev
Sea (Figure 1). This event has been well-studied by several previous workers
(e.g., Chapman and Solomon, 1976; Savostin and Karasik, 1981; Jemsek et al.,
1986; Koz’min, 1984; Cook, 1988), and calculated mechanisms agree only that
some extensional component is present (Figure 21). It appears that centroid—
moment tensor studies (i.e., Chapman and Solomon, 1976) favor a dilatational
mechanism with a northwest - southeast striking plane, while first-motion
studies favor mechanisms with more strike-slip component, and a more
north-south striking fault plane. Faults of the western side of the Omoloi
graben may be reconciled with either of these fault plane trends.

This study picks first-motion information and matches seven long-period
and six short-period station records in order to constrain a focal mechanism
(strike 314 dip 62 slip 301; Figures 22 and 23). Waveform fits were very good
for both long-period and short-period station records. The source time function
used (1.0 0.0 2.0) was adopted as closely as possible from Jemesk et al. (1986),
who also modelled long-period records and produced good fits for this event.
The crustal model used was a half-space of continental material overlain by a
one kilometer thick layer. The good matches for so many station records
suggests the error margin for this mechanism is low. The reason for the
inconsistency among the previous solutions for this event is not clear. The
mechanism of this study generally resembles other solutions derived from
first-motion and waveform-matching studies when comparing dip and slip angle.
The strike of the present mechanism agrees. with the Jemsek et al. (1986)

mechanism (314° azimuth), but is rotated by as much as 66° from that of other

 

 

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39
mechanisms. The consistency of the fault plane between this study and that of

Jemsek et al. (1986) suggests this fault plane may be correct.
September 22, 1987

The September 22, 1987 event (mb 5.6) occurred on the continental shelf
of the Eurasian continent, below the western side of the Bel’kov graben, in the
northern Laptev Sea (Figure 1). Dziewonski (1988) publishes a centroid-moment
tensor solution for this event: strike 6 dip 41 slip 291 (Figure 24). This study
picks first-motion information and matches two long-period and four
short-period station records (Figure 25). First-motions were dilatations. Good
waveform fits were found for both long-period and short-period records using
the mechanism (strike 189 dip 45 slip 270). Either this or the CMT mechanism
can be reconciled with normal faults of the Bel’kov graben (Figure 1). Short

period modelling was possible using the source time function (1.0 0.2 1.0).
August 5, 1986

A small event (mb 4.7) occurred on the continental shelf of Eurasia,
north of the New Siberian Islands on August 5, 1986 (Figure 1). This event is
the most eastern event in the present study. Little information from this event
is found on WWSSN station records. Few first-motion picks were made and no
waveform modelling was successful. The focal mechanism is unconstrained. It
was observed on some station records that two phases appear consistently and
with a fairly consistent difference in time between them. These phases appear
' On the statiOn- records at about the right time to be part of the P wavetrain

from this event. These otherwise poor records were digitized on the hope that

 

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42
these phases could later be named as the phases P and pP for this event, and

thereby be useful to determine focal depth for this event (Figure 26). If one
does assume these phases are P and pP, then assuming a reasonable crustal
model (mantle half-space, continental layer), a focal depth of 18 km is
calculated. More detailed crustal structure information is available for the New
Siberian Islands region (Avetisov, 1983), but is not implemented here because
the focal mechanism is unconstrained regardless. No other useful information

was obtained for this event.
April 23, 1977

An event occurred on April 23, 1977 (mb 5.0) in the eastern Laptev Sea,
below the eastern margin . of the Kykov graben (Figure 1). A centroid-moment
tensor solution from Dziewonski, (1987) is strike 0 dip 45 slip 270 for this
event (Figure 27). In this study, first-motion picks are made and waveform
matching of five short-period station records was done (Figure 28). All
first-motions are dilatations. The waveform modelling produced fair fits for the
mechanism (strike 30 dip 45 slip 270). The observed records were generally
noisy. Either this or the CMT mechanisms could be easily reconciled with the
normal faults of the north-northeast trending Kykov graben. The crustal model
used for study of this event is a half space of continental material created by
Jemsek et a1. (1986). The crustal model for the New Siberian Islands region
from Avetisov, (1983) is not used. The April, 1977, event is small, so that
regardless of crustal structure refinement adequate fits should be obtained. The
Jemsek et al. (1986) half-space crustal model was used.

