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Body-wave analysis for shallow intraplate earthquakes in the Korean Peninsula and Yellow Sea
Tectonophysics,
345
192 (1991) 345-357
Elsevier Science Publishers
B.V., Amsterdam
Body-wave analysis for shallow intraplate earthquakes in the Korean Peninsula and Yellow Sea Myung-Soon Seismological
Department,
Jun ’
Uppsala University, Box 12019, S-75012,
(Received
May 4,199O;
accepted
January
Uppsala, Sweden
4,199l)
ABSTRACT Jun,
M.-S., 1991. Body-wave analysis Tectonophysics, 192: 345-357.
for
shallow
intraplate
earthquakes
in the
Korean
Peninsula
and
Yellow
Sea.
Four medium-size shallow intraplate earthquakes from the Korean Peninsula and Yellow Sea have been studied in order to find their source mechanisms, focal depths, source time-functions and seismic moments. Focal depths obtained by matching short-period synthetic seismograms to the observations are found to be very shallow, between 6 and 9 km. To determine the source mechanisms, source time-functions and seismic moments, we inverted teleseismic long-period P- and SH-waves from GDSN data. Although the inversions are performed on only a few teleseismic body waves, the radiation patterns of the moment tensors are consistent with most of the P-wave first-motion polarities observed at regional and teleseismic distances. The overall agreement in relative amplitudes and waveforms between observations and synthetics indicates that the earthquake source parameters are reasonably well determined. For two earthquakes from the Yellow Sea and one from the north Korean Peninsula, the mechanisms are predominantly of strike-slip style, while the earthquake in the central Korean Peninsula shows a dip-slip faulting. Consistent ENE-WSW trending compression (P-axis) and NNW-SSE tension (T-axis) are found for all studied earthquakes.
We read the first-motion polarity data from vertical-component seismograms to find the preliminary mechanism. The focal depth was constrained from the short-period P-wave modeling, assuming a point-source dislocation. Long-period P- and SH-waves were then inverted to find the earthquake source-parameters, i.e., seismic moment, strike, dip and rake of the fault planes and source time-function. Finally, we plotted the constructed synthetics and observed seismograms to provide for a visual comparison of the results. The events studied are listed in Table 1, which gives the origin times, epicentral coordinates and magnitudes. The epicentral locations are also plotted in Fig. 1.
The occurrence of intraplate earthquakes is often enigmatic but their mechanism and focal depth together with epicentral location provide important information on the state of stress within the plate. The main objective of this study is to derive basic source parameters for four larger intraplate earthquakes (May 21,1984; Feb. 14,1982; Jan. 07, 1980; Oct. 06, 1976) in the Korean Peninsula and Yellow Sea by studying the teleseismic body-waves. So far, these four earthquakes are the largest events that have occurred within the two regions and the only ones permitting a quantitative study of seismic moments, focal depths and source time-functions,
Tectonic settings
’ Present Daedok
address:
Korea
Institute
P.O. Box 5, Daejon,
0040-1951/91/%03.50
of Energy
Republic
and
The Korean Peninsula and Yellow sea occupy the eastern margin of the Eurasian plate where
Resources,
of Korea.
0 1991 - Elsevier Science Publishers
B.V.
M.-S.
346
y
2m4
JUN
PHILIPPINE PLATE I 140F
I 130’E
Fig. 1. Location of studied earthquakes (dots) in the Korean Peninsula and Yellow Sea. Active plate margins are depicted by heavy lines.
three major tectonic plates, namely, the Pacific, the Philippine and the Eurasian plates are colliding. In eastern Asia, China and Korea constituted a single craton until the early Cretaceous. Since that time, significant tectonic activity has occurred due to the interaction between neighbouring tectonic plates. Since Mid-Cretaceous time, the western Pacific seafloor has been subducting beneath the craton. Subduction of the Kula-Pacific ridge system, at different times, resulted in a change of the direction from north-nor~west to west-northwest
at about 45 Ma (Hilde et al., 1977), when the Korean Peninsula had emerged from the shallow Yellow Sea {Ben-A~aham and Uyeda, 1973; Juan, 1986). The ridge system became well established by about 24 Ma. Back-arc spreading began as a result of the faster subduction and formed marginal seas like the Sea of Japan (Hilde et al., 1977; Juan, 1986). The northward migrating Indian plate collided with the Asian continent in the Eocene. This collision resulted in large scale intra-continental deformation, extending to the north and east, with
TABLE 1 Hypocenters of studied earthquakes from the Korean Penninsula and Yellow Sea * Event
Date
No
1 2 3 4
21-05-84 14-02-82 07-01-80 06-10-76
Origin time 15h39mOOs 14h37m33s 23h44m25s OlbOlm08s
Lat.
Long.
Depth
(ON)
(OE)
(km)
32.67 38.46 40.22 35.31
121.51 125.65 125.02 124.18
6 9 9 9
Magnitude mb
M,
5.7 5.1 4.9 5.2
6.3 5.2 5.2
* Epicentral locations and magnitudes are from ISC, and focal depths are obtained in this study.
Epicentral region Yellow Sea Central Korea North Korea Yellow Sea
$O,,Y-WAVE
ANALYSIS
FOR SHALLOW
INTRAPLATE
347
EARTHQUAKES
decreasing intensity t~ou~out much of the Asia. Current tectonism in the studied region is believed to be the combined result of the Pacific plate subducting along the Japan trench in the east, along the Nankai trough and Ryukyu trench in the southeast (Shimazaki, 1984), together with the continental collision along the Himalayan front in the southwest (Molnar and Tapponnier, 1975, 1977; Tapponnier et al., 1982). The complex interaction of these convergent plates controls the state of the stress and earthquake mechanisms in the studied area.
bfwzl
e%4
Method The analysis of data proceeded in several steps. First, we read the first-motion polarities from vertical-component P-wave records at teleseismic and regional distances and deduced the preliminary fault-plane solutions. We mainly determined the polarity from the long-period vertical-component seismo~~s, In several cases, however, clear supplementary short-period records were employed when the long-period records from a certain region were not available. The SH-wave polarity was also considered whenever possible. The final nodal planes for major double-couple solutions obtained in the inversions are plotted (lower-hemisphere equal-area projection) together with the P-wave first-motion data sets in Fig. 2. The dotted two nodal lines for the May 21, 1984, Yellow Sea event, show the fault plane solution by Chung and Brantley (1989) from the modeling of teleseismic body-waves. We will discuss the difference of these two solutions later in this paper. The next step in the analysis was to constrain focal depths using pP-P and sP-P travel-time differences. Previous studies have shown that the moment tensor inversion of long-period bodywaves is relatively insensitive to variations in the source depth (Baker and Langston, 1981, 1982). We computed short-period vertical-~mponent synthetic seismo~~s for various focal depths, compared them with observed records and determined the focal depth through a trial-and-error approach. To compute the synthetics, the direct P and two free-surface reflected phases, pP and sP, were considered. Since crustal structures in the
Fig. 2. Lower-hemisphere equal-area projections of doubiecouple components (fault planes) of the moment tensor for the four events from the Korean Peninsula and Yellow Sea, superimposed on the P-wave first-motion polarities (dots for compression, and circles for dilatation). Also shown are the compressional (P) and tensional (T) stress axes of the moment tensor.
