seismic source characteristics of nine strong earthquakes from...

Pure appl. geophys. 157 (2000) 1423–14430033–4553/00/091423–21 $ 1.50+0.20/0

Seismic Source Characteristics of Nine Strong Earthquakes from1988 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 veryactive 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 apparenttime differences (aTD) is adopted to derive rupture characteristics for the strong earthquakes.Combining the source characteristics with aftershock data, regional tectonics and geology, this paperexamines the migration of strong earthquakes. The Qinghai earthquakes in this study were found tohave strong reverse-slip faulting, which is different from the strike-slip focal mechanisms of pastearthquakes in the region. The steepness of compressional axes of Sichuan earthquakes is related to thelocal complicated tectonics. Finally, the single-link cluster (SLC) method is used to analyze thespatial-temporal behavior of the all strong earthquakes that occurred in the region since 1970. The SLCanalysis suggests that the patterns of earthquake activity can be identified well and that continentalearthquakes occur seemingly with basic regularity.

Key words: Sichuan-Qinghai-Xizang (Tibet), seismic source, rupture, earthquake activity, single-linkcluster.

1. Introduction

The Qinghai-Xizang (Tibetan) Plateau holds particular interest among geolo-gists and geophysicists because of the intense and broad deformation arising fromthe collision between the Indian (Australian) and Eurasian plates (Fig. 1a). Eastand southeast of the Plateau, the crustal block body removes itself towards thesoutheast. However the north and northeast of the Plateau, the crustal block bodyremoves itself towards the northeast (DING and LU, 1989; ZENG and SUN, 1992),which extremely influences the strong earthquake activity in this zone. Qinghaiprovince is located at the northeast of the Plateau, and Sichuan province is locatedto 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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Figure 1Interplate and intraplate movements and tectonic setting. (a) Indian-Eurasian plate (also see DING and LU, 1989). The intense uplift in figure means theelevation above sea level \3500 m or the relative elevation \1500 m. Solid rectangle outlines the zone shown in Figure 1b. Earthquakes are labeled inFigure 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.

Yuan Gao et al.1426 Pure appl. geophys.,

region of the Chinese mainland nearly every year. From 1988 to 1990, ninestrong earthquakes, with magnitudes larger than 5.9, struck regions of Sichuanand Qinghai. It is possible to study the detail of the rupture process of strongearthquakes using broadband body-wave data (CHOY and KIND, 1987; GAO andWU, 1995). Initially, in this paper, the broadband data from the Global Seis-mograph Network (GSN) are adopted to analyze the rupture process of thesenine earthquakes. In the second part of this paper, we examine the spatial andtemporal behavior of source parameters and the occurrence of aftershocks toidentify regularities in the occurrence of strong earthquakes. In particular, weadopt the SLC (Single-link Cluster) method (FROHLICH and DAVIS, 1990; ZHOU

et al., 1998) to examine the earthquake catalog of large earthquakes that haveoccurred 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 ofSichuan and Qinghai, China. Source parameters of the earthquakes are listed inTable 1. The parameters come from the Monthly Listing of the PDE (Prelimi-nary Determination of Epicenters) published by the National Earthquake Infor-mation Center (NEIC), United States Geological Survey (USGS).

From 15 April to 3 May in 1989, a strong swarm of four strong earthquakesstruck Batang County of Sichuan (i.e., events SC8901-04 in Table 1). Theirepicenters are located on a fault system that strikes north-south (Fig. 1b). Moredetailed distribution of faults is shown in Figure 2. On 22 September 1989, anearthquake of magnitude 6.1 occurred north of Xiaojin County of Sichuan (i.e.,the event SC8905). Event SC8905 was located north of the bifurcation formedfrom two faults, and located just at the juncture of a smaller bifurcation formedfrom two smaller faults (see diagram S in Fig. 2). Another set of four strongearthquakes occurred in Qinghai province in 1988 and 1990 (see diagram Q inFig. 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, 1990which actually consisted of two strong events (events QH9002 and QH9003)which occurred 30 seconds apart. All four earthquakes occurred in large or smallbasins. The predominant strike of geological features and faults appears in theregion trend NW or NWW. All of these earthquakes were well recorded by theGSN (Global Seismograph Network). Broadband waveform data from the GSNare used in this paper.

