Pure appl. geophys. 157 (2000) 1423–1443
0033–4553/00/091423–21 $ 1.50+0.20/0
Seismic Source Characteristics of Nine Strong Earthquakes from
1988 to 1990 and Earthquake Activity since 1970 in the
Sichuan-Qinghai-Xizang (Tibet) Zone of China
YUAN GAO,1,2 ZHONG-LIANG WU,3 ZHENG LIU1 and HUI-LAN ZHOU1
Abstract—The Chinese provinces of Sichuan, Qinghai and Xizang (Tibet) are situated in a very
active seismic zone. From 1988 to 1990, nine strong earthquakes (M\ 5.9) occurred in these provinces.
A method of analyzing seismic waveforms using apparent source time functions (aSTF) and apparent
time differences (aTD) is adopted to derive rupture characteristics for the strong earthquakes.
Combining the source characteristics with aftershock data, regional tectonics and geology, this paper
examines the migration of strong earthquakes. The Qinghai earthquakes in this study were found to
have strong reverse-slip faulting, which is different from the strike-slip focal mechanisms of past
earthquakes in the region. The steepness of compressional axes of Sichuan earthquakes is related to the
local complicated tectonics. Finally, the single-link cluster (SLC) method is used to analyze the
spatial-temporal behavior of the all strong earthquakes that occurred in the region since 1970. The SLC
analysis suggests that the patterns of earthquake activity can be identified well and that continental
earthquakes occur seemingly with basic regularity.
Key words: Sichuan-Qinghai-Xizang (Tibet), seismic source, rupture, earthquake activity, single-link
cluster.
1. Introduction
The Qinghai-Xizang (Tibetan) Plateau holds particular interest among geologists and geophysicists because of the intense and broad deformation arising from
the collision between the Indian (Australian) and Eurasian plates (Fig. 1a). East
and southeast of the Plateau, the crustal block body removes itself towards the
southeast. However the north and northeast of the Plateau, the crustal block body
removes itself towards the northeast (DING and LU, 1989; ZENG and SUN, 1992),
which extremely influences the strong earthquake activity in this zone. Qinghai
province is located at the northeast of the Plateau, and Sichuan province is located
to the east of the Plateau. Strong earthquakes hit the Sichuan-Qinghai-Xizang
1
Graduate School, University of Science and Technology of China, Beijing 100039, China. E-mail:
[email protected]
2
Center for Analysis and Prediction, China Seismological Bureau, Beijing 100036, China.
3
Institute of Geophysics, China Seismological Bureau, Beijing 100081, China.
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Fig. 1.
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Seismic Source Characteristics
Figure 1
Interplate and intraplate movements and tectonic setting. (a) Indian-Eurasian plate (also see DING and LU, 1989). The intense uplift in figure means the
elevation above sea level \3500 m or the relative elevation \ 1500 m. Solid rectangle outlines the zone shown in Figure 1b. Earthquakes are labeled in
Figure 1b. (b) Sichuan-Qinghai-Xizang (Tibet) zone studied in this paper (also see ZHENG et al., 1989) and distribution of nine strong earthquakes from 1988
to 1990. The rectangles S and Q are shown in Figure 2 in detail.
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region of the Chinese mainland nearly every year. From 1988 to 1990, nine
strong earthquakes, with magnitudes larger than 5.9, struck regions of Sichuan
and Qinghai. It is possible to study the detail of the rupture process of strong
earthquakes using broadband body-wave data (CHOY and KIND, 1987; GAO and
WU, 1995). Initially, in this paper, the broadband data from the Global Seismograph Network (GSN) are adopted to analyze the rupture process of these
nine earthquakes. In the second part of this paper, we examine the spatial and
temporal behavior of source parameters and the occurrence of aftershocks to
identify regularities in the occurrence of strong earthquakes. In particular, we
adopt the SLC (Single-link Cluster) method (FROHLICH and DAVIS, 1990; ZHOU
et al., 1998) to examine the earthquake catalog of large earthquakes that have
occurred in this region since 1970.