epicentral regions are not well known, a half-space model with regional average P-wave velocity of 6.2 km/s, S-wave velocity of 3.6 km/s and density of 2.9 g/cm3 was used. In Fig. 3, we compare the observed short-period P-waves and corresponding synthetics for various depths for the four events studied. The best fit was found for a depth of 6 km for event 1, and 9 km for events 2, 3 and 4 (Table 1). For the two Yellow Sea events (event 1 and 4 in Table l), we omitted the water layer since the water depth at the epicentral region is less than 50 m (Wageman et al., 1970). Our waveform modeling experience is that the water produces a negligible difference in the synthetic waveforms. Omitting the water layer also simplifies the calculation. Crustal structures beneath indi~dual stations were approximated by a half-space model with P-wave velocity of 6.0 km/s and S-wave velocity of 3.5 km/s for all stations. The moment tensor formalism of the generalized inverse technique has become a standard tool to determine the seismic source parameters
M.-S. JUN
348
May 21,1984 h (km)
Feb.14‘1982
UME
UPP
ANT0
h (k@ 5
OBS 6
IO SEC
5 SEC
Jan.07,1980 h (km)
Oct.06,1976 SHIO
GRFO
h (km)
5
5
7
7
OBS
OBS
9
9
11
11
13
13 I
M$O
PO0
,
10 SEC
10 SEC
Fig. 3. Short-period P-wave synthetics for various focal depths. Dashed lines show arrivals of peaks and/or
troughs in observed
traces (OBS) and in corresponding synthetics.
different waveform data (Stump and Johnson, 1977; Langston, 1981). Inversion of the source parameters is based upon the fact that an arbitrarily oriented point-source dislocation can be represented by a summation of three Green’s functions (Langston and Helmberger, 1975). We used a generalized inversion procedure (Wiggins, 1972) to determine the source parameters. The method used is described in Langston (1981), and using
examples of applications to teleseismic body-wave data are given in Baker and Langston (1981,1982). To avoid the upper mantle triplication and core shadow effects, records were selected from epicentral distances between 29 o and 80 O, Green’s functions for a point source were calculated for a vertical strike-slip, vertical dip-slip and 45” dipslip source for P-waves and vertical strike-slip and a vertical dip-slip source for SH-waves in a half-
BODY-WAVE
ANALYSIS
FOR SHALLOW
INTRAPLATE
space model applying the generalized ray method by Langston and Helmberger (1975). Responses from three P-rays (P, pP and sP) and two SH-rays (S and sS) were calculated and convolved with known instrumental responses for each station (GDSN: Global Digital Seismograph Network) and for Futterman’s attenuation operator with t * = 0.7 for P-waves and t* = 4.0 for SH-waves (Futterman, 1962). In carrying out the inversion, the source timefunction was parameterized as a number of boxcars with 1 s duration each. Five or six boxcar timefunction elements were allowed, although an average scheme was employed such that only three were free parameters in the inversion. The timefunction elements were weighted by 0.001 relative to moment tensor elements for all studied events, in order to help insure that the moment tensor
TABLE Results
would be well resolved and that any parameter trade-off would take place in the source timefunction. Five iterations were performed although convergence was usually obtained within the first three steps. During the inversion, the covariance matrix for the observation was assumed to be the identity matrix and the eigenvalue cut-off was defined by an arbitrarily selected value to allow maximum variation of the parameter change. In this way, reliable solutions were obtained within five iterations. Indication of the performance of the inversion is the r.m.s. error, which is a direct measure of the fitness between data and synthetics. The final moment tensor was decomposed into a major double-couple, being an average of the maximum and minimum principal components and a remainder, which is the compensated linear vector dipole
2 of inversion 84-05-21
Event: Moment
349
EARTHQUAKE3
82-02-14
80-01-07
76-10-06
tensor a
MO
11.9
Ml1
- 4.698
0.9069 - 0.8993
0.420 - 0.3002
0.4082 -0.1715 0.3684
M22
8.451
0.09074
0.4190
MI,
7.597
0.1241
0.1159
0.1801
Ys
- 3.376
0.1563
0.1175
0.1637
4.235
0.2529
0.1342
- 0.05488
M23
Time function
b -0.177
0.106
S,
0.0088
s2
0.362
0.280
0.299
s3
0.716
0.737
0.491
s4
0.210 0.297
-
S5
0.299 -0.139
0.153 -0.186 0.137
S, D.C. mechanism
’
strike
15.3
245.2
215.7
dip rake
74.1 148.5
43.8 242.8
62.3
61.8
172.8
146.6
8.2
1.7
27.5
9.5
0.1566
0.0573
C.L.V.D.
(W)
r.m.s. fit
0.0342
199.5
0.04583
No of station
P: 3
P: 4
P: 3
P: 2
used for inversion
SH: 3
SH: 3
SH: 3
SH: 2
a Units of moment
are 1O24 dyn cm.
b Units of time function
are seconds.
’ Units of double-couple
mechanism
are degrees.
M.-S. JUN
350
(CLVD). For the major double-couple, the seismic moment, M,, and the strike, dip and rake of the two nodal planes were computed. Data and results We were forced to use only a limited number of high-quality records for each event due to the size of the events. Combining the SH- and P-waves constrains better the source orientation. Later, when we compared the observed records with corresponding synthetic seismograms, constructed by determined parameters from the inversion, we added stations not included in the inversions. The use of SH-waves is often indispensable in constraining the orientation of the sources, especially in the case of a dip-slip mechanism (Langston et al., 1982). In the present analysis, detecting the earliest SH-arrival against the background noise was often difficult since the S-wave, in general, has a longer period than the P-wave. To avoid this problem, we made careful comparison with the
SV-wave to determine the true SH-wave arrival time. Table 2 lists the inversion results for each event we have studied here. The moment-tensor elements, time-function elements, the standard errors and major double-couple parameters are given in the table. The major double-couple nodal planes and their P- and T-axes are plotted in Fig. 2 together with polarity info~ation. The Yellow Sea event of May 21, 1984 This event, which occurred at the western margin of the Yellow Sea is the largest event among the events studied. According to ISC bulletin, this event was preceded by a smaller event, 8%b= 5.4, southeast of the main shock. The origin-time difference is 69 s and the distance between these two events is about 10 km. Amplitudes of the foreshock are smaller and suppressed by the later larger event. Separation between the two shocks from P- and SH-waves at teleseismic
May 21, 1984
P - Waves
I
L.
