Vol.16 No.5 (479~491)

ACTA SEISMOLOGICA SINICA

Sept., 2003

Article ID: 1000-9116(2003)05-0479-13

Studies on seismic source∗ LI Shi-yu (李世愚)

CHEN Yun-tai (陈运泰)

Institute of Geophysics, China Seismological Bureau, Beijing 100081, China

Abstract During the period of 1999~2002, the Chinese seismologists made a serious of developments in the study on seismic sources including observations, experiments and theory. In the field of observation, the methods of the accuracy location of earthquake sources, the inversion of seismic moment tensor and the mechanism of earthquake source are improved and developed. A lot of important earthquake events are studied by using these methods. The rupture processes of these events are inverted and investigated combined with the local stress fields and the tectonic moment by using the measurements of surface deformation. In the fields of experiments and theory, many developments are obtained in cause of seismic formation, condition of stress and tectonics, dynamics of earthquake rupture, rock fracture and nucleation of strong earthquakes. Key words: seismic moment tensor; source parameter of earthquake; physics of earthquake source CLC number: P315.3 Document code: A

1 Studies on seismic source parameters With the development of China Digital Seismograph Network (CDSN) and the setting up of local digital seismograph networks, the seismologists of China have got a lot of global data from GDSN and internal digital seismic wave records. Based on the data, the rupture processes of many strong earthquake events were studied by the seismologists of China. Combined with the measurement of surface deformation, they also studied the relationship of the rupture of earthquake source with the local stress field and the tectonic moment. 1.1 Inversion for seismic moment tensor Seismic moment tensor inversion that provides parameters and image of seismic source process is an important way to study seismic source. In the past four years, studies on seismic moment tensor inversion have substantially increased in China. On Nov. 6, 1988, the MS=7.6 and MS=7.2 earthquakes occurred in Lancang-Gengma, Yunnan, China. Mozaffari, et al (1999b) inverted the moment tensor of these earthquakes and got the source mechanism and source function. HE, et al (2001b) studied the ground fissure of the MS=7.2 Lancang-Gengma earthquake in the epicentral area and inverted the direction of stress tensors inducing the band of ground fissure by using fault slip vector method. The results are consistent with the stress field character indicated by the focal mechanism. HE, et al (2001a) investigated the rupture process of the March 19, 1996 Artux, Xinjiang earthquake (MS=6.9). Their results show that this event is a unilateral-rupture process from the west to the east. According to their results, the 30 km difference between the instrumental and ∗

Received date: 2003-02-19; revised date: 2003-05-21; accepted date: 2003-06-01. Foundation item: State Natural Science Foundation of China (49774217) and Joint Seismological Foundation of China (9507435). Contribution No. 03FE1013, Institute of Geophysics, China Seismological Bureau. E-mail: [email protected]

