LI Shiyu and CHEN Yuntai
Institute of Geophysics, China Seismological Bureau, Beijing 100081, China
I. STUDIES ON SEISMIC SOURCE PARAMETERS
1. Inversion of Seismic Moment Tensor
Seismic moment tensor inversion, that provides parameters and image of seismic source process, is an important way of study on seismic source. In the past four years, studies on seismic moment tensor inversion have substantially increased in China.
An earthquake of MS7.4 occurred in Mani, Qinghai-Xizhang 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.（2000）inverted higher degree moment tensor of this earthquake using broad-band body wave of Global Seismograph Network (GSN).
Zhou (2002) inverted the moment tensor solutions of the March 25, 1998, Antarctic plate earthquake (MW8.1, IRIS) and the June 4, 2000, Southern Sumatra, Indonesia earthquake (Ms8.0) using far field seismic body waves recorded by long period seismograph stations of the IRIS Global Seismic Network. Zhou developed a new method of fitting directly the observed far-field body-waves to invert for the temporal-spatial source process. The moment radio function of every sub-fault can be directly solved in this new method.
Mozaffari et al. (1999) carried out moment tensor inversion to invert the source mechanism and source function of the MS7.6 and MS7.2 November 6, 1988, Lancang-Gengma, Yunnan, China, earthquake. Waveforms of long-period body-waves recorded by China Digital Seismograph Network (CDSN)were used in the inversion. He et al. (2001) studied the ground fissure of the Lancang-Gengma, MS7.2 earthquake in the epicentral area. He inverted the direction of stress tensors inducing the band of ground fissure using fault slip vector method. The result is consistent with the stress field character indicated by the focal mechanism.
A large earthquake (Mw7.6) occurred in Jiji (Chi-Chi), Taiwan, China on Sep. 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 of Mw6.1 and Mw6.2 as empirical Green's function (EGF). Chen (2000) collected and cleaned up the observation data of Fujian and Taiwan digital seismic network, seismo-geology, crust deformation survey and plate movement. Chen analyzed and researched the data. Ma and Xu (1999) studied the focal mechanism of the May 4, 1998, in South (and East) China sea, Taiwan strong earthquake by using digital seismic waveforms of IRIS.
Liu Ruifeng et al. (1999a, 1999b) inverted rapidly the source mechanisms of 10 earthquakes with Ms5.2 that occurred in China from November 1996 to January 1998. The Results were sent as “Bulletins of Source Mechanism Parameters of Earthquake” to the Seismic Regime Guards' Office, China Seismological Bureau, and the relevant provincial seismological bureaus. These bulletins have played considerable role in the fast response to large earthquakes.
Ma et al. (1999) developed a method of estimating focal mechanisms of moderate and small earthquakes using surface waveform 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 (2002) developed a new method of inverting non-uniform deform on the surface of fault in the time field using long-period and broad-band frequency waveforms of CDSN. He analyzed the time-spatial process of the April 26, 1990, Gonghe, Qinghai Province seismic rupture. Based on first motion solution, Du (2001) considered Gonghe earthquake as mainly reverse thrust and strike-slip.
Wang et al.(2001) introduced the destructive degree and source parameters of the Nov. 14, 2001 strong earthquake in the boundary between Xinjiang and Qinghai. 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 Rongmao et al.(1999) inverted the Dongfang (Hainan) earthquake swarm occurring from June to August 1992. The results indicate that these earthquakes occurred within the same ambient stress field. Zhou et al.(1999) also inverted the Ms6.0 and Ms6.1 earthquakes of Beibuwan occurring in Dec. 31, 1994 and Jan. 10, 1995.
Xu et al.（2001）precisely determined source parameters of the July 20, 1995 ML4.1 earthquake sequence. These earthquakes are recorded by Sino-European Cooperative Huailai Digital Seismograph Network. The hypocenters were precisely located. Using 8 aftershocks as empirical Green's function (EGF), Xu et al. calculated the source time function (STF) of the ML4.1 event respectively, and by superposing these results, he obtained the average time function which has better ratio of signal to noise.
Rong et al. (2000a,b) inverted the Tianzhu-Gulang earthquake (M5.4, June 1, 1996) using the 3 components digital record of single station and compared difference between 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. Finally he gave a calculating result of an instance.