2. Rupture Characteristics of the 1988 – 1990 Strong Earthquakes
2.1 Data
From 1988 to 1990 nine strong earthquakes occurred in the provinces of
Sichuan and Qinghai, China. Source parameters of the earthquakes are listed in
Table 1. The parameters come from the Monthly Listing of the PDE (Preliminary Determination of Epicenters) published by the National Earthquake Information Center (NEIC), United States Geological Survey (USGS).
From 15 April to 3 May in 1989, a strong swarm of four strong earthquakes
struck Batang County of Sichuan (i.e., events SC8901-04 in Table 1). Their
epicenters are located on a fault system that strikes north-south (Fig. 1b). More
detailed distribution of faults is shown in Figure 2. On 22 September 1989, an
earthquake of magnitude 6.1 occurred north of Xiaojin County of Sichuan (i.e.,
the event SC8905). Event SC8905 was located north of the bifurcation formed
from two faults, and located just at the juncture of a smaller bifurcation formed
from two smaller faults (see diagram S in Fig. 2). Another set of four strong
earthquakes occurred in Qinghai province in 1988 and 1990 (see diagram Q in
Fig. 2). These earthquakes were the Totoheyan earthquake MS 6.3 of 5 October,
1988 (the event QH8801); the Mangya earthquake MS 6.1 of 14 January, 1990
(the event QH9001); and the Gonghe earthquake sequence of 26 April, 1990
which actually consisted of two strong events (events QH9002 and QH9003)
which occurred 30 seconds apart. All four earthquakes occurred in large or small
basins. The predominant strike of geological features and faults appears in the
region trend NW or NWW. All of these earthquakes were well recorded by the
GSN (Global Seismograph Network). Broadband waveform data from the GSN
are used in this paper.
Vol. 157, 2000
Table 1
Catalogues of strong earthquakes in Sichuan-Qinghai region of China from 1988 to 1990 used in this study ( from PDE)
QH8801
SC8901
SC8902
SC8903
SC8904
SC8905
QH9001
QH9002
QH9003
Date
Origin Time
Location of Earthquakes
Yr-mh-dy
hr-min-sec (UTC)
lat. (°N)
long (°E)
1988-11-05
1989-04-15
1989-04-25
1989-05-03
1989-05-03
1989-09-22
1990-01-14
1990-04-26
1990-04-26
02-14-30.30
20-34-08.93
02-13-20.83
05-53-01.17
15-41-30.88
02-25-50.88
03-03-19.23
09-37-15.04
09-37-45.38
34.354
29.987
30.048
30.091
30.053
31.583
37.819
35.986
36.239
91.880
99.195
99.419
99.475
99.499
102.433
91.971
100.245
100.254
Depth (km)
8
13.3
7.7
14.0
7.5
14.6
12
8
10
Magnitudes*
Radiated Energy
Mb
Ms
M
value (J)
order
5.9
6.2
6.2
6.1
5.8
6.1
6.1
6.5
6.3
6.3
6.2
6.0
6.1
5.9
6.1
6.1
6.9
7.1
6.9
6.9
6.6
6.4
6.8
6.8
7.1
0.8 9 2.1
1.0 90.2
5.1 90.7
0.5 90.1
0.4 9 0.1
0.8 90.3
2.4 90.7
2.1 9 0.9
1.0 9 0.3
13
14
13
14
14
14
13
14
14
Notes
Totoheyan, Qinghai
Batang, Sichuan
Batang, Sichuan
Batang, Sichuan
Batang, Sichuan
Xiaojin, Sichuan
Mangya, Qinghai
Gonghe, Qinghai
Gonghe, Qinghai
Seismic Source Characteristics
Event Codes
* Notes: Values of Magnitude M are from the database of the Center for Analysis and Prediction, China Seismological Bureau, determined by Chinese
Seismic Network.