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macro-seismic epicenters obtained by the other authors can be interpreted. An earthquake of MS=7.9 occurred in Mani, Qinghai-Xizang plateau (Tibet), China on Nov. 8, 1997. XU and CHEN (1999) inverted the moment tensor of this earthquake using the long period body waveform data from China Digital Seismograph Network (CDSN). LIU, et al (2000b) inverted the higher degree moment tensor of this earthquake using broad-band body wave of global seismograph network (GSN). By using the method of mode summation, GAO, et al (2001b) investigated the source mechanism of this earthquake. ZHOU (2002) inverted the moment tensor solutions of the March 25, 1998, Antarctic plate earthquake (MW=8.1, IRIS) and the June 14, 2000, southern Sumatra, Indonesia earthquake (MS=8.0) using far field seismic body waves recorded by long period seismograph stations of the IRIS Global Seismic Network. ZHOU (2002) developed a new method of fitting directly the observed far-field body-waves to invert the temporal-spatial source process. The moment rate of change of every sub-fault can be directly solved in this new method. A large earthquake (MW=7.6) occurred in Jiji (Chi-Chi), Taiwan, China on Sept. 20, 1999. XU, et al (2002) obtained the source time functions (STF) of the main shock from long-period waveform data of the Global Digital Seismograph Network (GDSN) by using two of the largest aftershocks with the magnitudes MW=6.1 and MW=6.2 as empirical Green′s function (EGF). CHEN (2000) collected and arranged the observation data of Fujian and Taiwan Digital Seismic Network, seismo-geology, crust deformation survey and plate movement. CHEN (2000) also analyzed and researched the data. MA and XU (1999) studied the focal mechanism of the May 4, 1998 in the southeast sea area of Taiwan MS=7.7 earthquake by using digital seismic waveforms of IRIS. LIU, et al (1999b) inverted rapidly the source mechanisms of 10 earthquakes with MS=5.2 that occurred in China from November 1996 to January 1998. From 1995, the results of tensor moment and source mechanism of large earthquakes in China (MS≥5.5) and global (MS≥6.0) were published in Catalog of Earthquake by Seismic Stations of China and Seismic Moment Tensor Solution and Annual Bulletins of China Seismological Network of Earthquake (LIU, et al, 1999c). MA, XU (1999) and MA, et al (1999) developed a method of estimating focal mechanisms of moderate and small earthquakes by combining surface waveform with period band 45~100 s fitting method and first motion signs of P waves. Two earthquake examples that formed normal faults and three examples that formed strike-slip faults are given. XU (2000) developed a new method of inverting non-uniform deformation on the surface of fault in the time field using long-period and broad-band frequency waveforms of CDSN. He analyzed the tempo-spatial process of the April 26, 1990, Gonghe, Qinghai MS=6.9 earthquake rupture. Based on first motion solution, DU (2001) considered the Gonghe earthquake as mainly reverse thrust with strike-slip movement. DU, et al (2002) analyzed the characteristic deformations in Himalaya block. Their results show that there is not only compressional deformation in north-south direction, but also obvious tensile moment in east-west direction, and the magnitude of deformation is in accord with the results of observation of GPS and the data of geology. On Nov. 14, 2001 the MS=8.1 strong earthquake occurred in the boundary between Xinjiang and Qinghai, China. WANG, et al (2001a) introduced the destructive degree and source parameters of this earthquake. CHEN and XU (2002) briefly introduced the predictions of this earthquake, as well as the distribution of aftershocks and variation of load/unload response ratio with time. By using near-source broadband data recorded, ZHOU, et al (1999a) inverted the Dongfang (Hainan) earthquake swarm occurred from June to August of 1992. The results indicate that these earthquakes occurred within the same ambient stress field. ZHOU, et al (1999b) also inverted the

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MS=6.0 and MS=6.1 earthquakes of Beibuwan occurred in Dec. 31, 1994 and Jan. 10, 1995. XU, et al (2001) precisely determined the locations of the after-shock sequence of the July 20, 1995 earthquake with ML=4.1. Using 8 aftershocks as empirical Green′s function (EGF), XU, et al (2001) calculated the source time function (STF) of the ML=4.1 event, and obtained the higher average ratio of signal to noise by using summation. RONG, et al (2000b) inverted the Tianzhu-Gulang earthquake (M=5.4, June 1, 1996) using the 3 components digital record of single station and compared the stress field before and after the main shock. WAN, et al (2000) deducted the transform relation between seismic source parameters from seismic fault plane solution in double couple point source model and from the seismic moment. They finally gave a calculating result as an example. Based on the waveform data recorded by 3 DR-200 digital seismometers during the 1989 Batang earthquake swarm (from 16 of April to 4 of May), LI and CHEN (1999) obtained the mechanisms of 5 earthquakes with MS=4.1~4.3. Their results show the complexity of fault activities associated with the swarm. LIU, et al (1999) studied the near-field records of the three destructive mining shocks occurred in a shaft of the Fangshan, Beijing coal mine on May 15, 1999. The largest shock is ML=2.3. The source mechanism of the three earthquakes are analyzed by using theoretical seismograph. The main shock and a series of events prior to it are abnormal. There are not only high-frequency vibration, but also low-frequency vibration. By using nucleation theory of seismic fracture, LIU, et al (1999d) indicated that the low-frequency vibration maybe the plastic wave of sub-critical extension of coal bed. 1.2 Focal mechanism solutions Using precise location and focal-mechanism solution of seismic source to analyze causative fault and stress field, is an important way in the studies of source parameters. HOU, et al (1999) discussed the seismo-tectonic environment that located at the deep and shallow crust and seismology and geology model causing the 1927 Gulang earthquake according to the recent research about the active fault, surface rupture, fault plane solution, seismic activity, as well as the deep geophysical exploration data analysis in the epicentral area. ZENG and SONG (1999) inverted the geodetic measurements of seismic deformation of the 1989 MS=7.1 Loma Prieta earthquake to determine its source mechanism by using the Green′s function. The results indicated that the slipping of the fault plane was very heterogeneous. ZHENG, et al (1999) inverted the source process of three strong-moderate earthquakes occurred at the Himalayan convergence zone according to the generalized ray theory. The consistency of focal stress field of earthquake sequence is paid close attention to. WANG, et al (2001c) analyzed tempo-spatial distribution of source mechanism solution of moderate-strong earthquakes in 5 years before 8 earthquakes with M=7 in Sichuan-Yunnan areas since 1970s. WANG, et al (2002a) analyzed Yao′an, Yunnan earthquake sequence. By using the synthetic focal mechanism solutions of small earthquakes in Xinjiang GAO, et al (2001a) studied the background of stress field before mid-strong earthquakes in northeastern Pamir. ZHOU, et al (2001) analyzed the temporal and spatial changes of stress field of 4 mid-strong earthquakes in North China before strong earthquakes and found some anomaly changes. ZHANG and JIANG (2001) and ZHANG, et al (2001a) studied the Xiuyan-Haicheng MS=5.4 earthquake on Nov. 29, 1999. The results show that focal mechanisms of aftershocks are mostly similar to that of the mainshock.