Based on the waveform data recorded by 3 DR-200 digital seismometers during the 1989 Batang earthquake swarm (form1 6 April to 4 May), Li and Chen (1999) employed the obtained mechanisms of 5 earthquakes of Ms4.0-4.3. It shows that there is the complexity of fault activities associated with the swarm.
Liu et al. (1999) studied three destructive mining shocks occurring in a shaft of the Fangshan (Beijing) coal mine in 4 minutes on May 15, 1993. The largest shock is of ML2.3. Three mining source mechanism was analyzed using synthetic seismograph. The near-field records of the main shock and a series of events prior to it are abnormal, it had not only high-frequency vibration, but also low-frequency vibration. By using nucleation theory of seismic fracture, they indicated that the low-frequency vibration may be as the result of sub-critical extension of fault.
2. Focal Mechanism Solutions
Using precise location and focal-mechanism solution of seismic source to analyze seismic 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 depth and shallow crust and seismology and geology model caused 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 Ms7.1 Loma Prieta earthquake to determine its source mechanism by using the Green's function. The results indicated that the slipping of the fault plane is very heterogeneous.
Shan et al. (2002) studied the source mechanism of the Jiashi Xinjiang strong earthquake swarm in 1997. The results show that the directions of principal stresses are different from directions of the regional modern pressure stress field.
Zheng et al. (1999) presented the inversion results of three strong-moderate earthquakes which occurred at the Himalayan convergence zone according to the general ray theory.
The consistency of focal stress field of earthquake sequence is paid close attention. Wang et al. (2002) analyzed Yao'an, Yunnan earthquake sequence. Wang et al. (2001) analyzed time-space distribution of source mechanism solution of moderate-strong earthquakes 5 years before 8 earthquakes of M7 in Sichuan-Yunnan areas since 1970s. Gao et al. (2001) using the synthetic focal mechanism solutions of small earthquakes in Xinjiang 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 northern China before strong earthquakes and found some anomaly changes. Zhang et al. (2001a,b) studied the Xiuyan-Haicheng Ms5.4 earthquake on Nov. 29, 1999. The results show that focal mechanisms of aftershocks are mostly similar to that of the mainshock.
Du and Liu (2000, 2001) provided focal mechanisms of 3 earthquakes (M>5.0) occurring in Qinghai Province in 1999 and the 2000, Zaduo, Qinghai earthquake M5.3.
Zhao et al. (1999) studied Reservoir induced Seismicity Structure revealed by the earthquake sequences at Sichuan Ertan Reservoir area before filling. They solved the focal mechanism of ML3.0 in Tianwan sequence. Zhao et al.（2000）studied the connection between seismicity features of the 1998 Luding, Sichuan earthquake squence of ML3.4 and fault zone structure and regional stress field of Longmenshan. Zhao et al. (2001a, b) studied the Nov. 30, 1999 Mianzhu, Sichuan earthquake Sequence of Ms5.0 and the Sep. 14, 1999 Qingping, Mianzhu, Sichuan earthquake sequence of Ms5.0. The results show that the direction of earthquake generating fault is not consistent with that of Longmenshan Fault. Wei et al.（2001）studied the Feb., 2001 Kanding and Yajiang, Sichuan Ms5.0 and Ms 6.0 earthquakes.
Han and Miao et al. (2000) studied the surface rupture zone of Izmit, Turkey earthquake in 1999.
Jiashi strong earthquake swarm occurred in 1997 in Xinjiang. As compared to the location results of MS>3.0 earthquake of Jiashi swarm obtained by using the main event location method, it shows more reasonable and more consistent with that from the focal mechanism solutions. Wang et al.(2001) analyzed the Jiashi strong earthquake swarm.
Gao and Nie (2000) studied the space-time distribution patterns of seismic activity and some focal mechanism data since 1990 in Tulufan area, as well as the Toksun earthquake (M5.6) on Jan. 30, 1999.
Many isolated focal mechanisms have been studied. Xie et al. (2001) studied the ML4.7 Neixiang-Zhenping earthquake and characteristic of seismicity before earthquake. Wu et al.(2000) studied some characteristics of the Jingyang MS4.8 earthquake in 1998. Wang and Wang (1999) studied the characteristic of space-time distribution of seismic wave velocity ratio before the Jingyang earthquake with MS4.8. Wang et al.（2001）improved the selection of locating model, locating method and distribution of seismic network and obtained precise locating of Changshu earthquake of M5.1 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 M7.3 Taiwan Strait earthquake of Sep. 16, 1994.