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Table 1

Catalogues of strong earthquakes in Sichuan-Qinghai region of China from 1988 to 1990 used in this study (from PDE)

Location of Earthquakes Depth (km) Magnitudes* Radiated Energy NotesEvent Codes Date Origin Time

Mb Ms M value (J) orderlong (°E)lat. (°N)hr-min-sec (UTC)Yr-mh-dy

QH8801 91.880 8 5.9 6.3 7.1 0.892.1 13 Totoheyan, Qinghai1988-11-05 02-14-30.30 34.35413.3 6.2 6.2 6.9 1.090.2 1499.195 Batang, SichuanSC8901 29.98720-34-08.931989-04-15

99.419SC8902 7.7 6.2 6.0 6.9 5.190.7 13 Batang, Sichuan1989-04-25 02-13-20.83 30.04814.0SC8903 6.1 6.1 6.6 0.590.1 14 Batang, Sichuan1989-05-03 05-53-01.17 30.091 99.475

7.5 5.8 5.9 6.4 0.490.1 1499.499 Batang, Sichuan30.05315-41-30.881989-05-03SC890414.6 6.1 6.1 6.8 0.890.3 14 Xiaojin, SichuanSC8905 1989-09-22 02-25-50.88 31.583 102.43312 6.1 6.1 6.8 2.490.7 1391.971 Mangya, Qinghai37.81903-03-19.231990-01-14QH9001

35.986 100.245 8 6.5 6.9 7.1 2.190.9 14 Gonghe, QinghaiQH9002 1990-04-26 09-37-15.0410 6.3 1.090.3 14100.254 Gonghe, Qinghai36.239QH9003 1990-04-26 09-37-45.38

* Notes: Values of Magnitude M are from the database of the Center for Analysis and Prediction, China Seismological Bureau, determined by ChineseSeismic Network.


Figure 2Earthquakes 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 ofthe GSN are processed into displacement records using the method of HARVEY andCHOY (1982). To avoid interference from upper mantle or core triplications, dataare restricted to stations in the distance range of 30 degrees to 90 degrees. Thedisplacement 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 opera-tors. 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 whichare approximations to the parabolic rupture and healing phases of BOATWRIGHT’s(1980) causal rupture model. For complex earthquakes the displacement is synthe-sized by the summation of sources lagged in time as a function of azimuth. Usingthe 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 thecorrelation function between the modeling waveforms as a kind of measurement forerror, the results suggest that under the 95% of correlation function the errors ofstrikes, dips, rakes of these strong earthquake are about 15° and the errors of thedepths of seismic sources are about 2 km. The synthetic broadband waveform andthe apparent source time function (abbreviated as aSTF) for some of these stationsare shown in Figure 3. From the aSTF patterns, it is clear that some earthquakesconsist of several subevents, say, events of QH8801 and SC8901, whereas otherevents 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 thedirection of the horizontal rupture of the source, whereas the stations that recordthe longest pulse widths are in its reverse direction. It is possible to detect thedirection of rupture propagation of a seismic source by measuring the azimuthalvariations of the aSTFs (GAO and WU, 1995). For a source that consists of a singlefinite rupture, the analysis of the pulse of apparent time widths (abbreviated asaTW) can determine the predominant direction of rupture. From the aTW patternfor every earthquake, it is easy to discover stations with relatively small aTWs. Theazimuths of these stations correspond to the predominant direction of unilateralrupture (GAO et al., 1998a). Similarly, for an earthquake that consists of more thanone subevent, the apparent time difference (abbreviated as aTD) between subeventsat every station can be inverted to find relative locations. The azimuths of stationswith the smallest aTDs again indicate the direction of rupture. GAO et al. (1998a)


applied the aSTF method to certain earthquakes in the Sichuan region. Here, themain 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 withChinese seismograph networks were able to record the aftershock sequences ofthese earthquakes. Three aftershock sequences were recorded, they are the Batangsequence for approximately 4 weeks, which included SC8901-04; the Xiaojinsequence for about 2 weeks, which included only SC8905; the Gonghe sequence forabout 11 weeks, which included QH9002-03. When these sequences showed obviousattenuation trends, i.e., the number and magnitude of earthquake activities tendedto continuously diminish, the Temporary Arrays were closed. The smallest magni-

Figure 3The aSTF (black triangular functions at the left) used to model the waveform (at the right, where thedashed 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.