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DU and LIU (2000) provided focal mechanisms of 3 earthquakes with M>5.0 occurred in Qinghai Province in 1999 and the 2000, Zaduo, Qinghai earthquake with M=5.3. ZHAO, et al (1999) studied reservoir induced seismicity tectonics revealed by the earthquake sequences at Ertan Reservoir area in Sichuan before filling water. They solved the focal mechanism of ML=3.0 in Tianwan sequence. ZHAO, et al (2000, 2001b, c) successively studied the earthquake sequences of the 1998 Luding, Sichuan ML=3.4, the Sept. 14, 1999 Qingping Mianzhu, Sichuan MS=5.0 and the Nov. 30, 1999 Mianzhu, Sichuan MS=5.0 earthquakes. Their results showed that the direction of earthquake generating faults are not consistent with that of Longmenshan fault. WEI, et al (2001) studied the Feb. of 2001 Kangding and Yajiang, Sichuan MS=5.0 and MS=6.0 earthquakes. Their results show that the source mechanism of the two earthquakes are alike, but the distributions of aftershocks are obvious different, forming conjugate rupture. HAN and MIAO (2000) studied the surface rupture zone of Izmit, Turkey M=7.4 earthquake in 1999. Their results show that the micro-epicenter coincides with the location of maximal surface dislocation, and the characteristics of dynamic and geometry are comported with the results of source mechanism. In 1997, there occurred strong earthquake swarm in Jiashi, Xinjiang. Using the master event method, ZHOU, et al (1999c) inverted the precise location of the MS>3.0 earthquakes of Jiashi swarm and compared the results with those obtained by using the traditional methods. By using the focal spectrum, ZHOU and XU (2000) deduced the rupture characteristic of Jiashi swarm. Their results show the low stress drop of most of the earthquakes in this swarm. They deduced that the Jiashi strong earthquakes would continue for a long time. SHAN, et al (2002) studied the source mechanism of the Jiashi swarm. Their results show that the directions of principal stresses differ from that of the regional modern pressure stress field. WANG, et al (2001b) analyzed the rupture characteristic of the MS>5.0 earthquakes of Jiashi swarm. GAO and NIE (2000) studied the space-time distribution patterns of seismic activity and some focal mechanism data since 1990 in Turpan area, as well as the Toksun M=5.6 earthquake on Jan. 30, 1999. Some isolated earthquakes have been studied. XIE, et al (2001) studied the Neixiang-Zhenping, Henan ML=4.7 earthquake occurred on April 29, 2000. WU, et al (2000) studied some characteristics of the Jingyang MS=4.8 earthquake in 1998. WANG and WANG (1999) studied the source mechanism and the seismogeology condition of the Jingyang MS=4.8earthquake. WANG, et al (2001d) improved the selection of locating model, locating method and distribution of seismic network and obtained precise locating of the Changshu earthquake of M=5.1 on Feb. 10, 1990. DIAO, et al (2001) analyzed Ibaraki earthquake sequence of 1982 in Japan. PENG and ZHOU (2000) studied the strike of the main fractured surface of the M=7.3 Taiwan Strait earthquake of Sept. 16, 1994. Using a relative relocation technique, YANG, et al (1999, 2002) relocated the hypocenters of the main shock and the aftershocks with ML>3.0 of the Zhangbei MS=6.2 earthquake. SHI, et al (1999) studied the aftershocks of the Zhangbei MS=6.2 earthquake on Jan. 10, 1998. LIU and DIAO (2000) investigated the macroscopic intensity of Zhangbei M=5.6 aftershock occurred on March 11, 1999. They thought that the mechanism of this event is different from that of the MS=6.2 earthquake. HU, et al (2002) inferred the focal fault of famous Hongtong M=8.0 earthquake (1303) and Linfen M=7 earthquake (1695) by using the records of small earthquakes occurred recently. They relocated and analyzed the mechanism of the 1670 small earthquakes occurred from 1987 to 1999