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Figure 2
Earthquakes and faults in zones of two solid rectangles S and Q shown in Figure 1b.
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2.2 Data Processing and Wa6eform Modeling
For each earthquake, broadband P-waves from digitally recording stations of
the GSN are processed into displacement records using the method of HARVEY and
CHOY (1982). To avoid interference from upper mantle or core triplications, data
are restricted to stations in the distance range of 30 degrees to 90 degrees. The
displacement records are fit by forward modeling, applying the method of CHOY
and KIND (1987), in which synthetic waveforms of the far-field P-wave group
(P+ pP + sP) are obtained by the convolution of source and propagation operators. Propagation effects are dominated by geometrical spreading and attenuation.
For attenuation we employ the frequency-dependent dispersive operator of CHOY
and CORMIER (1986). The source operators are triangular source functions which
are approximations to the parabolic rupture and healing phases of BOATWRIGHT’s
(1980) causal rupture model. For complex earthquakes the displacement is synthesized by the summation of sources lagged in time as a function of azimuth. Using
the fault plane solutions and depths published in the Monthly Listings of the PDE,
forward modeling is used for fitting the waveforms at each station. Using the
correlation function between the modeling waveforms as a kind of measurement for
error, the results suggest that under the 95% of correlation function the errors of
strikes, dips, rakes of these strong earthquake are about 15° and the errors of the
depths of seismic sources are about 2 km. The synthetic broadband waveform and
the apparent source time function (abbreviated as aSTF) for some of these stations
are shown in Figure 3. From the aSTF patterns, it is clear that some earthquakes
consist of several subevents, say, events of QH8801 and SC8901, whereas other
events could be modeled with only a single event, say, the event SC8902.
2.3 The aSTF Analysis and Rupture Direction
For a propagating finite fault, waveform patterns vary as a function of azimuth.
From GAO (1998), the stations that record the shortest pulse widths are in the
direction of the horizontal rupture of the source, whereas the stations that record
the longest pulse widths are in its reverse direction. It is possible to detect the
direction of rupture propagation of a seismic source by measuring the azimuthal
variations of the aSTFs (GAO and WU, 1995). For a source that consists of a single
finite rupture, the analysis of the pulse of apparent time widths (abbreviated as
aTW) can determine the predominant direction of rupture. From the aTW pattern
for every earthquake, it is easy to discover stations with relatively small aTWs. The
azimuths of these stations correspond to the predominant direction of unilateral
rupture (GAO et al., 1998a). Similarly, for an earthquake that consists of more than
one subevent, the apparent time difference (abbreviated as aTD) between subevents
at every station can be inverted to find relative locations. The azimuths of stations
with the smallest aTDs again indicate the direction of rupture. GAO et al. (1998a)
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applied the aSTF method to certain earthquakes in the Sichuan region. Here, the
main results of the nine earthquakes of this study are listed in Table 2.
3. Earthquake Acti6ity
3.1 Aftershock Sequences
Temporary arrays of the China Seismological Bureau in combination with
Chinese seismograph networks were able to record the aftershock sequences of
these earthquakes. Three aftershock sequences were recorded, they are the Batang
sequence for approximately 4 weeks, which included SC8901-04; the Xiaojin
sequence for about 2 weeks, which included only SC8905; the Gonghe sequence for
about 11 weeks, which included QH9002-03. When these sequences showed obvious
attenuation trends, i.e., the number and magnitude of earthquake activities tended
to continuously diminish, the Temporary Arrays were closed. The smallest magni-
Figure 3
The aSTF (black triangular functions at the left) used to model the waveform (at the right, where the
dashed line is data and solid line is the synthetic). Small arrows point to subevents used in the analysis.
Only a representative few of the aSTFs are shown here. The ‘‘Event code: Station code’’ is indicated at
the upper of each group of diagrams.