60 SEC
Fig. 4. The observed (tup) and synthetic (bottom) P-wave seismograms for a focal depth of 6 km and a 3 s source time-function duration. The seismic moment, MQ, at each station is given to the right of the corresponding trace. The inversion tome-endows are indicated by arrows. Also shown are lower-hemisphere equai-area projections of the major double-couple moment-tensor nodalsurfaces.
BODY-WAVE
ANALYSIS
FOR
SHALLOW
INTRAPLATE
351
EARTHQUAKES
distances was clear. We analyzed only the later event. For inversion, we used three P- and three SHwaves from three different quadrants at distances between 50 o to 80 O. The applied time function consisted of five boxcars with 1 s duration each. We generated Green’s functions using a focal depth of 6 km, estimated from short-period waveform modeling (Fig. 3). The major double-couple corresponds to a strike-slip motion, with a small amount of thrust component with the following fault parameters: strike (8) = 15 O; dip (6) = 74“; rake (A) = 149 o and seismic moment, M, = 1.2 x 10z5 dyn cm, and about 3 s of the source timefunction duration. The P-axis is nearly horizontal with plunging angle 9“ in east-northeast (azimuth 68’ ) direction and the T-axis plunges with a moderate angle (33”) to the west-northwest (azimuth 332“). The observed P- and SH-waves and the synthetics resulting from the inversion (source depth of 6 km, 3 s source time-function duration) are plotted
in Figs. 4 and 5, respectively. The major doublecouple mechanism is also exhibited for both Pand SH-waves. The synthetics show a very good fit, both in amplitude and waveform for all stations considered. Even though we inverted only a rather limited number of stations, the decomposition of the moment tensor into a double-couple is consistent with all of the P-wave first-motion observations. The moment tensor obtained by inverting a few long-period, teleseismic P- and SH-waves accounts for radiation observed at both the regional and teleseismic distances. In an earlier study, Chung and Brantley (1989) studied this event extensively. They applied forward waveform modeling technique using teleseismic body-waves from WWSSN data. They used a focal depth of 12 km and a 3.5 s long trapezoidal source time-function and found a strike-slip motion (strike = 120”, dip = 88”, rake = 28”) with seismic moment of 1.1 X 10” dyn cm. Since they used a different data set, direct comparison of the
May 21, 1984
SH - Waves
Ld
60 SEC Fig. 5. The observed (top) and synthetic (bottom) SH-wave seismograms for a focal depth of 6 km and a 3 s source time-function duration. Notation as in Fig. 4.
M.-S. JUN
352
from 30’ to 80” and the applied source timefunction was composed of five boxcars with 1 s duration each. We generated Green’s functions using a focal depth of 9 km, estimated from short-period waveform modeling (Fig. 3). The major double-couple corresponds to a predominant dip-slip motion on moderately dipping nodal plane with the following fault parameters: strike (13) = 245’; dip (6) = 44O; rake (A) = 243”. The seismic moment, MO= 9.1 x 10z3 dyn cm, and the source time-function duration about 3 s were estimated. The T-axis is nearly horizontal with a very low angle (4O) in N-S direction (azimuth 174O) and the P-axis is very steep with plunging angle 71” in ENE-WSW (azimuth 72O) direction. The observed P- and SH-waves and the resulting synthetics, from six stations and for a source depth of 9 km, are plotted in Figs. 6 and 7, respectively. The major double-couple mechanism is also plotted in the figures (lower-hemisphere equal-area projection). The synthetics show a good fit, both in amplitude and overall waveform shape for all stations considered, thereby indicating re-
reliability of their and the present solution is difficult. However, the source time-functions and seismic moments deduced by Chung and Brantley (1989) and in the present study are consistent with each other. Both solutions satisfy respective data sets quite well for P-waves. However, for the SHradiation patterns, the two solutions differ. For the !%I-waves, one common station, GDH in Greenland, which is due north of the epicenter, was employed in both studies. From our GDSN digital data, the SH-wave from GDH shows clear negative polarity and this waveform was modelled excellently using the solution of this study (Fig. 5). However, in Chung and Brantley (1989), the SHsynthetic polarity for GDH is positive whereas the observed seismo~am is cont~nat~ by the background noise. The central Korean Peninsula event
qf Feb.
14, 1982
This event occurred in the central Korean Peninsula (Fig. 1). For inversion, we used four Pand three SH-waves within the distance range
Feb. 14, 1982 P - haves 4NMO
ioir x lOaddyne-cm N
I
J
60 SEC Fig. 6. Tbe observed (top) and synthetic (bottom) P-wave seismograms for a focal depth of 9 km and a 3 s source time-function duration. Notation as in Fig. 4.
BODY-WAVE
ANALYSIS
FOR SHALLOW
INTRAPLATE
353
EARTHQUAKES
Feb. 14, 1982
SH
-
Waves
193
i
x lO'dyne-cm
I
60
SEC
Fig. 7. The observed (top) and synthetic (bottom) SH-wave seismograms for a focal depth of 9 km and a 3 s source time-function duration. Notation as in Fig. 4.
alistic determination (mechanism, source depth).
of the basic time-function
parameters and focal
The northern Korean Peninsula event of Jan, 7, 1980 For inversion of this event, we used three Pand three SH-waves within the distance range from 30 o to 45 o and a source time-function consisting of six boxcars with 1 s duration each. We generated Green’s functions using a focal depth of 9 km, estimated from short-period waveform modeling (Fig. 3). The major double-couple corresponds to a predo~n~tly strike-slip motion with the following fault parameters: strike (19) = 216”; dip (6) = 62”; rake (A) = 173”. The seismic moment, M, = 4.2 x 1O23 dyn cm, and the source time-function duration about 4 s were estimated. The P-axis is almost horizontal with a very low angle (15 “) in E-W direction (azimuth 79O) and the T-axis plunges with moderate angle 24” in the N-S direction (azimuth 1760). The large non-double-couple (CLVD) compo-
nent of the moment tensor may be due to the poor azimuthal coverage of data. Johnston and Langston (1984) and Chen et al. (1981) described similar results. All stations we used for the inversion of this event were west of the epicenter and limited in azimuthal range. The observed P- and SH-waves and the resulting synthetics, for a focal depth of 9 km, are plotted in Fig. 8 for both the P- and SH-waves. The major double-couple mechanism is also plotted in the figure (lower-he~sphere equal-area projection). The synthetics show a good fit, both in amplitude and overall waveform shape, for all four stations considered. The Yellow Sea event of Oct. 4, 1976 This event occurred in the central Yellow Sea in the very beginning of operation of the Global Digital Seismograph Network (GDSN). Hence, for the moment tensor inversion, we were able to use only two P- and two SH-waves from two stations (GUM0 and MAIO) at distances 29” and 52”, respectively. Due to the limited data and
M.-S.