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recorded by Linfen Telemeter Seismic Network. The results are consistent with the earthquake-generating faults of the two strong earthquakes occurred in Hongtong and Linfen. WANG, et al (2002b) studied the MS>5.0 earthquakes occurred in Datong in 1989, 1991 and 1999. ZHU, et al (1999) analyzed three-dimensional characteristic of focal faults of the two earthquakes at Datong-Yanggao in 1989 and 1991. MA, et al (2001) studied the earthquakes in Tianjin and nearby during 1999 to 2000. CHENG (2000) developed the F-K (wave-number frequency integration) techniques which can quickly determine source mechanism of local and near events that allows for better use of the entire broadband seismogram when only a few stations data are available by using the complete wave theory solution including near-field P-SV, SH term and far-field P-SV terms. She determined the source mechanisms of 1998 Zhangbei MS=6.2 earthquake and its part aftershocks. CHENG, et al (2000) determined the focal mechanism of the three earthquakes occurred in Tengchong volcano area by using the F-K techniques. YE, et al (2000) analyzed the spectrum of 24 microseisms occurred in Tengchong volcano areas using Brune model to calculate the source parameters. CHUAI, et al (2001) studied 11 earthquake sequences with ML>3.0 occurred in Shanxi since 1980s. They think that the characteristics of mechanism consistency of different type of earthquake sequences are different. The variation of contrary note ratio can be regarded as the criterion to determine whether there will be strong earthquake after this earthquake sequence. Mozaffari, et al (1999a) studied high frequency fall-off of source spectra using Q-free spectra estimation. Using this algorithm, the influence of Q-value to the seismograph is wiped out. They processed the near-source broadband seismographs of five aftershocks of the 1988 Lancang-Gengma, Yunnan earthquakes. Their results show that one can get the same tendency of high frequency attenuation from the records of different stations. RONG, et al (2000a) studied the variation characteristics of coda Q values of micro-earthquakes in the source areas before and after Su′nan earthquake (M=5.7, Nov. 22, 1988) and Tianzhu-Gulang earthquake (M=5.4, June 1, 1996). Their results show that the scatter 1/Q values in the source areas before M=5.7 turned to be of the same view. They considered this phenomenon as the precursor of the M=5.7 earthquake. RONG, et al (2000c) also studied the characteristics of the stress field change in the short-term and imminent-term before the M=5.4 Tianzhu-Gulang earthquake. JIN, et al (2000) inverted the source mechanisms of the earthquake sequence of Su′nan earthquake (Nov. 22, 1988, M=5.7) by using the ratio of P wave and S wave amplitudes. Their results showed that the source mechanisms of foreshocks were coincident with the fault plane of the main shock in general, whereas the fault plane solutions of after-shocks are dispersed. Continuously monitoring the composite focal mechanism, coda Q and seismic wave velocity ratio, ZHANG, et al (2000) found that the conflict note ratio of composite focal mechanism in 1997 reached its lowest value over the years and its direction is similar to the 1998 Zhangbei MS=6.2 earthquake. DIAO, et al (1999a) inverted the focal mechanism and the focal locations of Shacheng, Hebei earthquake swarm in July of 1995. DIAO, et al (1999b) studied the spatial orientation and activity characteristics of focal fault of the 1995 Wuding, Yunnan earthquake of ML=6.5. DIAO, et al (1999c) also studied the data of 8 earthquake sequences in Central Asia during 1970~1984. ZHANG, et al (2001b) inverted the shear velocity structure under 3 stations around the old earthquake region of Shacheng. Combining with the velocity structures in Xingtai and Tangshan region, they pointed out that the occurrence of strong earthquake was closely related to the low