Vol. 157, 2000
Table 2
Code
Number
Start Depth (km)
Direction
Code
Number
Start Depth (km)
Direction
SC8901
SC8902
SC8903
SC8904
SC8905
8
8
8
4
4
10
8
12
5
5
NEE
NE & SW
NW
NWW
NW
QH8801
QH9001
QH9002
QH9003
2
3
6
6
12
10
8
10
NW or NWW
East-to-West
SEE (or NEE?)
?
Seismic Source Characteristics
Results of source rupture**
** Notes: Code, Number, Start, Depth and Direction mean the event code, number of stations, start depth of rupture and
propagation direction of rupture respectively.
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tude threshold is about M =1.0. However, the location of these aftershocks was
imprecise, it was at times possible to reach an error of 5 km.
In Figure 4(a), aftershock distributions are plotted together with their corresponding main shock. In some cases, such as a large earthquake, the rupture
direction correlates with the longest dimension of the aftershocks, say, event
QH9002. In other cases, however, rupture direction does not coincide with the
lengthwise direction of the aftershock, say, event SC8902.
The STRONG EARTHQUAKE INVESTIGATION GROUP OF QINGHAI SEISMOLOGICAL BUREAU OF CHINA (1990) investigated the Mangya earthquake (event
QH9001) and found that most strong aftershocks are located at two sides of the
main rupture fault plane, and those aftershocks are of low frequency and low
intensity. Here, the low frequency means a small number of earthquakes within a
specific time interval, the low intensity means not only a small number of earthquakes but also their small energy release, i.e., small magnitudes.
Fig. 4.
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Figure 4
Earthquake sequences and their statistical features. (a) Distribution of main shock (large circle) and
aftershocks (small circles). The black solid arrows represent inferred propagational directions of rupture.
(b) Magnitude-time diagram of several sequences. (c) Magnitude-frequency relation which is used to
calculate the b value. China magnitude (DCAP catalog) is used here.
The predominant strike of the aftershock distribution of Qinghai Gonghe
earthquakes, i.e., events QH9002 and QH9003, coincides with the main rupture
fault plane. This suggests that the dominant strike in the spatial distribution of
aftershocks of a very strong earthquake, such as larger than MS 6.9 could directly
indicate the strike of the main rupture plane of the main shock. The advantage of
examining aftershocks is that they are usually numerous and have a long duration.
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Figure 4(b) shows the aftershock sequences for the Batang strong earthquake
group from April to May of 1989 (Batang sequence), the northern Xiaojin
earthquake of 22 September 1989 (Xiaojin sequence) and double Gonghe earthquakes of 26 April 1990 (Gonghe sequence). It is obvious that these three types of
aftershock sequences are different. The sequence can be very short, such as the
Xiaojin sequence; or the sequence can be of a very long duration, such as the
Gonghe sequence. The duration of the Batang sequence is just between the Xiaojin
sequence and the Gonghe sequence. Particularly, for the Xiaojin earthquake, there
was no event larger than magnitude 4.0 in its aftershock sequence, although the
main shock was of MS 6.1. For this reason the strange flat line appears on the
bottom of the middle diagram of Figure 4(c).
The statistical results of these three sequences are shown in Figure 4(c). It is
easy to fit the magnitude-frequency relation of these sequences with a straight line
according to the Gutenberg-Richter relation
log N=a− bM.
(1)
Thus there is a relation for the Batang sequence,
log N= 4.09− 0.61 M
(2)
log N= 2.88−0.77 M
(3)
for the Xiaojin sequence,
and for the Gonghe sequence,
log N= 3.74−0.56 M.
(4)
For the Gonghe, the Batang and the Xiaojin sequences, the b values and standard
variances are 0.569 0.02, 0.6190.03 and 0.7790.06, respectively. The highest and
lowest b values are, respectively, for the Xiaojin sequence and the Gonghe sequence. It seems that the b value is directly proportional to the magnitude of the
main shock, while the standard variance of b values is inversely proportional to the
magnitude. The distribution range and duration time of aftershocks possibly reflects
the pattern and size of the earthquake rupture. Precisely located aftershocks could
indicate the information of the rupture direction of the strong earthquake. However, it is sometimes difficult to obtain a well-located aftershock sequence of a
strong earthquake because there is insufficient azimuthal coverage by a temporary
or permanent seismic network or array in actual operation.