354
JUN
Jan. 07, 1980 SH
P - Waves
Fig. 8. The observed
(top)
and
synthetic
(bottom)
P- and
time-function
SH-wave
duration.
seismograms
Notation
-
Waves
for a focal
depth
of 9 km and
a 4 s source
as in Fig. 4.
depth of 9 km, estimated from short-period waveform modeling (Fig. 3). The major double-couple corresponds to a predominantly strike-slip motion on moderately dipping nodal planes with the following fault parameters: strike (0) = 200”; dip
azimuthal coverage, the inversion did not converge. Thus, we constrained the source time-function as a 4 s long trapezoid and inverted only for the moment tensor. We generated Green’s functions using a focal
Oct. 06,1976 P - Waves
SH - Waves
60SEC
Fig. 9. The observed
(top) and synthetic
(bottom)
P- and SH-wave
source time-function
seismograms
duration.
Notation
for a focal depth of 9 km and a 4 s long trapezoidal as in Fig. 4.
BODY-WAVE
ANALYSIS
FOR
SHALLOW
INTRAPLATE
355
~RTHQUAK~
four shallow intraplate earthquakes in the Korean peninsula and Yellow Sea. Although the inversions were based on data from few teleseismic bodywaves, radiation patterns of the moment tensors are consistent with most of the P-wave first-motion polarities at regional and teleseismic distances. The overall agreement in polarity of signals and relative amplitude between observations and synthetics are also good (Fig. 4-9), thereby indicating that the source parameters, i.e., focal mechanism, seismic moment, source timefunction and focal depth determined in the present study are reasonably well determined. Focal depths from 6 to 9 km were estimated from the short-period P-wave modeling for the four events studied here. Two earthquakes from the Yellow Sea and one from northern Korean Peninsula exhibit a strike-slip motion, while the event from the central Korean Peninsula shows a dip-slip motion. The studied region is subject to an ENEWSW compression and a NNW-SSE extension which is consistent with the tectonic stress orientation is neighbouring northeastern China (Shimazaki, 1984) and southwestern Japan (Shiono, 1977). Figure 10a shows the distribution of P- and T-axes of the four studied earthquakes. It can be seen from the figure, that the P-axes are oriented in east-northeast direction whereas T-axes are oriented in north-northwest direction. An analysis of the shallow-earthquake mecha-
62”; rake (X) = 147 O. The seismic moment of M,, = 4.1 x 1O23 dyn cm was estimated. The P-axis is nearly horizontal with a very low angle (1”) in E-W direction (azimuth 253O) and the T-axis plunges with a moderate angle of 43” in N-S direction (azimuth 163” ). The observed P- and SI-I-waves and the resulting synthetics, for a 4 s trapezoidal source timefunction and for a focal depth of 9 km, are plotted in Fig. 9. The major double-couple mechanism is also exhibited in the figure (lower-hemisphere equal-area projection). Even though the number of stations available for comparison of the recorded data and corresponding synthetics is small, Fig. 9 shows a reasonable fit both in amplitude and overall waveform shape. The major double-couple mechanism is consistent with P-wave polarity data shown in Fig. 2. Thus, the suggested mechanism, focal depth and source time-function seem plausible. Since the station ANMO (see Fig. 9) lies at an epicentral distance of about 95 O, the observed waveform may to some extent be affected by the diffraction along the core-mantle boundary. We included ANMO data for qualitative comparison due to scarcity of good data from other stations. (6)
=
Disunion
and eonelusions
Focal depths, fault parameters (strike, dip and rake), source time-functions and seismic moments were deduced from teleseismic body-waves for al
bl
Fig. 10. (a) Lower-hemisphere equal-area projection of the compressional (P) and tensional (7’) axes for earthquakes considered in this study. (b) Composite plot of the determined focal mechanisms (Fig. 2). The black and hatched sectors are common areas of dilatation and compression, respectiveIy. Arrows show the direction of the maximum (P) and minimnm (7’) compressive stress on a lower-he~phere equal-area projection.
M.-S. JUN
356
nisms in the study region indicates a certain portion of compressional and dilatational quadrants to be common for all four earthquakes considered here. As pointed out by Teague et al. (1986), this “sector method” partially overcomes the shortcoming of considering the average P-axis orientation as being indicative of in situ stress directions. The “sector method” is not directly biased by pre-existing fault orientations and thus limits the region of the maximum compressive stress. If we assume that the intraplate stress field is uniform over a certain region, we can justify the combination of mechanisms shown in Fig. lob. The composite plot of the earthquake sourcemechanisms depicted in the figure suggests that the maximum compressive stress, nearly horizontal and oriented E-W, is representative in this area. This trend is consistent with that observed in southwestern Japan and in northeastern China. Results of the inversion can be dependant on the station coverage (Chen et al., 1981) as well as on the source complexity (Scott and Kanamori, 1985). The improved azimuthal coverage reduces the size of the non-double-couple component (Johnston and Langston, 1984) which may be interpreted as a measure of the deficiency in the inversion procedure. It is obvious that this also depends on how well the theoretical source and the structural model resemble the true physical situation. Since the complexity of the source and/or the structure and the azimuthal coverage of the data set control the CLVD component, care should be taken in the interpretation of the nondouble-couple source terms.
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with first-motion
of data.
Inter., 37: 97-107.
1984.
Mid-plate,
earthquakes
A Collection
H., 1985. On the consistency
source
Planet.
K.,
boundary
of Papers Seismicity
plate-margin,
and stress transition of International and Earthquake
and
plate-
in Far East. In: Symposium Prediction.
on
Seism.
Press, Beijing, pp. 132-147. Shiono,
Baker, J.S. and Langston, waves
Am.,
J., 68: 777-803.
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J.L., Fitch,
An intermediate
Continental
body
Sot.
Sci. Lett., 18: 365-376.
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References
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Geophys.
and the Mesozoic
of eastern
I would like to thank 0. Kulhanek, M. Bath and G.F. Panza for critically reading the manuscript and for constructive comments. I am also grateful to M. Zirbes who kindly provided GDSN data. This research has been carried out at the Seismological Department, Uppsala, Sweden.
Bull.
C.A., 1982. Moment
earthquakes.
Ben-Avraham,
Molnar,
Acknowledgments
earthquake.
1423-1444.