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velocity layer, whereas the shear wave is sensitive to the low velocity tectonics. By using the S-wave splitting calculation of the deep source earthquake sounding, LIU, et al (2000a) analyzed the profile of Tai′an-Xinzhou, and got the credible evidence of the anisotropic of medium in the crust in this area and the abnormity of regional stress field direction. LI, et al (2001d) analyzed the relationship between the horizontal tide force of the sun and moon act on the seismogenic zone and the focal mechanism. The azimuth of the horizontal tide force is compared respectively with those of the axes P and T and two groups of fault trends, and it can be shown that they are almost parallel to or perpendicular with one another and the parallel or perpendicular accordance is very well. WANG and LU (2002) discovered the tile disturbance before earthquakes in initial recording diagrams of horizontal pendulum tiltmeters from Lijiang stations, etc. According to the focal mechanism and rock mechanics, they considered that tilt disturbance is intrinsically micro-rupture and stick-slip phenomena and effective precursors.

2 Experimental studies on physics of seismic source The seismologists of China have done many foundational research of physical mechanism of earthquake rupture, including the revolution of micro-cracks in rock and the process of nucleation, the 3-dimensional fracture in rock related to the in-plate earthquakes, the space-time distribution of AE source in rock related to the tectonics and the stress fields, the synchronism or non-synchronism of the revolution of micro-cracks with the electro-magnetic radiation and the transverse wave splitting response to the differential stress variation, etc. LI, et al (2000c, 2001c) obtained the image of micro-cracks and location of AE source of the 3-D fracture of non-penetrated notched rock specimen. These results were got in the cooperation with Kuksenko, et al in the Laboratory of Earthquake Source Physics in Beijing. In this cooperate experiments, they also got the in time locations of the AE sources of rock samples with decimeter scale and typical tectonics under the condition of loading and unloading. LI, et al (2000b, 2001c) and TENG, et al (2001) investigated the evolution and nucleation of microcracks in typical tectonics during the fracture of marble. The principal effect of tectonics on evolution of microcracks is analyzed by using theory of fracture and damage. Using a new set of AE devices, LIU, et al (1999a) studied the time-space distribution of micro-cracks in two types of granite under tri-axial compress. They found that in India granite, it is not obvious that the AE sources appear concentrate together in time-space, while in Mayet granite, locations of AE appear crowd together in time and distribute along main fracture. The difference is attributed to the style of deformation with different tectonics. Ma, et al (2002) considered that there are evidences of seismic nucleation phase according to the research of the AE wave records of rock samples. LI, et al (2001b) analyzed the location results of AE sequences in the process of large rock samples rupture. They found the character image of general and direct fore-shocks. The AE sequences exhibit the characteristics of after-shocks after local damage. They pointed out that the location of denseness center of character magnitude of AE differs from that of the lower magnitude, and it is nearer to the main rupture. LI, et al (2002) investigated the physical mechanism of low-frequency events before strong earthquake experimentally. Their results show that in the later period of rock sample rupture, the low frequency wave components can propagate out to the air and can be received by mike as

