REVIEW ON STUDY OF SEISMOTECTONICS
WANG Chunyong and LIU Qionglin
Institute of Geophysics, China Seismological Bureau, Beijing 100081, China
The study on seismotectonics is concerned with earthquake data analysis, which relates to active faults, crustal structures, tectonic deformation, and to physical processes operative within the earth. The structural and tectonic setting conditions of earthquake occurrence are mainly obtained in the comprehensive analysis of the data from geology, geophysics and seismological phenomena. The study on the seismotectonics in crustal structure, active faults, crustal deformation and numerical simulation in China during the last four years is briefly reviewed in this paper.
I. CRUSTAL STRUCTURES
Based on the crustal structures of deep seismic sounding profiles conducted in last 30 years in China and its surrounding region, Teng, Zeng and Yan et al. (2002) presented the topography of Moho depth beneath the Eastern Asia Continent. Wang, Hearn and Xu et al. (2001) inverted the Pn velocity structure in the uppermost mantle beneath Chinese mainland by using seismic tomography on the basis of the Pn ray travel times recorded in the China seismic network.
Wang, Lou and Wei et al. (2001) and Wang, Zou and Shi et al. (2001) presented a 2-D fine crustal structure along the deep seismic reflection profile in the north margin of Tianshan Mountains, which passes through the epicentral area of the Manas M7.7 earthquake. Based on the crustal structure, they analyzed its relationship with the reverse fault-fold zones on the ground surface, and discussed the crustal tectonic environment of the Manas earthquake occurrence. Zhao, Sun and Gu et al. (2001) studied the shallow crust structure characteristics of Manas anticline by using high precision shallow seismic prospecting method. Xu, Liu and Liu et al. (2000) investigated the deep structure of the Tianshan earthquake belt on the basis of the seismic tomography of the northwestern China.
Liu, Chen and Li et al. (2000) studied 3-D S-wave velocity structure of the crust and upper mantle down to the depth of 100 km beneath a portable broadband seismic array in Jiashi region, Xinjiang by using the teleseismic receiver function migration, and discussed the tectonic causing of Jiashi strong earthquake sequence. Li, Zhang and Mooney et al. (2002) determined the hypocenter locations and 3-D velocity structure in Jiashi earthquake region, and discussed the generating mechanism and deep structure background of the Jiashi strong earthquake swarm. Fan, Li and Lai et al. (2001) inverted 3-D S-wave Q structure on the basis of the attenuation of seismic waves in the 1998 Jiashi earthquake region. Yang, Zhao and Zhang et al. (2002) reconstructed the image of 3-D velocity structure of upper crust beneath Jiashi strong earthquake swarm area by using the inversion method without blocks and on the basis of P and S reflections on the Moho at critical distance gathered from a 3-D temporary seismic array, and discussed the tectonic background of Jiashi strong earthquake swarm.
Ding, Di and Yuan et al. (2000) and Yuan, Ding and Di (1999) studied the 3-D crustal S wave velocity in the Weihe fault basin on the basis of travel times, and obtained the ratio Vp/Vs from P and S crustal velocity models. Jin, Yang and Zhao et al. (1999) presented a 3-D velocity structure by using the travel time data of direct P wave, reflected Pm and refracted Pn phases and discussed the deep tectonic background occurring to earthquake in Ningxia and its neighborhood.
Niu, Lu and Jiang et al. (2000) studied the crustal and upper mantle structure feature and the seismic activity of the main tectonic units in North Tanlu fault zone. On the basis of the tomographic image of Tangshan and Xingtai areas, Mei, Xue and Yin et al. (1999) respectively analyzed the relation between the characteristics of the earthquake sequence and the 3-D velocity structure, and discussed the prediction of strong earthquake sequence. Zhao, Sun and Liu et al. (1999) studied the shallow structural characteristics and the coupling relation between deep and shallow structures in Xingtai earthquake area. Zhu, Zhang and Zhang et al. (1999a) conducted a comparative interpretation with respect to the data from 5 deep seismic sounding profiles in the central and southern parts of Shanxi Province, and presented that in Linxian, Linfen and Xingtai earthquake regions, there exist anomalous crustal and upper mantle structure and crustal faults extending to the Moho, all being regarded as the deep indications for earthquake occurrence. On the basis of the seismic data obtained from wide angle reflection/refraction profile passing through Zhangjiakou area in Hebei Province, Zhu, Zhang and Zhang et al. (1999b) studied the crustal and upper mantle structure, the low velocity anomaly inside crust and the distribution of deep fault, and the relation to the activity of Zhangbei earthquakes.