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Table 2

Results of source rupture**

Number Start Depth (km)Code DirectionNumber Start Depth (km) Direction Code

2 12 NW or NWWNEE QH8801108SC89013 10 East-to-WestSC8902 8 8 NE & SW QH90016 8 SEE (or NEE?)QH9002NW12SC8903 8

QH9003SC8904 6 10 ?4 5 NWWNW4SC8905 5

** Notes: Code, Number, Start, Depth and Direction mean the event code, number of stations, start depth of rupture andpropagation direction of rupture respectively.


tude threshold is about M=1.0. However, the location of these aftershocks wasimprecise, it was at times possible to reach an error of 5 km.

In Figure 4(a), aftershock distributions are plotted together with their corre-sponding main shock. In some cases, such as a large earthquake, the rupturedirection correlates with the longest dimension of the aftershocks, say, eventQH9002. In other cases, however, rupture direction does not coincide with thelengthwise direction of the aftershock, say, event SC8902.

The STRONG EARTHQUAKE INVESTIGATION GROUP OF QINGHAI SEISMOLOGI-

CAL BUREAU OF CHINA (1990) investigated the Mangya earthquake (eventQH9001) and found that most strong aftershocks are located at two sides of themain rupture fault plane, and those aftershocks are of low frequency and lowintensity. Here, the low frequency means a small number of earthquakes within aspecific time interval, the low intensity means not only a small number of earth-quakes but also their small energy release, i.e., small magnitudes.

Fig. 4.


Figure 4Earthquake sequences and their statistical features. (a) Distribution of main shock (large circle) andaftershocks (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 Gongheearthquakes, i.e., events QH9002 and QH9003, coincides with the main rupturefault plane. This suggests that the dominant strike in the spatial distribution ofaftershocks of a very strong earthquake, such as larger than MS 6.9 could directlyindicate the strike of the main rupture plane of the main shock. The advantage ofexamining aftershocks is that they are usually numerous and have a long duration.


Figure 4(b) shows the aftershock sequences for the Batang strong earthquakegroup from April to May of 1989 (Batang sequence), the northern Xiaojinearthquake of 22 September 1989 (Xiaojin sequence) and double Gonghe earth-quakes of 26 April 1990 (Gonghe sequence). It is obvious that these three types ofaftershock sequences are different. The sequence can be very short, such as theXiaojin sequence; or the sequence can be of a very long duration, such as theGonghe sequence. The duration of the Batang sequence is just between the Xiaojinsequence and the Gonghe sequence. Particularly, for the Xiaojin earthquake, therewas no event larger than magnitude 4.0 in its aftershock sequence, although themain shock was of MS 6.1. For this reason the strange flat line appears on thebottom of the middle diagram of Figure 4(c).

The statistical results of these three sequences are shown in Figure 4(c). It iseasy to fit the magnitude-frequency relation of these sequences with a straight lineaccording 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)

for the Xiaojin sequence,

log N=2.88−0.77 M (3)

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 standardvariances are 0.5690.02, 0.6190.03 and 0.7790.06, respectively. The highest andlowest b values are, respectively, for the Xiaojin sequence and the Gonghe se-quence. It seems that the b value is directly proportional to the magnitude of themain shock, while the standard variance of b values is inversely proportional to themagnitude. The distribution range and duration time of aftershocks possibly reflectsthe pattern and size of the earthquake rupture. Precisely located aftershocks couldindicate the information of the rupture direction of the strong earthquake. How-ever, it is sometimes difficult to obtain a well-located aftershock sequence of astrong earthquake because there is insufficient azimuthal coverage by a temporaryor 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 theseismic source characteristics of the 1988–1990 earthquakes in the Sichuan-Qing-hai-Xizang (Tibet) zone of China. The studied zone is simply divided into three