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hearable sound. Their fracture experiments show that the initial of tensile fracture of notched rock specimens can produce low frequency stress pulse. They analyzed the mechanism of this kind of low-frequency pulse by using the theory of fracture mechanics. GAO, et al (1999) observed the shear-wave splitting in the specimen of marble of Laizhou, Shandong, China. They observed that in most specimens, with the increasing of loading, the delay time of shear-wave splitting increases quickly just before failure. Sometimes the time delay falls down before it increases. Under some pressure, time delay also increases when the level of press keeps even unloading. GAO, et al (2000b) and GAO (2001) measured velocity anisotropy of Laizhou marbles of Shandong in China. The results show that there is obvious azimuth anisotropy both for intact marble with parallel laminations and for parallel cracked marble. Many studies have been done in the electromagnetic emission during the evolution of cracks in rock. GUO, et al (1999) studied electromagnetic (EM) and acoustic emissions (AE) and their relations during the process of fracturing of various rock samples. The results show that the emissions and properties of EM and AE were affected strongly by the sizes and mineral compositions of rock samples. The relationships between EM and AM emissions were various; for example, some EM emissions were accompanied by AE, but some were not. Their result suggests that in addition to micro fracturing, there may exist some non-fracturing mechanism responsible for EM. GUO and QIAN (2001) explored whether the acoustic wave generated by micro-cracks before earthquake are able to change water content of surface soil. Their results show that when the acoustic wave enters the surface soil the water content here increases on the background of decreasing because of natural evaporation, whereas the temperature here decreases. A lot of studies have been done to find the relations between the micro-crack nucleation and its surrounding stress field. XIONG, et al (2000, 2002) simulated a rupture process of homogeneous rock mass by using the transparent samples. A real-time holographic optical setup is to record the distribution and variations of the samples′ stress fields and the transient waveform automatic recorder is used to record the location and intensity of the micro-fractures. Their method provided a new approach to make a seismic experimental research. JIANG, et al (2000) show granite rock strength increases with the depth until 30 km. The characteristics of failure as well as that of AE sequence are different in different depth. SANG, et al (2001) performed an experimental study on brittle-ductile transition in Panzhihua fine-grained gabbro. Experiments were conducted in 450~800 MPa confining pressure. The results show that the brittle-plastic transition is at 700~900 °C. The major factors that influence the brittle-plastic transition in the gabbro are temperature, strain rate, as well as confining pressure. LIU and Shimada (1999) did systematic micro-structural analyses with a series of granite samples deformed under dry conditions up to 300 °C and 3 GPa. Several styles of micro-fractures with different characteristics, and therefore deformation mechanisms, are recognized from their optical and SEM characteristics. Their micro-structure correlation to the high-press and low-press type fractures is evaluated. By using the synthetic analysis of faulted limestone from the Waterberg fault zone in the Damara Origin, Namibia, Southwest Africa, LIU, et al (2001) indicated a duplicity of the deformation of the limestone: brittle and crystalline plasticity. The duplicity is shown by the co-existence of fractures and micro-fractures, and dynamically crystallized grains distributing along the factures. ZHAO, et al (2001a) studied abnormal variation of wave velocity before rock rupture. The results show that the P wave velocity increases to the value more than twice of previous maximum

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just before the main-rupture. The velocity ratio (vP/vS) also increases to 2.5~3.0. They discussed the reason of these phenomena.