C.A., 1981. Inversion
for the moment
tensor
of teleseismic
of the 1978 Thessa-
K., 1977. Focal
southwest
Japan
Earth, 25: l-26.
and
meachnism their
of major
tectonic
earthquakes
significance.
in
J. Phys.
BODY-WAVE
ANALYSIS
FOR
SHALLOW
INTRAPLATE
EARTHQUAKES
Stump, B. and Johnson, L., 1977. The determination of source properties by the linear inversion of seismograms. Bull. Seismol. Sot. Am., 67: 1489-1502. Teague, A.G., Bolhnger, G.A. and Johnstone, A.C., 1986. Focal mechanism analysis for eastern Tennessee earthquakes (1981-1983). Bull. Seismol. Sot. Am., 76: 95-109. Tapponnier, P., Peltzer, G., Le Dam, A.Y., Armijo, R. and Cobbold, P., 1982. Propagating extrusion tectonics in Asia:
357 new insights from simple experiments with Plasticine. Geology, 10: 611-616. Wageman, J.M., Hilde, T.W.C. and Emery, K.O., 1970. Structural framework of East China Sea and Yellow Sea. Am. Assoc. Pet. Geol. Bull., 54: 1611-1643. Wigpins, R.A., 1972. The general linear inverse problem: imphcations of surface waves and free oscillations for earth structure. Rev. Geophys. Space Phys., 10: 251-285.
345
192 (1991) 345-357
Elsevier Science Publishers
B.V., Amsterdam
Body-wave analysis for shallow intraplate earthquakes in the Korean Peninsula and Yellow Sea Myung-Soon Seismological
Department,
Jun ’
Uppsala University, Box 12019, S-75012,
(Received
May 4,199O;
accepted
January
Uppsala, Sweden
4,199l)
ABSTRACT Jun,
M.-S., 1991. Body-wave analysis Tectonophysics, 192: 345-357.
for
shallow
intraplate
earthquakes
in the
Korean
Peninsula
and
Yellow
Sea.
Four medium-size shallow intraplate earthquakes from the Korean Peninsula and Yellow Sea have been studied in order to find their source mechanisms, focal depths, source time-functions and seismic moments. Focal depths obtained by matching short-period synthetic seismograms to the observations are found to be very shallow, between 6 and 9 km. To determine the source mechanisms, source time-functions and seismic moments, we inverted teleseismic long-period P- and SH-waves from GDSN data. Although the inversions are performed on only a few teleseismic body waves, the radiation patterns of the moment tensors are consistent with most of the P-wave first-motion polarities observed at regional and teleseismic distances. The overall agreement in relative amplitudes and waveforms between observations and synthetics indicates that the earthquake source parameters are reasonably well determined. For two earthquakes from the Yellow Sea and one from the north Korean Peninsula, the mechanisms are predominantly of strike-slip style, while the earthquake in the central Korean Peninsula shows a dip-slip faulting. Consistent ENE-WSW trending compression (P-axis) and NNW-SSE tension (T-axis) are found for all studied earthquakes.
We read the first-motion polarity data from vertical-component seismograms to find the preliminary mechanism. The focal depth was constrained from the short-period P-wave modeling, assuming a point-source dislocation. Long-period P- and SH-waves were then inverted to find the earthquake source-parameters, i.e., seismic moment, strike, dip and rake of the fault planes and source time-function. Finally, we plotted the constructed synthetics and observed seismograms to provide for a visual comparison of the results. The events studied are listed in Table 1, which gives the origin times, epicentral coordinates and magnitudes. The epicentral locations are also plotted in Fig. 1.
The occurrence of intraplate earthquakes is often enigmatic but their mechanism and focal depth together with epicentral location provide important information on the state of stress within the plate. The main objective of this study is to derive basic source parameters for four larger intraplate earthquakes (May 21,1984; Feb. 14,1982; Jan. 07, 1980; Oct. 06, 1976) in the Korean Peninsula and Yellow Sea by studying the teleseismic body-waves. So far, these four earthquakes are the largest events that have occurred within the two regions and the only ones permitting a quantitative study of seismic moments, focal depths and source time-functions,
Tectonic settings
’ Present Daedok
address:
Korea
Institute
P.O. Box 5, Daejon,
0040-1951/91/%03.50
of Energy
Republic
and
The Korean Peninsula and Yellow sea occupy the eastern margin of the Eurasian plate where
Resources,
of Korea.
0 1991 - Elsevier Science Publishers
B.V.
M.-S.
346
y
2m4
JUN
PHILIPPINE PLATE I 140F
I 130’E
Fig. 1. Location of studied earthquakes (dots) in the Korean Peninsula and Yellow Sea. Active plate margins are depicted by heavy lines.
three major tectonic plates, namely, the Pacific, the Philippine and the Eurasian plates are colliding. In eastern Asia, China and Korea constituted a single craton until the early Cretaceous. Since that time, significant tectonic activity has occurred due to the interaction between neighbouring tectonic plates. Since Mid-Cretaceous time, the western Pacific seafloor has been subducting beneath the craton. Subduction of the Kula-Pacific ridge system, at different times, resulted in a change of the direction from north-nor~west to west-northwest
at about 45 Ma (Hilde et al., 1977), when the Korean Peninsula had emerged from the shallow Yellow Sea {Ben-A~aham and Uyeda, 1973; Juan, 1986). The ridge system became well established by about 24 Ma. Back-arc spreading began as a result of the faster subduction and formed marginal seas like the Sea of Japan (Hilde et al., 1977; Juan, 1986). The northward migrating Indian plate collided with the Asian continent in the Eocene. This collision resulted in large scale intra-continental deformation, extending to the north and east, with
TABLE 1 Hypocenters of studied earthquakes from the Korean Penninsula and Yellow Sea * Event
Date
No
1 2 3 4
21-05-84 14-02-82 07-01-80 06-10-76
Origin time 15h39mOOs 14h37m33s 23h44m25s OlbOlm08s
Lat.
Long.
Depth
(ON)
(OE)
(km)
32.67 38.46 40.22 35.31
121.51 125.65 125.02 124.18
6 9 9 9
Magnitude mb
M,
5.7 5.1 4.9 5.2
6.3 5.2 5.2
* Epicentral locations and magnitudes are from ISC, and focal depths are obtained in this study.