43

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46
June 10, 1983

A large (mb 5.5) event occured on June 10, 1983, beneath the Ust’ Lena
graben, in the western Laptev Sea (Figure 1). It has been studied previously by
several workers including Dziewonski et al. (1983) and Jemsek et al. (1986)
(Figure 29). The mechanisms determined in these studies are very consistent,
azimuths within i 4° of each other, dip _+_ 6°, slip 1; 4°, with a dilatational
component: Jemsek et al. (1986), strike 144 dip 72 slip 238; Dziewonski et al.
(1983), strike 142 dip 59 slip 242; Cook, (1988), strike 150 dip 70 slip 234.

This study considers first-motion picks and matches seven long-period and
seven short-period station records (Figures 30 and 31). The calculated
mechanism is (strike 159 dip 76 slip 235) and is similar to previous solutions.
Long-period waveform matching quality is generally good, using a source-time
function approximating as closely as possible that of J emsek et al. (1986).
Short-period waveform matching was of fair quality. The calculated focal depth
is 25 km. The crustal structure used is taken from Lander (1990). The fault
plane strike is very similar to the trend of faults bounding the Ust’ Lena
graben. Because so many stations are modelled with good quality and the
solution is similar to solutions obtained in previous waveform modelling and

centroid-moment tensor studies, high confidence is placed in this solution.
January 1, 1988

An event occuring on January 1, 1988, below the eastern Laptev Sea,
beneath the Omoloi graben was studied (mb 5.1). Virtually no information was
obtained. from WWSSN Station records for this event.‘ This event is noted

because the magnitude of this event is large enough that it should have been

47

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50
recorded on at least some stations and in fact, other events of smaller

magnitude have been better recorded. Reasons for this difference are not clear.
The remaining events under study occurred in the vicinity of the Lena

River delta.
May 20, 1963

The event of May 20, 1963 (mb 5.0) occurred below the Lena River delta
(Figure 1). It was studied previously by Cook (1988) using first-motion
information (Figure 32). He offers two possible mechanism solutions (strike 174
dip 60 slip 349, and strike 12 dip 80 slip 180), both mechanisms having large
strike-slip components. This study considers first-motion picks and matches
four short-period station .records for this event (Figure 33). The resulting
mechanism (strike 270 dip 75 slip 339) has a large strike-slip component and is
very similar to one of the Cook (1988) solutions. Waveform fits are good. The
crustal model assumed for the epicentral region, simplified from Vinogradov
(1984) consists of a kilometer-thick layer over a half space. The east-west
striking fault planes of these mechanism are not aligned with a prominent

graben trend.
July 21, 1964

A fairly large event (mb 5.4) occured below Bour Khaya Bay (Bour Khaya
graben; Figure 1), to the east of the Lena River delta on July 21, 1964. This
event has been studied by Cook (1988) using first-motion information,
surface-wave radiation studies and short-period aneform modelling of various

station records. The mechanism calculated is (strike 170 dip 45 slip 286), which

51

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52

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mun him 2 .5 2a 9.55

 

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53
has a large dilatational component, with a slight strike-slip motion (Figure 34).

He utilizes crustal model information from the area of Bour Khaya Bay obtained
from a refraction line shot across that area by Kogan (1974).

In this study first motion picks are considered and waveform modelling of
one long period and six short period station records was done (Figures 35 and
36). The waveform fits are generally good and the mechanism obtained (strike
177 dip 44 slip 255) is in agreement with the Cook (1988) mechanism. The
north-south striking fault plane aligns well with the bounding faults of the Bour
Khaya graben.