3 Theoretical studies and numerical simulation LI, et al (2001c) developed the theory of 3-D fracture mechanics. Based on the theory, they analyzed the image of the micro-cracks and AE sources distribution of non-penetrated notched rock samples, and pointed out that these results differ from the 2-D model essentially. LI, et al (2000a) discussed nucleation of earthquakes and its implication to precursors. They pointed out that the recent argument about nucleation phase of earthquakes reminds us to completely study the concept of earthquake nucleation. They reviewed the applications of concerning theoretical studies and earthquake nucleation. LI, et al (2001a) reviewed the physical measure of earthquake preparation state in the ground, and discussed the role of experiment in researches on seismic source physics. LI and CHEN (1999) studied the transient S-wave velocity rupture (TSVR), which means the velocity of fault rupture propagation is between S-wave velocity vS and P-wave velocity vP. In 2-D LEFM (linear elastic fracture mechanics) model, there are two difficulties in transient S-wave velocity rupture, i.e., initialization difficulty and divergence difficulty in interpreting the realization of TSVR. By introducing the concept of fractal faults and tunnel effect of faults, they gave the proof theoretically and overcome two difficulties successfully. Using the numerical method, Cai, et al (2000) simulated the dynamic process of the Tangshan earthquake. LI, et al (1999) make a further discussion on the physical characteristics of crustal fracture (earthquake) process, putting emphasis on approaching the non-linearity of strain accumulation and release in rock mass. By using a rock failure process analysis software RFPA2D, LIN, et al (1999) studied numerically the deformation and failure behavior of a double rock sample system including marble-granite series connection. The results were compared with an experiment result with a similar setup of the double sample system (marble-granite sample system). Based on the studies, the precursors prior to the main-shock in the real geological system composed of the failure area and the non-failure area are explained theoretically, including the seismic migration and the deformation localization. Using elastic fracture mechanics theory, ZHANG, et al (2002) analyzed the stress intensity factor and total stress intensity factor produced by the three kinds of stresses through the distributing law of earth stress, pressure from reservoir water and pressure of penetrative water from fault. They also analyzed the law of reservoir-induced earthquakes. WANG and SUN (1999) theoretically and experimentally investigated the different crack propagation behaviors for open or closed model II crack. Meanwhile, the problems of whether crack propagation will cause earthquakes and how tectonic distribution influences earthquake sequences are investigated. Based on the anomalous variation of the local geomagnetic field and electromagnetic radiation phenomenon induced by the strain and fracture of rock subjected to force, WANG, et al (2000) studied the traced predictions of imminent-short-medium anomalies for earthquakes using the techniques of transfer function for short period geomagnetic and response ratio of loading/unloading for vertical component of geomagnetic field, together with the analysis on the electromagnetic radiation phenomenon.

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On the basis of fracture mechanics earthquake rupture model, CHEN, et al (2001) derived relations between source parameters and τ0, the value of tectonic ambient shear stress in the place where the earthquake occurs. Thus, they calculated a large number of values of tectonic ambient shear stress or values of background stress in the place where the earthquake occurs. If nuclear explosions are treated as earthquakes in the calculation, they found that τ0 values of nuclear explosions are about 20 MPa, that is obviously higher than average τ0 values of earthquakes with the same magnitude. This result can be used to discriminate nuclear explosions from earthquakes. Using the source spectra of the aftershocks of the 1988 Lancang-Gengma, Yunnan, China earthquake, WU, et al (1999) studied scaling of stress drop and high-frequency fall-off of source spectra. The results show that the high-frequency fall-off of source spectra and its variation with the size of earthquake can be well explained by model that for large earthquakes the stress drop is a constant while for ″small″ earthquakes stress drop increases with the size of the earthquake. SHEN, et al (2000) studied strain portion model in the middle-upper crust and its application to seismic research. Strain partitioning is a phenomenon that the oblique strain in the lower crust and lithospheric mantle is partitioned into near pure tangential and normal strain in the middle-upper crust. Being a main model on fault interaction, strain partitioning model is directly supported by seismic data. On the other hand, a reasonable strain partitioning model will be helpful to determine seismic sequence and seismic tendency. GUO, et al (2001) discussed some controversial issues of earthquakes triggered by external factors, such as magnitude of external factor, counteraction between negative effect and positive effect, triggering of opposite phase, multiple triggering, triplet method and double method. The combinatorial model of earthquake source development is used in discussion on above-mentioned problems. CHE, et al (2000) advanced a hypothesis about the seismogenic process in a hard intercalary strata in the crust and promoting earthquakes by fluids (BEH-PEF). This hypothesis is that there are two (upper and lower) fluid activity system and a hard intercalary strata between them in the crust, in which the stress is accumulated and the earthquakes are bred. In the dilatancy stage, under the effect of vacuum attract bump, the fluids is absorbed from the lower level system to the expanded area and cause earthquakes. TANG, et al (2002) reviewed the records of low or extra low-frequency events before earthquake and current theoretical research. In general, in the fields of experiments and theory, many developments are obtained by the experts of China, including the causation of seismic formation, condition of stress and tectonics, dynamics of earthquake rupture, rock fracture and nucleation of strong earthquakes. These works will be helpful to the development of seismology and decreasing earthquake hazards. Acknowledgements The authors would like to express their thanks to Mr. HE Xue-song for his help in collecting the materials during the writing of this paper. References Cai Yong-en, He Tao, Wang Ren. 2000. Numerical simulation of dynamic process of the Tangshan earthquake by a new method—LDDA [J]. Pure Appl Geophys, 157(11-12): 2 083~2 104. CHE Yong-tai, LIU Wu-zhou, YU Jin-zi, et al. 2000. A hypothesis on seismogenic process in a hard intercalary strata in the crust and promoting earthquakes by fluids for intra-plate strong earthquake [J]. Acta Seismologica Sinica, 13(1): 105~114. CHEN Pei-shan, CHENG Jin, BAI Tong-xia. 2001. Discrimination between nuclear explosions and earthquakes based on consideration of tectonic ambient shear stress values [J]. Acta Seismologica Sinica, 14(3): 259~262. CHEN Xiang-xiong. 2000. Synthesis research on the Jiji earthquakes [J]. Fujian Seismology, 16(Suppl. 2): 74~89 (in Chinese).