Epicentral region Yellow Sea Central Korea North Korea Yellow Sea
$O,,Y-WAVE
ANALYSIS
FOR SHALLOW
INTRAPLATE
347
EARTHQUAKES
decreasing intensity t~ou~out much of the Asia. Current tectonism in the studied region is believed to be the combined result of the Pacific plate subducting along the Japan trench in the east, along the Nankai trough and Ryukyu trench in the southeast (Shimazaki, 1984), together with the continental collision along the Himalayan front in the southwest (Molnar and Tapponnier, 1975, 1977; Tapponnier et al., 1982). The complex interaction of these convergent plates controls the state of the stress and earthquake mechanisms in the studied area.
bfwzl
e%4
Method The analysis of data proceeded in several steps. First, we read the first-motion polarities from vertical-component P-wave records at teleseismic and regional distances and deduced the preliminary fault-plane solutions. We mainly determined the polarity from the long-period vertical-component seismo~~s, In several cases, however, clear supplementary short-period records were employed when the long-period records from a certain region were not available. The SH-wave polarity was also considered whenever possible. The final nodal planes for major double-couple solutions obtained in the inversions are plotted (lower-hemisphere equal-area projection) together with the P-wave first-motion data sets in Fig. 2. The dotted two nodal lines for the May 21, 1984, Yellow Sea event, show the fault plane solution by Chung and Brantley (1989) from the modeling of teleseismic body-waves. We will discuss the difference of these two solutions later in this paper. The next step in the analysis was to constrain focal depths using pP-P and sP-P travel-time differences. Previous studies have shown that the moment tensor inversion of long-period bodywaves is relatively insensitive to variations in the source depth (Baker and Langston, 1981, 1982). We computed short-period vertical-~mponent synthetic seismo~~s for various focal depths, compared them with observed records and determined the focal depth through a trial-and-error approach. To compute the synthetics, the direct P and two free-surface reflected phases, pP and sP, were considered. Since crustal structures in the
Fig. 2. Lower-hemisphere equal-area projections of doubiecouple components (fault planes) of the moment tensor for the four events from the Korean Peninsula and Yellow Sea, superimposed on the P-wave first-motion polarities (dots for compression, and circles for dilatation). Also shown are the compressional (P) and tensional (T) stress axes of the moment tensor.
epicentral regions are not well known, a half-space model with regional average P-wave velocity of 6.2 km/s, S-wave velocity of 3.6 km/s and density of 2.9 g/cm3 was used. In Fig. 3, we compare the observed short-period P-waves and corresponding synthetics for various depths for the four events studied. The best fit was found for a depth of 6 km for event 1, and 9 km for events 2, 3 and 4 (Table 1). For the two Yellow Sea events (event 1 and 4 in Table l), we omitted the water layer since the water depth at the epicentral region is less than 50 m (Wageman et al., 1970). Our waveform modeling experience is that the water produces a negligible difference in the synthetic waveforms. Omitting the water layer also simplifies the calculation. Crustal structures beneath indi~dual stations were approximated by a half-space model with P-wave velocity of 6.0 km/s and S-wave velocity of 3.5 km/s for all stations. The moment tensor formalism of the generalized inverse technique has become a standard tool to determine the seismic source parameters
M.-S. JUN
348
May 21,1984 h (km)
Feb.14‘1982
UME
UPP
ANT0
h (k@ 5
OBS 6
IO SEC
5 SEC
Jan.07,1980 h (km)
Oct.06,1976 SHIO
GRFO
h (km)
5
5
7
7
OBS
OBS
9
9
11
11
13
13 I
M$O
PO0
,
10 SEC
10 SEC
Fig. 3. Short-period P-wave synthetics for various focal depths. Dashed lines show arrivals of peaks and/or
troughs in observed
traces (OBS) and in corresponding synthetics.
different waveform data (Stump and Johnson, 1977; Langston, 1981). Inversion of the source parameters is based upon the fact that an arbitrarily oriented point-source dislocation can be represented by a summation of three Green’s functions (Langston and Helmberger, 1975). We used a generalized inversion procedure (Wiggins, 1972) to determine the source parameters. The method used is described in Langston (1981), and using
examples of applications to teleseismic body-wave data are given in Baker and Langston (1981,1982). To avoid the upper mantle triplication and core shadow effects, records were selected from epicentral distances between 29 o and 80 O, Green’s functions for a point source were calculated for a vertical strike-slip, vertical dip-slip and 45” dipslip source for P-waves and vertical strike-slip and a vertical dip-slip source for SH-waves in a half-
BODY-WAVE
ANALYSIS
FOR SHALLOW
INTRAPLATE
space model applying the generalized ray method by Langston and Helmberger (1975). Responses from three P-rays (P, pP and sP) and two SH-rays (S and sS) were calculated and convolved with known instrumental responses for each station (GDSN: Global Digital Seismograph Network) and for Futterman’s attenuation operator with t * = 0.7 for P-waves and t* = 4.0 for SH-waves (Futterman, 1962). In carrying out the inversion, the source timefunction was parameterized as a number of boxcars with 1 s duration each. Five or six boxcar timefunction elements were allowed, although an average scheme was employed such that only three were free parameters in the inversion. The timefunction elements were weighted by 0.001 relative to moment tensor elements for all studied events, in order to help insure that the moment tensor
TABLE Results
would be well resolved and that any parameter trade-off would take place in the source timefunction. Five iterations were performed although convergence was usually obtained within the first three steps. During the inversion, the covariance matrix for the observation was assumed to be the identity matrix and the eigenvalue cut-off was defined by an arbitrarily selected value to allow maximum variation of the parameter change. In this way, reliable solutions were obtained within five iterations. Indication of the performance of the inversion is the r.m.s. error, which is a direct measure of the fitness between data and synthetics. The final moment tensor was decomposed into a major double-couple, being an average of the maximum and minimum principal components and a remainder, which is the compensated linear vector dipole
2 of inversion 84-05-21
Event: Moment
349
EARTHQUAKE3
82-02-14
80-01-07
76-10-06
tensor a
MO
11.9
Ml1
- 4.698
0.9069 - 0.8993
0.420 - 0.3002
0.4082 -0.1715 0.3684
M22
8.451
0.09074
0.4190
MI,
7.597
0.1241
0.1159
0.1801
Ys
- 3.376
0.1563
0.1175
0.1637
4.235
0.2529
0.1342
- 0.05488
M23
Time function
b -0.177
0.106
S,
0.0088
s2
0.362
0.280
0.299
s3
0.716
0.737
0.491
s4
0.210 0.297
-
S5
0.299 -0.139
0.153 -0.186 0.137
S, D.C. mechanism
’
strike
15.3
245.2
215.7
dip rake
74.1 148.5
43.8 242.8
62.3
61.8
172.8
146.6
8.2
1.7
27.5
9.5
0.1566
0.0573
C.L.V.D.
(W)
r.m.s. fit
0.0342
199.5
0.04583
No of station
P: 3
P: 4
P: 3
P: 2
used for inversion
SH: 3
SH: 3
SH: 3
SH: 2
a Units of moment
are 1O24 dyn cm.
b Units of time function
are seconds.
’ Units of double-couple
mechanism
are degrees.