Figure 5Activity of strong earthquakes in the studied zone from 1988 to 1990. (a) Distribution of earthquakesis 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 ofthe 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 similarazimuths and dips of the P and T axes. However, the P and T axes are differentamong the three subzones. The focal mechanisms in subzones I and II are generallyreverse faulting. Those in subzone III are predominantly normal faulting ofstrike-slip component. The radiated energies of the nine earthquakes, which areshown in Figure 5(b), derive from the PDE and are calculated from the method of


BOATWRIGHT and CHOY (1986). Between 1988–1990, there were at least threeepisodes of earthquake energy release of greater than 1013 N·m which occurred inaverage intervals of 130 days. Furthermore, the earthquakes migrated along thepath from subzone II to III to I to II, in the direction of NW-to-SE as shown inFigure 4a. Because nine earthquakes constitute a small sample, we also examine allthe strong earthquakes that have occurred in this zone since 1970. These data comefrom the Database of Center for Analysis and Prediction, China SeismologicalBureau, i.e., DCAP. The earthquake catalog in DCAP is complete after 1970because of better station distribution and data processing. Since the Chinesemagnitude 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 largerthan 6.4.

Figure 6, depicts the migration of strong earthquakes from 1 January 1970 to 31August 1996 by linking them in chronological order with lines and arrows. Strongearthquakes in general migrated to and fro NW-to-SE from 1970 to 1990 andNE-to-SW after 1990. The three-year period of 1988-to-1990 marked a period ofhigh occurrence of strong earthquakes.

4. Single Link Cluster (SLC) Pattern

The fundamental feature of the SLC method is to link an earthquake withanother according to the shortest time-space distance (see FROHLICH and DAVIS,

Figure 6Migration of strong earthquakes. The left diagram illustrates strong earthquakes from 1970 to 1984, theright 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.


1990; DAVIS and FROHLICH, 1991). The time-space distance dST between two pointsis given as

dST=d2+c2(tk2− tk1)2 (5)

where d is the normal spatial distance between two points k1 and k2, t is time, cis a constant correlated to space and time. In the SLC analysis, an importantparameter is adopted to recognize the main event and its aftershock sequence,which is identified as the characteristic link length, noted as LC. This value isnormally 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 valuesof all earthquake events with magnitudes larger than 4.0 since 1970. In addition, acharacteristic link length of 150 km is adopted to analyze the relationship amongevents of relatively longer time-space distances. Figure 7 shows the time-spacepattern 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 AverageLink Length LA(t) as SLC parameters, and studied the time-spatial clusteringfeatures of earthquakes in the top area of Kunlun-Altun-Arc of China. Studies ofGAO et al. (1997) also verified that it is effective to study the earthquake activity byusing these SLC parameters. Here are the definitions of these SLC parameterswhich 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) DLBmaxi-mum link length5kL 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. TheEvent Point Density is,

P(t)=NS/V (8)

where V means the time-space volume in a specific zone; NS is the number of eventswithin the specific time-space volume. The condition satisfies that there is morethan one link, in which link length LST is shorter than a specific value LS, connectedto each of these events. Here, let LS=LC. The Average Link Length is defined as,


Figure 7SLC frame of earthquakes larger than M 4.0 from 1970 to 1996. Single-link clusters are connected byshort 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 thelengths 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) descendswhile P(t) rises up. From 1972 to 1975, these SLC parameters have obviousvariations. This was the period of very high intensity and frequency of earthquakes,

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Figure 8Variations 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.


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 theperiod of weak seismicity, H(t), R(t) and LA(t) are relatively high while P(t) isrelative low. Obviously, for the same reason, 1988-to-1990 is the period of strongseismicity and 1991-to-1996 is the period of weak seismicity. Furthermore, thepattern of Figure 8 suggests that the earthquake activity in this zone will possiblyincrease gradually and will transform into another period of strong seismicity inthe future, perhaps within approximately 10 years.

5. Tectonic Setting and Implications

DING and LU (1989) divided the Chinese mainland into different blocks anddescribed recent interplate and intraplate movements. In the Sichuan-Qinghai-Xi-zang zone, faults have two predominant strike directions. NW and NE (or NNE),which means a very complicated system of faults. The occurrences of these ninestrong earthquakes from 1988 to 1990 were related to these large faults. FromFigure 1(a), southwest of the studied zone is compressed by the NNE movementof the Lhasa block. The northern part of the Lhasa block is more rigid than thestudied zone (also see the shaded area in Figure 1(a)). This results in widespreadintraplate deformation in the northern and eastern parts of the Sichuan-Qinghai-Xizang zone.