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CHEN Xue-zhong, XU Li-sheng. 2002. The Great earthquake of MS 8.1 occurring in the boundary between Xinjiang Uygur Autonomous Region and Qinghai Province [J]. Recent Developments in World Seismology, (3): 1~4 (in Chinese). CHENG Jin. 2000. Research of the Application of Broadband Digital Seismic Data to Determination of Source Mechanism of Local and Regional Earthquakes and Ambient Shear Stress Magnitudes [D]: [Ph D Thesis]. Beijing: Institute of Geophysics, SSB, 1~110 (in Chinese). CHENG Jin, CHEN Pei-shan, TU Yi-min, et al. 2000. Source estimation in Tengchong volcano area using F-K method [J]. Seismological and Geomagnetic Observation and Research, 21(6): 1~10 (in Chinese). CHUAI Yong-qing, WANG Zhao-li, MENG Yang-ying, et al. 2001. Distinguishing between foreshocks and earthquake swarm from the change of first motion sign [J]. Earthquake Research in China, 15 (1): 57~74. DIAO Gui-ling, ZHANG Si-chang, WANG Pei-de, et al. 1999a. 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Earthquake Research in Plateau, 12(1): 70 (in Chinese). DU Ji-liang, XU Cai-jun, LI Zhi-cai. 2002. Characteristic analysis of deformations in Himalaya block by inverting seismic moment tensors [J]. Geomatics and Information Science of Wuhan University, 27(3): 261~264 (in Chinese). GAO Guo-ying, NIE Xiao-hong. 2000. Analysis on seismic activity and stress field feature in Turpan area [J]. Seismological and Geomagnetic Obsveration and Research, 21(6): 39~44 (in Chinese). GAO Guo-ying, WANG Hai-tao, WEN He-ping, et al. 2001a. Analysis on the dynamic change character of the stress field before mid-strong earthquakes in northeastern Pamir [J]. Northwestern Seismological Journal, 23(4): 389~394 (in Chinese). GAO Yuan, LI Shi-yu, ZHOU Hui-lan, et al. 1999. Experimental studies of marbles on response of shear-wave splitting to differential stress variation [J]. Chinese J Geophys, 42(6): 778~784. GAO Yuan, LI Shi-yu, LIU Qi-liang, et al. 2000. Measurement of wave velocity anisotropy [J]. 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SHI Jin-hu, LAI Xiao-ling, DUAN Yong-hong, et al. 1999. A preliminary interpretation of near-field observation data of the Zhangbei earthquake [J]. Earthquake Research in China, 145(2): 179~187. TANG Lin-bo, LI Shi-yu, SU Fang, et al. 2002. Research on long-period events before earthquakes — History and current situation [J]. Recent Developments in World Seismology, (4): 1~6 (in Chinese). TENG Chun-kai, LI Shi-yu, HE Xue-song, et al. 2001. An experiment study on dynamic of the crack-system in rocks [J]. Chinese J Geophys, 44(Suppl.): 136~145. WAN Yong-ge, WU Zhong-liang, ZHOU Gong-wei, et al. 2000. How to get rake angle of the earthquake fault from known strike and dip of the two nodal planes [J]. Seismological and Geomagnetic Observation and Research, 21(5): 26~30 (in Chinese). WANG Gui-yao, SUN Zong-qi. 1999. Exploration for some problems in fracture mechanics analysis of earthquake mechanism [J]. Chinese Journal of Rock Mechanics and Engineering, 18(1): 55~59 (in Chinese). 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