M.-S. JUN
350
(CLVD). For the major double-couple, the seismic moment, M,, and the strike, dip and rake of the two nodal planes were computed. Data and results We were forced to use only a limited number of high-quality records for each event due to the size of the events. Combining the SH- and P-waves constrains better the source orientation. Later, when we compared the observed records with corresponding synthetic seismograms, constructed by determined parameters from the inversion, we added stations not included in the inversions. The use of SH-waves is often indispensable in constraining the orientation of the sources, especially in the case of a dip-slip mechanism (Langston et al., 1982). In the present analysis, detecting the earliest SH-arrival against the background noise was often difficult since the S-wave, in general, has a longer period than the P-wave. To avoid this problem, we made careful comparison with the
SV-wave to determine the true SH-wave arrival time. Table 2 lists the inversion results for each event we have studied here. The moment-tensor elements, time-function elements, the standard errors and major double-couple parameters are given in the table. The major double-couple nodal planes and their P- and T-axes are plotted in Fig. 2 together with polarity info~ation. The Yellow Sea event of May 21, 1984 This event, which occurred at the western margin of the Yellow Sea is the largest event among the events studied. According to ISC bulletin, this event was preceded by a smaller event, 8%b= 5.4, southeast of the main shock. The origin-time difference is 69 s and the distance between these two events is about 10 km. Amplitudes of the foreshock are smaller and suppressed by the later larger event. Separation between the two shocks from P- and SH-waves at teleseismic
May 21, 1984
P - Waves
I
L.
60 SEC
Fig. 4. The observed (tup) and synthetic (bottom) P-wave seismograms for a focal depth of 6 km and a 3 s source time-function duration. The seismic moment, MQ, at each station is given to the right of the corresponding trace. The inversion tome-endows are indicated by arrows. Also shown are lower-hemisphere equai-area projections of the major double-couple moment-tensor nodalsurfaces.
BODY-WAVE
ANALYSIS
FOR
SHALLOW
INTRAPLATE
351
EARTHQUAKES
distances was clear. We analyzed only the later event. For inversion, we used three P- and three SHwaves from three different quadrants at distances between 50 o to 80 O. The applied time function consisted of five boxcars with 1 s duration each. We generated Green’s functions using a focal depth of 6 km, estimated from short-period waveform modeling (Fig. 3). The major double-couple corresponds to a strike-slip motion, with a small amount of thrust component with the following fault parameters: strike (8) = 15 O; dip (6) = 74“; rake (A) = 149 o and seismic moment, M, = 1.2 x 10z5 dyn cm, and about 3 s of the source timefunction duration. The P-axis is nearly horizontal with plunging angle 9“ in east-northeast (azimuth 68’ ) direction and the T-axis plunges with a moderate angle (33”) to the west-northwest (azimuth 332“). The observed P- and SH-waves and the synthetics resulting from the inversion (source depth of 6 km, 3 s source time-function duration) are plotted
in Figs. 4 and 5, respectively. The major doublecouple mechanism is also exhibited for both Pand SH-waves. The synthetics show a very good fit, both in amplitude and waveform for all stations considered. Even though we inverted only a rather limited number of stations, the decomposition of the moment tensor into a double-couple is consistent with all of the P-wave first-motion observations. The moment tensor obtained by inverting a few long-period, teleseismic P- and SH-waves accounts for radiation observed at both the regional and teleseismic distances. In an earlier study, Chung and Brantley (1989) studied this event extensively. They applied forward waveform modeling technique using teleseismic body-waves from WWSSN data. They used a focal depth of 12 km and a 3.5 s long trapezoidal source time-function and found a strike-slip motion (strike = 120”, dip = 88”, rake = 28”) with seismic moment of 1.1 X 10” dyn cm. Since they used a different data set, direct comparison of the
May 21, 1984
SH - Waves
Ld
60 SEC Fig. 5. The observed (top) and synthetic (bottom) SH-wave seismograms for a focal depth of 6 km and a 3 s source time-function duration. Notation as in Fig. 4.
M.-S. JUN
352
from 30’ to 80” and the applied source timefunction was composed of five boxcars with 1 s duration each. We generated Green’s functions using a focal depth of 9 km, estimated from short-period waveform modeling (Fig. 3). The major double-couple corresponds to a predominant dip-slip motion on moderately dipping nodal plane with the following fault parameters: strike (13) = 245’; dip (6) = 44O; rake (A) = 243”. The seismic moment, MO= 9.1 x 10z3 dyn cm, and the source time-function duration about 3 s were estimated. The T-axis is nearly horizontal with a very low angle (4O) in N-S direction (azimuth 174O) and the P-axis is very steep with plunging angle 71” in ENE-WSW (azimuth 72O) direction. The observed P- and SH-waves and the resulting synthetics, from six stations and for a source depth of 9 km, are plotted in Figs. 6 and 7, respectively. The major double-couple mechanism is also plotted in the figures (lower-hemisphere equal-area projection). The synthetics show a good fit, both in amplitude and overall waveform shape for all stations considered, thereby indicating re-
reliability of their and the present solution is difficult. However, the source time-functions and seismic moments deduced by Chung and Brantley (1989) and in the present study are consistent with each other. Both solutions satisfy respective data sets quite well for P-waves. However, for the SHradiation patterns, the two solutions differ. For the !%I-waves, one common station, GDH in Greenland, which is due north of the epicenter, was employed in both studies. From our GDSN digital data, the SH-wave from GDH shows clear negative polarity and this waveform was modelled excellently using the solution of this study (Fig. 5). However, in Chung and Brantley (1989), the SHsynthetic polarity for GDH is positive whereas the observed seismo~am is cont~nat~ by the background noise. The central Korean Peninsula event
qf Feb.
14, 1982
This event occurred in the central Korean Peninsula (Fig. 1). For inversion, we used four Pand three SH-waves within the distance range
Feb. 14, 1982 P - haves 4NMO
ioir x lOaddyne-cm N
I
J
60 SEC Fig. 6. Tbe observed (top) and synthetic (bottom) P-wave seismograms for a focal depth of 9 km and a 3 s source time-function duration. Notation as in Fig. 4.
BODY-WAVE
ANALYSIS
FOR SHALLOW
INTRAPLATE
353
EARTHQUAKES
Feb. 14, 1982
SH
-
Waves
193
i
x lO'dyne-cm
I
60
SEC
Fig. 7. The observed (top) and synthetic (bottom) SH-wave seismograms for a focal depth of 9 km and a 3 s source time-function duration. Notation as in Fig. 4.
alistic determination (mechanism, source depth).