February 1, 1980

The final event under study occured on February 1, 1980 (mb 5.4) below
the western edge of the Lena River delta (Figure 1), below the Olenek
aulacogen. This event was studied previously by Cook (1988) (Figure 37), using
first-motion information and surface-wave radiation patterns, producing two
mechanisms: strike 274 dip 71 slip 222, and strike 274 dip 72 slip 344. The
strike of the fault planes for these mechanisms align well with the east-west
striking faults associated with the aulacogen. A centroid-moment tensor
solution, strike 315 dip 55 slip 282 by Dziewonski et al. (1988a) (Figure 37)
could also be reconciled with probable faults in the area.

This study considers first motion picks and models six short-period
station records for this event (Figure 38). The resulting mechanism (strike 288
dip 60 slip 228) is in general agreement with the previously offered mechanisms.
Waveform fits are good. The crustal model used includes a sediment layer to
approximate ' the effect of the nearby Lena River delta. The source-time
function used (1.0 0.0 2.0) was adopted from Jemsek et al. (1986).

54

.805 39 .Fu :2. 2: .2 «cozaom .3 2:9“.

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>0D._.m m_I._.

#mmw .PN >42.

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mmm— .xooo HZO_._._.jOm mDO_>mmn_

 

55

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Nam 42<

 

 

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g. N. a... N m >9. N9 205

.mmN .1. NC. 2m_Z<IUw2

 

56

 

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>0D._.m m_I._.

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Nam o<m

Nam Nom

Nam .Im

57

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mmmr .xooo HmZOEjOm mDO.>umn_

F >m<Dmmmm 0mm—

 

mNN n__.._m om n=0 mmm mx.m._.m
>02.m m_I._.

P >m<Dmmmm 0mm?

 

 

58

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mum 2.6. 8 .6 «mu mem 193.:
536. mi» _ _ .

P >m<3mmmm owmp mozooumon : :

Nam 1.00 Nam Jim

I? It?
5:255

Nam 030 Nam Sm

2...:

 

 

ad fin to

3 ca NWDWM
3 od 0.. E m2: .258 E _ 5:? ;(

5.. «a Emma
63 .8 .mnw 2925.8: Nam 00.. Nam ozm

59
DISCUSSION

Calculated and plotted focal depths (Figure 2), show a pattern of shallow
focal depth across the Laptev Sea region. Events of the north-central and
south-central Laptev Sea have focal depths of 11 km (April, 1969), 12 km (July,
1964), and 10 km (May, 1963). The events of the northwestern Laptev Sea,
excluding those north of the continental slope, and of the southwestern Laptev
Sea have focal depths of 25 km (June, 1983) and 22 km (February, 1980).
Events near the New Siberian Islands (eastern Laptev Sea) have focal depths,
given in order from east to west, of 15 km (September, 1987), 17 km (April,
1977) and 18 km (August, 1986). There is a pattern of increasing focal depths
both toward the east and. west, away from the 130° meridian. The grabens in
the 130° East area and aligned north-south are the Omoloi graben to the north
and the Bour Khaya graben to the south (Figure 38). It is interpreted that
these represent the current locus of spreading in the Laptev Sea area as
suggested by Fujita et al. (1990b) and Kim (1986). The data are sufficient to
resolve the current locus of spreading, but do not resolve which style of rifting
(pure, simple, or a combination of these) is presently active in the area. This
is due to the similarity of heat flow regimes (and therefore shallow earthquake
trends) arising from both types of rifting, as modelled by Buck et al. (1988).

All events studied occurring beneath the Laptev Sea or to the north
include an extensional component, with some strike-slip motion (Figure 1). The
events occurring in the Cherskii mountains have considerable strike-slip and
compressional components (January, 1976: strike 56 dip 70 slip 11; April, 1962:
strike 80 dip 39 slip 58; November, 1984: strike 58 dip 90 slip 6)., This change '

in stress field on going north to south within the study area indicates that

60
some events occurred north of the pole of rotation in this area, and other

events occurred south of the pole. However, grabens or fault-bounded
depressions extend through the Laptev Sea area and through the Cherskii Range
(extending out of the study area). These features, as well as high heat flow,
have been cited by Savostin and Karasik (1981) as the basis for concluding that
this entire region is presently undergoing extension. This is not the case,
based on seismic information. Compression is probably a fairly recent
development on a previously extensional margin.