Yang et al.(1999,2002) used a relative relocation technique to relocate the hypocenters of the main shock and the aftershocks with ML>3.0. Shi et al.(1999) studied the aftershock of Zhangbei Ms6.2 earthquake on Jan. 10, 1998. Liu and Diao (2000) investigated for the macroscopic intensity of Zhangbei M5.6 aftershock occurred on March 11, 1999.
Hu et al.(2002) inferred the focal fault of famous Hongdong M8.0 earthquake (1303) and Linfen M7 earthquake (1695) using recent small earthquakes. They relocated and analyzed 1670 small earthquakes from 1987 to 1999 recorded by Linfen telemeter seismic network. The results show that the earthquake-generating faults of the two strong earthquakes are consistent.
Wang et al.（2002）studied the MS>5.0 earthquakes occurred in Datong in 1989, 1991and 1999. Zhu et al. (1999) analyzed three-dimensional characteristic of focus faults of the two earthquakes at Datong-Yanggao in 1989 and 1991. Ma et al.(2001) studied earthquake focus mechanism in Tianjin and nearby during 1999 to 2000.
Li et al. (2001) analyzed the relation between the horizontal tide force which the sun and moon imposes on the seismogenic zone and the focal mechanism. The azimuth of the horizontal tide force is compared respectively with those of the axes Pand Tand two groups of fault trends, and it can be shown that they are almost parallel or perpendicular one another and the parallel or perpendicular accordance is very well.
Cheng (2000) developed the technique 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 (wave-number-frequency integration techniques) including near-field P-SV, SH term and far-field P-SV terms. He 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 occurring in Tengchong volcano area using F-K method. Ye et al. (2000) analyzed the spectrum of 24 microseisms occurring in Tengchong volcano areas using Brune model to calculate the source parameters.
Chuo et al.(2000) studied the initial motion characteristics of the earthquake sequence with ML>3.0 of 11times of Shanxi since 1980s.
Mozaffari et al.(1999) studied high frequency fall-off of source spectra using Q-free spectra estimation. Using this algorithm, the near-source broadband seismographs of five aftershocks of the 1988 Langcang-Gengma, Yunnan, earthquake were processed.
Rong et al. (2000c) studied the variation characteristics of the coda Qc values of micro- earthquakes in the source areas before and after Su'nan earthquake(M5.7, Nov. 22, 1988) and Tianzhu-Gulang earthquake (M5.4, June 1, 1996). The relation between the coda Qc values near the source area of Tianzhu-Gulang earthquake and the frequencies have been studied.
Jin et al.(2000) inverted the source mechanisms of the seismic sequence of Sunan earthquake (Nov. 22, 1988, M5.7) by using the method of the ratio of P wave and S wave amplitudes.
Continuously monitoring the composite focal mechanism, coda Q and seismic wave velocity ratio, Zhang et al.(2000) have found that the inconsistency ratio of composite focal mechanism in 1997 reached its lowest value over the years and its direction similar to the Zhangbei earthquake.
Diao et al.(1999a) inverted the focal mechanism and the focal locations of Shacheng earthquake swarm in July 1995, Hebei.
Diao et al.(1999b) studied the space orientation and activity characteristics of focal fault of 1995 Wuding Yunnan earthquake of ML6.5. Diao et al.(1999c) studied the data of 8 earthquake sequences in Central Asia during 1970-1984.
Zhang et al. (2001) inverted the shear velocity structure under 3 stations around the old earthquake region of Shacheng. Combined with the velocity structures in Xingtai and Tangshan regions, they pointed out that the occurrence of strong earthquake is closely related to the low velocity layer, whose shear wave is sensitive to its structure.
II. EXPERIMENTAL STUDIES ON PHYSICS OF SEISMIC SOURCE
Many studies have been done in the electromagnetic emission during the evolution of cracks in rock. Guo Ziqiang 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 Ziqi et al.(2001) explored whether the acoustic wave generated by micro-cracks before earthquake are able to change water content of surface soil. The results shows that when the acoustic wave enters into the surface soil the water content here increases on the background of decreasing because of natural evaporation. In the meantime, temperature here decreases.