of the basic time-function
parameters and focal
The northern Korean Peninsula event of Jan, 7, 1980 For inversion of this event, we used three Pand three SH-waves within the distance range from 30 o to 45 o and a source time-function consisting of six boxcars with 1 s duration each. We generated Green’s functions using a focal depth of 9 km, estimated from short-period waveform modeling (Fig. 3). The major double-couple corresponds to a predo~n~tly strike-slip motion with the following fault parameters: strike (19) = 216”; dip (6) = 62”; rake (A) = 173”. The seismic moment, M, = 4.2 x 1O23 dyn cm, and the source time-function duration about 4 s were estimated. The P-axis is almost horizontal with a very low angle (15 “) in E-W direction (azimuth 79O) and the T-axis plunges with moderate angle 24” in the N-S direction (azimuth 1760). The large non-double-couple (CLVD) compo-
nent of the moment tensor may be due to the poor azimuthal coverage of data. Johnston and Langston (1984) and Chen et al. (1981) described similar results. All stations we used for the inversion of this event were west of the epicenter and limited in azimuthal range. The observed P- and SH-waves and the resulting synthetics, for a focal depth of 9 km, are plotted in Fig. 8 for both the P- and SH-waves. The major double-couple mechanism is also plotted in the figure (lower-he~sphere equal-area projection). The synthetics show a good fit, both in amplitude and overall waveform shape, for all four stations considered. The Yellow Sea event of Oct. 4, 1976 This event occurred in the central Yellow Sea in the very beginning of operation of the Global Digital Seismograph Network (GDSN). Hence, for the moment tensor inversion, we were able to use only two P- and two SH-waves from two stations (GUM0 and MAIO) at distances 29” and 52”, respectively. Due to the limited data and
M.-S.
354
JUN
Jan. 07, 1980 SH
P - Waves
Fig. 8. The observed
(top)
and
synthetic
(bottom)
P- and
time-function
SH-wave
duration.
seismograms
Notation
-
Waves
for a focal
depth
of 9 km and
a 4 s source
as in Fig. 4.
depth of 9 km, estimated from short-period waveform modeling (Fig. 3). The major double-couple corresponds to a predominantly strike-slip motion on moderately dipping nodal planes with the following fault parameters: strike (0) = 200”; dip
azimuthal coverage, the inversion did not converge. Thus, we constrained the source time-function as a 4 s long trapezoid and inverted only for the moment tensor. We generated Green’s functions using a focal
Oct. 06,1976 P - Waves
SH - Waves
60SEC
Fig. 9. The observed
(top) and synthetic
(bottom)
P- and SH-wave
source time-function
seismograms
duration.
Notation
for a focal depth of 9 km and a 4 s long trapezoidal as in Fig. 4.
BODY-WAVE
ANALYSIS
FOR
SHALLOW
INTRAPLATE
355
~RTHQUAK~
four shallow intraplate earthquakes in the Korean peninsula and Yellow Sea. Although the inversions were based on data from few teleseismic bodywaves, radiation patterns of the moment tensors are consistent with most of the P-wave first-motion polarities at regional and teleseismic distances. The overall agreement in polarity of signals and relative amplitude between observations and synthetics are also good (Fig. 4-9), thereby indicating that the source parameters, i.e., focal mechanism, seismic moment, source timefunction and focal depth determined in the present study are reasonably well determined. Focal depths from 6 to 9 km were estimated from the short-period P-wave modeling for the four events studied here. Two earthquakes from the Yellow Sea and one from northern Korean Peninsula exhibit a strike-slip motion, while the event from the central Korean Peninsula shows a dip-slip motion. The studied region is subject to an ENEWSW compression and a NNW-SSE extension which is consistent with the tectonic stress orientation is neighbouring northeastern China (Shimazaki, 1984) and southwestern Japan (Shiono, 1977). Figure 10a shows the distribution of P- and T-axes of the four studied earthquakes. It can be seen from the figure, that the P-axes are oriented in east-northeast direction whereas T-axes are oriented in north-northwest direction. An analysis of the shallow-earthquake mecha-
62”; rake (X) = 147 O. The seismic moment of M,, = 4.1 x 1O23 dyn cm was estimated. The P-axis is nearly horizontal with a very low angle (1”) in E-W direction (azimuth 253O) and the T-axis plunges with a moderate angle of 43” in N-S direction (azimuth 163” ). The observed P- and SI-I-waves and the resulting synthetics, for a 4 s trapezoidal source timefunction and for a focal depth of 9 km, are plotted in Fig. 9. The major double-couple mechanism is also exhibited in the figure (lower-hemisphere equal-area projection). Even though the number of stations available for comparison of the recorded data and corresponding synthetics is small, Fig. 9 shows a reasonable fit both in amplitude and overall waveform shape. The major double-couple mechanism is consistent with P-wave polarity data shown in Fig. 2. Thus, the suggested mechanism, focal depth and source time-function seem plausible. Since the station ANMO (see Fig. 9) lies at an epicentral distance of about 95 O, the observed waveform may to some extent be affected by the diffraction along the core-mantle boundary. We included ANMO data for qualitative comparison due to scarcity of good data from other stations. (6)
=
Disunion
and eonelusions
Focal depths, fault parameters (strike, dip and rake), source time-functions and seismic moments were deduced from teleseismic body-waves for al
bl
Fig. 10. (a) Lower-hemisphere equal-area projection of the compressional (P) and tensional (7’) axes for earthquakes considered in this study. (b) Composite plot of the determined focal mechanisms (Fig. 2). The black and hatched sectors are common areas of dilatation and compression, respectiveIy. Arrows show the direction of the maximum (P) and minimnm (7’) compressive stress on a lower-he~phere equal-area projection.
M.-S. JUN
356
nisms in the study region indicates a certain portion of compressional and dilatational quadrants to be common for all four earthquakes considered here. As pointed out by Teague et al. (1986), this “sector method” partially overcomes the shortcoming of considering the average P-axis orientation as being indicative of in situ stress directions. The “sector method” is not directly biased by pre-existing fault orientations and thus limits the region of the maximum compressive stress. If we assume that the intraplate stress field is uniform over a certain region, we can justify the combination of mechanisms shown in Fig. lob. The composite plot of the earthquake sourcemechanisms depicted in the figure suggests that the maximum compressive stress, nearly horizontal and oriented E-W, is representative in this area. This trend is consistent with that observed in southwestern Japan and in northeastern China. Results of the inversion can be dependant on the station coverage (Chen et al., 1981) as well as on the source complexity (Scott and Kanamori, 1985). The improved azimuthal coverage reduces the size of the non-double-couple component (Johnston and Langston, 1984) which may be interpreted as a measure of the deficiency in the inversion procedure. It is obvious that this also depends on how well the theoretical source and the structural model resemble the true physical situation. Since the complexity of the source and/or the structure and the azimuthal coverage of the data set control the CLVD component, care should be taken in the interpretation of the nondouble-couple source terms.
loniki,
Greece,
Baker, J.S. and Langston, of complex basin Planet. Chen,
Z. and Uyeda,
W.P., Nabelek,
depth
characteristics
71:
tensor inversion
S., 1973. The evolution
of China
of Borneo.
Earth
T.J. and Molnar,
earthquake
beneath
of the event of September
P., 1981.
Tibet;
source
14, 1976. J. Geo-
phys. Res., 86: 2863-2876. Chung,
W. and Brantley,
Sea earthquake
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