The style of rifting in the region of the Laptev Sea is discussed by Fujita
et al. (1990b). They describe the characteristics of both the pure and simple
shear rifting models. The pure shear model involves a rift which is separating
symmetrically about the spreading axis, with faults on either side extending into
the ductile zone of the crust (McKenzie, 1978). The simple shear model rift
(Wernicke, 1985; Lister et al., 1986) is distinctly asymmetric, separating on a
single detachment fault which extends through the crust. This detachment
produces an upper and lower plate in the rifting margin. The polarity of this
detachment plane may reverse across transfer faults in the region. Fujita et al.
(1990b) determine that the simple shear model of rifting is most appropriate for

describing the rifting of the Laptev Sea region.
GEOLOGIC INTERPRETATIONS

In the Laptev Sea region, the Eurasian and North American plate form a
two-plate system, with the North American plate rotating clockwise with respect
to the Eurasian plate about a pole lying near Bour Khaya Bay, southern Laptev

Sea (Cook et al., 1986). Rifting is in progress north of the pole. The northern
I Moma Rift system, to the south of the Laptev Sea, is currently undergoing

 

 

61
compression. The presence of the Moma Rift system, with high heat flow and

Cenozoic bimodal volcanism suggest the northern Cherskii Range area (Moma
rift) was recently under extension and possibly connected with the Laptev Sea
rift system through the Ust’ Yana graben (Fujita et al., 1990b). The high heat
flow of the Moma rift system should be dissipating after extension ended in the

area.
TECTONIC IMPLICATIONS

From this study, all Laptev Sea events occur on high angle faults.
Because no low angle normal faulting (detachment plane) events are found, it
seems unlikely that the simple shear model is presently active in the area.
However arguments from Fujita et al. ( 1990b), based on topography, etc.,
suggest that simple shear rifting was active in the area at one time. It is
possible that rifting in the area began as simple shear motion and has since
either changed to pure shear motion or has developed beyond the point where
any single mode of rifting is recognizable. The deep focal depths occurring in
the western Laptev Sea (22 km, 25 km) are difficult to explain with either pure
or simple shear rifting and may support the idea that continental material in
the area is being broken up by the extensional stress, and that the Laptev Sea
rift system, which is a system in transition from oceanic to continental affinity,
may be complex and un-modellable with a single rifting model. The presence of
more than one north-south trend in seismicity in the area may be delimiting the
edges of various continental blocks. The trend of seismicity farthest to the
east, just west of the New Siberian Islands (Bel’kov, Kykov grabens), include
nearly pure normal faulting mechanisms and may be occurring on a major .

bounding normal fault of the "solid" continent to the east. Other events in the

62
Laptev Sea have greater strike-slip components and may be occurring between

various smaller continental blocks in the rift zone.

CONCLUSIONS

A trend exists in the calculated focal depths for seismic events occurring
in the region of the Laptev Sea, with shallower events occuring along the
130°E meridian. This corresponds with and aligns with the entry point of the
Mid-Arctic ridge into the Eurasian continent and is thought to be caused by the
propagation of the ridge extensional regime into the continent. It is
interpreted that the Omoloi graben (north Laptev Sea) and the Bour Khaya
graben (south Laptev Sea) comprise the current locus of spreading in this area,
and that the Eurasian-North American plate boundary lies along the 130°
meridian between 77° and 72° north latitude.

Focal mechanisms are extensional in the north of the study area and
gradually have more strike-slip motion to the south, with a compressive
component existing in the southern-most events. This is interpreted as
confirmation that the pole of rotation for Eurasia-North America lies in the

study area.

53
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