3.2 Migration of Strong Earthquakes
Using the fault plane solutions published in PDE, it is possible to compare the
seismic source characteristics of the 1988–1990 earthquakes in the Sichuan-Qinghai-Xizang (Tibet) zone of China. The studied zone is simply divided into three
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Figure 5
Activity of strong earthquakes in the studied zone from 1988 to 1990. (a) Distribution of earthquakes
is divided into three subzones. The lower hemisphere focal mechanisms for each main shock are shown.
The P and T axes are also summarized. The solid line with arrow represents the migration direction of
the large earthquakes. (b) Relation between the broadband radiated energy of the large earthquakes as
a function of time.
subzones in Figure 5(a). Earthquakes within each of the subzones have similar
azimuths and dips of the P and T axes. However, the P and T axes are different
among the three subzones. The focal mechanisms in subzones I and II are generally
reverse faulting. Those in subzone III are predominantly normal faulting of
strike-slip component. The radiated energies of the nine earthquakes, which are
shown in Figure 5(b), derive from the PDE and are calculated from the method of
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BOATWRIGHT and CHOY (1986). Between 1988–1990, there were at least three
episodes of earthquake energy release of greater than 1013 N·m which occurred in
average intervals of 130 days. Furthermore, the earthquakes migrated along the
path from subzone II to III to I to II, in the direction of NW-to-SE as shown in
Figure 4a. Because nine earthquakes constitute a small sample, we also examine all
the strong earthquakes that have occurred in this zone since 1970. These data come
from the Database of Center for Analysis and Prediction, China Seismological
Bureau, i.e., DCAP. The earthquake catalog in DCAP is complete after 1970
because of better station distribution and data processing. Since the Chinese
magnitude M in DCAP is systematically about 0.4 units larger than the MS or mb
magnitudes reported in the PDE, we consider here only earthquakes of M larger
than 6.4.
Figure 6, depicts the migration of strong earthquakes from 1 January 1970 to 31
August 1996 by linking them in chronological order with lines and arrows. Strong
earthquakes in general migrated to and fro NW-to-SE from 1970 to 1990 and
NE-to-SW after 1990. The three-year period of 1988-to-1990 marked a period of
high occurrence of strong earthquakes.
4. Single Link Cluster (SLC) Pattern
The fundamental feature of the SLC method is to link an earthquake with
another according to the shortest time-space distance (see FROHLICH and DAVIS,
Figure 6
Migration of strong earthquakes. The left diagram illustrates strong earthquakes from 1970 to 1984, the
right one for those from 1985 to 1996. The digits in brackets indicate the year of occurrence. The solid
line with the arrow represents the migration direction of the strong earthquakes.
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Seismic Source Characteristics
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1990; DAVIS and FROHLICH, 1991). The time-space distance dST between two points
is given as
dST =
d 2 + c 2(tk2 − tk1)2
(5)
where d is the normal spatial distance between two points k1 and k2, t is time, c
is a constant correlated to space and time. In the SLC analysis, an important
parameter is adopted to recognize the main event and its aftershock sequence,
which is identified as the characteristic link length, noted as LC. This value is
normally equalized to 70 km (FROHLICH and DAVIS, 1990; ZHOU et al., 1998).
Using the algorithm of LIU and ZHOU (1997), this paper calculates the LC values
of all earthquake events with magnitudes larger than 4.0 since 1970. In addition, a
characteristic link length of 150 km is adopted to analyze the relationship among
events of relatively longer time-space distances. Figure 7 shows the time-space
pattern of SLC, in which all links between events have been cut away if their dST
values are larger than LC =150 km.