A lot of studies have been done to find the relationships between the micro-crack nucleation and its surrounding stress field. Using the fracture process of transparent samples Xiong et al. (2000, 2002) simulate a homogeneous rock mass. 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. The method provided a new approach to make a seismic experimental research.
Jiang et al. (2000) show that granite rock strength increases with the depth until 30 km. The characteristics of failure is different from the depth, as well as that of AE sequence.
Sang et al. (2001) performed an experimental study of brittle-ductile transition in Panzhihua fine-grained gabbro. Experiments were conducted in 450-800MPa confining pressure. The results show that the brittle-plastic transition is at 700-900℃. The major factors that influence the brittle-plastic transition in the gabbro are temperature，strain rate, as well as confining pressure.
Liu and Shimada (2000) did systematic micro-structural analyses with a series of granite samples deformed under dry conditions up to 300℃ and 3GPa. 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-P and low-P 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.(2001) Studied abnormal variation of wave velocity before rock rupture. The results of the experiment show that the P wave velocity increases to the value more than times of previous maximum just before the main-rupture. The velocity ratio (VP/VS) also increases to 2.5-3.0. They discussed the reason of these phenomena.
Using a new set of AE device, Liu et al. (1999) studied the time-space distribution of micro-cracks in two types of granite under tri-axial compress. They found that in India granite, the AE crowd together in time-space is not obvious, 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.
Li (2000) 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 damage theory. Li Shiyu et al. (2001b) analyzed the locating results of AE of large sample rock rupture. They found the character image of general or direct fore-shocks before failure of rock and after-shocks after local rupture. They pointed out that the denseness center of character magnitude of AE in nearer to the main rupture than that of the other magnitude.
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 Yuan et al.(2000) measure 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.
In 1999, Li et al.(2000) and Kuksenko et al. carried out the experimental investigation of rock rupture with scale of decimeters in the laboratories of earthquake source physics in Beijing. During the experiment, multiple types of tectonics and load-unload were simulated.
Wang and Lu (2002) discovered tile disturbance before earthquakes in initial recording diagrams of horizontal of tile-meters from Lijiang stations etc. According to the focal mechanism and rock mechanics, it is considered that tile disturbance is intrinsically micro-rupture and stick–slip phenomena and effective precursors.
III. THEORETICAL STUDIES AND NUMERICAL SIMULATION
Li and Chen (1999) studied transient S-wave velocity rupture (TSVR), which means the velocity of fault rupture propagation is between S-wave velocityαand P-wave velocityβ. 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 and tunnel effect of fault, they gave the proof theoretically and overcome two difficulties successfully.
Li et al.(2000) discussed nucleation of earthquakes and its implication to precursors. They point 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.(2001) reviewed the physical measure of earthquake preparation state in the ground, and discussed the role of experiment in research of seismic source physics. Tang et al.(2002) reviewed the records of low or extra low-frequency events before earthquake and current theoretical research.
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 the deformation and failure behavior of a double rock sample system numerically. 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 by 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 gene and total stress intensity gene 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 analyze the law of reservoir-induced earthquake from breaking foundation of compounding stress intensity factor.
Wang and Sun (1999) theoretically and experimentally analyzed the different crack propagation behaviors for open or closed model Ⅱ crack. Meanwhile, the problems of whether crack propagation will cause earthquake and how tectonic distribution influences earthquake sequence 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 earthquake using the transfer function method and response ratio method of load and unload of vertical component of geomagnetic field and the analysis for the electromagnetic radiation phenomenon.
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 calculate 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 find that τ0 values of nuclear explosions have about 20 MPa, which 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 portioning model in the middle-upper crust and its application to seismic research. Strain partitioning is the 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 Zengjian 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.
Based on the up-to-date studies on relationship between crust fluids and strong earthquake activities at home and abroad, Che et al.(2000) conducted several aspects of search and illustrations and presented a hypothesis on seismogenic process in a hard intercalary strata in the crust and promoting earthquakes by a fluids (SPH－PEF). The points of the hypothesis are that there are two (upper and lower) fluid activity systems and a hard intercalary strata between them in the crust. When the stress is cumulated to the yield strength at some places, the micro-crack (dilatancy) stages will begin and earthquake source entity will be formed.
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