As seen in Figure 1(b), the strike of the large faults of the Bayankala Moun-tains, which originates from the juncture of Altun and Kunlun Mountains,changes from east-to-west to southeast. Thereafter in the Yunnan region it turnsinto a left-lateral north-south striking fault. This fault intersects with another faultand forms a bifurcation at the city of Kangding. The five Sichuan earthquakesoccurred in response to these tectonics. In their study of the rupture characteristicsof these nine strong earthquakes, GAO et al. (1988a,b) found that the earthquakesof the strike-slip focal mechanisms were predominantly left-lateral strike-slip,consistent with the regional compression. In the study of focal mechanisms ofearthquakes exceeding 6.0 that occurred in this zone from 1920 to 1980, YANG etal. (1989) also found that most strong earthquakes in the Sichuan-Qinghai-Xizangzone are basically strike-slip. However, the mechanism of several Qinghai earth-quakes in this study are obviously reverse-slip. This difference is possibly a resultof complicated local tectonics. The study of XU et al. (1992) indicates that thehorizontal pressure axis of the tectonic stress field in the northeastern Qinghai-Xi-zang 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-scaleresults in the occurrence of strong earthquakes west and southwest of the Chinesemainland.


6. Conclusions

Combining analysis of seismicity with plate movement, it is possible to correlateseismic source characteristics of strong earthquakes with active tectonics. Havinganalyzed the broadband waveforms, the aftershock sequences, the migration ofstrong earthquakes, and their SLC patterns, this paper makes the followingconclusions.

The rupture direction of strong earthquakes could be obtained by broadbandwaveform modeling of teleseismic data. It will be better if there are well-locatedaftershock data since they can provide strong additional constraints. The b value ofthe main shock-aftershock sequence of strong earthquakes is directly proportionalto the magnitude of main shock, while the standard variance of b values is inverselyproportional to the magnitude for large earthquakes in the Sichuan-Qinghai-Xizangzone. The stronger the earthquake, the more obvious is the predominant directionof aftershock distribution and the easier it is to verify the possible rupture directionfrom aSTF pattern. Different types of seismic source and different geological andtectonic settings influence the b value and the distribution of aftershock sequence.In the studied zone, strong earthquakes generally migrated back and forth in aNW-to-SE direction from 1970 to 1990 and along a NE-to-SW direction from 1990to 1996. The SLC method could be used to demonstrate the cluster pattern ofearthquake activity and the SLC parameters could indicate the intensity variationof earthquake activity. There have been two periods of strong seismicity since 1970.Seismic source parameters and rupture patterns of these nine strong earthquakesare consistent with the tectonic stress field and geological surroundings. The seismicsources of four Qinghai earthquakes have strong reverse-slip components, whichdiffer from the strike-slip sources found for past earthquakes by previous re-searchers. P axes of seismic sources of large Sichuan earthquakes, which basicallyare not near-horizontal, are related to the local bifurcation style of complicatedtectonic faults. Lastly, although only a small data set is available for this study, theresults could provide beneficial suggestion or indication for more detailed furtherresearch.

Acknowledgements

We thank Professors Yun-tai CHEN and Si-hua ZHEN for their invaluableguidance, assistance, and advice during the course of this research. Yuan GAOwishes to thank Dr. George Choy of the National Earthquake InformationCenter/USGS in USA for his guidance in seismic source rupture study andconsiderably helpful comments pertaining to the preliminary manuscript. YuanGAO also is grateful to Professor Domenico Giardini for assistance during his visitin ETH Zurich in Switzerland.


This research was supported by the Joint Seismological Science Foundation ofChina (95-07-425 and 196088) and partly by the National Science Foundation ofChina (49674214).

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STRONG EARTHQUAKE INVESTIGATION GROUP OF QINGHAI SEISMOLOGICAL BUREAU OF CHINA

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(Received March 24, 1999, accepted December 17, 1999)

seismic source characteristics of nine strong earthquakes from...

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