ZHOU et al. (1998) introduced four parameters of SLC, the Source Entropy
H(t), the Link Length Ratio R(t), the Event Point Density P(t) and the Average
Link Length LA (t) as SLC parameters, and studied the time-spatial clustering
features of earthquakes in the top area of Kunlun-Altun-Arc of China. Studies of
GAO et al. (1997) also verified that it is effective to study the earthquake activity by
using these SLC parameters. Here are the definitions of these SLC parameters
which are used in this study. The source entropy is given as,
H(t)= − % P(i ) log P(i )
(6)
where P(i ) =N(i )/N0; N(i ) is the number of links in which lengths are within
[(i – 1) DL, i DL], DL is the link length interval, i= 1, 2, . . . , kL, (kL –1) DL B maximum link length5 kL DL; N0 is the number of links within the time window T, i.e.,
in the time interval [j Dt, T + j Dt], Dt is the sliding step length of time window,
j= 0, 1, 2, . . . , kT, kT Dt is equal to the total time length of the earthquake catalog.
The Link Length Ratio is given as,
R(t)=Nr /N0
(7)
where Nr is the number of links in which link lengths LST are longer than LC. The
Event Point Density is,
P(t)=NS /V
(8)
where V means the time-space volume in a specific zone; NS is the number of events
within the specific time-space volume. The condition satisfies that there is more
than one link, in which link length LST is shorter than a specific value LS, connected
to each of these events. Here, let LS = LC. The Average Link Length is defined as,
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Figure 7
SLC frame of earthquakes larger than M 4.0 from 1970 to 1996. Single-link clusters are connected by
short lines. The 1988–1990 main shocks, indicated by black solid circles are connected with longer
straight lines. The time axis starts at 1 January 1970.
LA (t)= % N(i )l(i )/N0
(9)
where N(i ) is the number of single links, each of which lengths is l(i ), i= 1, 2, . . . .
In general, the sparser the time-space distribution of events, i.e., the longer the
lengths of SLC links, the larger the H(t), R(t) and LA (t) values, reversely however,
the smaller the P(t) values.
Using a time window of 1000 days and a sliding time step of 250 days, the H(t),
R(t), P(t) and LA (t) are calculated as a function of time (Fig. 8). The H(t), R(t)
and LA (t) rise up while P(t) descends. Conversely, H(t), R(t) and LA (t) descends
while P(t) rises up. From 1972 to 1975, these SLC parameters have obvious
variations. This was the period of very high intensity and frequency of earthquakes,
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Figure 8
Variations in SLC parameters. (a) Variation of SLC parameters in time slide. H, R, LA and P are the entropy of information source, the ratio of link length
number, the average link length and the point density of events, respectively. (b) Magnitude-time relation. The time axis starts 1 January 1970.
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that is, the period of strong seismicity. During this period, H(t), R(t) and LA (t)
are relatively low while P(t) is relative high. Between 1982 and 1985, however,
there was very low intensity and frequency of earthquakes, that is, this is the
period of weak seismicity, H(t), R(t) and LA (t) are relatively high while P(t) is
relative low. Obviously, for the same reason, 1988-to-1990 is the period of strong
seismicity and 1991-to-1996 is the period of weak seismicity. Furthermore, the
pattern of Figure 8 suggests that the earthquake activity in this zone will possibly
increase gradually and will transform into another period of strong seismicity in
the future, perhaps within approximately 10 years.
5. Tectonic Setting and Implications
DING and LU (1989) divided the Chinese mainland into different blocks and
described recent interplate and intraplate movements. In the Sichuan-Qinghai-Xizang zone, faults have two predominant strike directions. NW and NE (or NNE),
which means a very complicated system of faults. The occurrences of these nine
strong earthquakes from 1988 to 1990 were related to these large faults. From
Figure 1(a), southwest of the studied zone is compressed by the NNE movement
of the Lhasa block. The northern part of the Lhasa block is more rigid than the
studied zone (also see the shaded area in Figure 1(a)). This results in widespread
intraplate deformation in the northern and eastern parts of the Sichuan-QinghaiXizang zone.
As seen in Figure 1(b), the strike of the large faults of the Bayankala Mountains, which originates from the juncture of Altun and Kunlun Mountains,
changes from east-to-west to southeast. Thereafter in the Yunnan region it turns
into a left-lateral north-south striking fault. This fault intersects with another fault
and forms a bifurcation at the city of Kangding. The five Sichuan earthquakes
occurred in response to these tectonics. In their study of the rupture characteristics
of these nine strong earthquakes, GAO et al. (1988a,b) found that the earthquakes
of the strike-slip focal mechanisms were predominantly left-lateral strike-slip,
consistent with the regional compression. In the study of focal mechanisms of
earthquakes exceeding 6.0 that occurred in this zone from 1920 to 1980, YANG et
al. (1989) also found that most strong earthquakes in the Sichuan-Qinghai-Xizang
zone are basically strike-slip. However, the mechanism of several Qinghai earthquakes in this study are obviously reverse-slip. This difference is possibly a result
of complicated local tectonics. The study of XU et al. (1992) indicates that the
horizontal pressure axis of the tectonic stress field in the northeastern Qinghai-Xizang Plateau is oriented NE-SW. The studies of CHEN et al. (1996) and GAO et al.
(1998b) also support this conclusion. This nature of block movement in large-scale
results in the occurrence of strong earthquakes west and southwest of the Chinese
mainland.
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6. Conclusions
Combining analysis of seismicity with plate movement, it is possible to correlate
seismic source characteristics of strong earthquakes with active tectonics. Having
analyzed the broadband waveforms, the aftershock sequences, the migration of
strong earthquakes, and their SLC patterns, this paper makes the following
conclusions.
The rupture direction of strong earthquakes could be obtained by broadband
waveform modeling of teleseismic data. It will be better if there are well-located
aftershock data since they can provide strong additional constraints. The b value of
the main shock-aftershock sequence of strong earthquakes is directly proportional
to the magnitude of main shock, while the standard variance of b values is inversely
proportional to the magnitude for large earthquakes in the Sichuan-Qinghai-Xizang
zone. The stronger the earthquake, the more obvious is the predominant direction
of aftershock distribution and the easier it is to verify the possible rupture direction
from aSTF pattern. Different types of seismic source and different geological and
tectonic settings influence the b value and the distribution of aftershock sequence.
In the studied zone, strong earthquakes generally migrated back and forth in a
NW-to-SE direction from 1970 to 1990 and along a NE-to-SW direction from 1990
to 1996. The SLC method could be used to demonstrate the cluster pattern of
earthquake activity and the SLC parameters could indicate the intensity variation
of earthquake activity. There have been two periods of strong seismicity since 1970.
Seismic source parameters and rupture patterns of these nine strong earthquakes
are consistent with the tectonic stress field and geological surroundings. The seismic
sources of four Qinghai earthquakes have strong reverse-slip components, which
differ from the strike-slip sources found for past earthquakes by previous researchers. P axes of seismic sources of large Sichuan earthquakes, which basically
are not near-horizontal, are related to the local bifurcation style of complicated
tectonic faults. Lastly, although only a small data set is available for this study, the
results could provide beneficial suggestion or indication for more detailed further
research.
Acknowledgements
We thank Professors Yun-tai CHEN and Si-hua ZHEN for their invaluable
guidance, assistance, and advice during the course of this research. Yuan GAO
wishes to thank Dr. George Choy of the National Earthquake Information
Center/USGS in USA for his guidance in seismic source rupture study and
considerably helpful comments pertaining to the preliminary manuscript. Yuan
GAO also is grateful to Professor Domenico Giardini for assistance during his visit
in ETH Zürich in Switzerland.
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Pure appl. geophys.,
This research was supported by the Joint Seismological Science Foundation of
China (95-07-425 and 196088) and partly by the National Science Foundation of
China (49674214).
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(Received March 24, 1999, accepted December 17, 1999)