Zhao, Zhang and Zhang et al. (1999) studied the deep crustal fault background in the seismic area of Linxian, Henan province, and presented crustal and upper mantle structure beneath the seismic area and its vicinity. Liu, Zhu and Fang et al. (1999) studied comprehensively the crustal structures, the anomalies of geophysical field and the seismicity in Central-South Shanxi, and proposed that there may exist a deep tectonic background for occurring moderate earthquakes in the Lingshi-Jiexiu region between Linfen and Taiyuan.
Based on the first arrival P and S data of 4625 regional earthquake recorded at 174 stations dispersed in the Yunnan and Sichuan Provinces, Wang, Mooney and Wang et al. (2002) presented a 3-D velocity crustal model in the region, and discussed the feature of crustal structure in major seismic zones and deep environment of earthquake occurrence. Su, Liu and Cai et al. (1999) discussed the deep structure background for strong earthquakes occurrence in Yunnan area on the basis of the velocity structure, electrical conductivity structure, geothermal structure in the crust and upper mantle in Yunnan area.
II. ACTIVE FAULTS AND TECTONIC SETTING OF LARGE EARTHQUAKES
The active faults are defined as those faults, which have been active since Late Quaternary and are expected to be active within a future time span. Wen and Wang (1999) outlined the progress and problems in the research on active fault seismic hazard assessment in China.
Jiang, Xiao and Xie et al. (2000) analyzed the correlation between segments of the faults according to surface ruptures in 9 historical strong earthquakes occurring in downfaulted system and active structures around the Ordos block from the results of researches of active faults in recent years. Ran, Chen and Xu et al. (2001) used deterministic, probabilistic and composite-grading methods to get the possible locations of strong earthquakes in the future in Northwest Beijing and its vicinity by ordering on the basis of the quantitative data and their accuracy about active tectonics in the research area, and discussed some questions in the results. Hao and You (2001) conducted a shallow seismic survey for detailed detection of the Tangshan active fault.
Qian, Zhou and Ma et al. (1999) discussed a newly found north-south trending active fault and ground fracture of 1933 Diexi earthquake, which might be the south extending of Minjiang fault. He, Zhang and Huang (2002) studied the 2001 Yajiang-Kangding earthquake with M6.0, and suggested the occurring of the earthquake might be related to the activity of Litang fault.
Hou, Deng and Liu (1999) discussed the seismotectonic environment of 1927 Gulang earthquake, according to the recent research on active fault, surface rupture, fault plane solution, seismic activity, as well as the deep geophysical exploration in the earthquake area. They presented that 1927 Gulang great earthquake was caused by NE-SW-trending compress and thrust, and it was a latest event occurring in the reverse fault-folding belt that developed along the intracrustal decollement. Ren, Wang and Wu et al. (1999) studied the Quaternary faulting of eastern Kunlun fault (Xidatan-Dongdatan), northern Tibetan Plateau.
Xiang, Zhang and Guo et al. (1999) presented the main indicators for identifying the new-generated seismic rupture zone (NSRZ) on the basis of the surface geological and geomorphologic survey and the analysis of spatial-temporal image of seismic activity, and discussed the recognition and type division of NSRZ.
Wen (2001) studied the earthquake behavior of variable rupture-scale on active faults in Chinese mainland ¾ on an individual fault potion earthquake's rupture-scale varied cycle to cycle, and hence earthquake's strength changed with time. He introduced and improved the cascade-rupturing model, and described the variability and complexity of rupture scale on individual fault portions. Based on the summary of basic feature of some active strike-slip faults on which cascade-rupture had occurred, he proposed a principle of cascade-rupture segmentation for this type of faults. Yi and Wen (2000) studied the earthquake recurrence behaviors on entire active fault zones and their relations to those on individual fault-segments on the basis of the earthquake data of 11 active intraplate fault zones in Chinese mainland. Wen (1999a) analyzed quantitatively earthquakes history of 19 fault segments to investigate the recurrence behaviors of segment-rupturing earthquakes on active faults in Chinese mainland on the basis of earthquake data of multi-cycle recurrences. Wen (1999b) established corresponding empirical distributions for earthquake recurrence interval for the two main recurrence behaviors of segment-rupturing earthquakes on active faults in Chinese mainland.
Huang, Zhou and He et al. (2000) studied the Holocene activity on Yunongxi fault and 1975 Liuba M6.2 earthquake in Kangding, Sichuan, and presented that the occurrence of the earthquake resulted from the newest activity of the fault. Zhang and Xie (2001) divided the seismotectonic areas of historical strong earthquakes with magnitude Ms≥7.0 along the Xianshuihe-Xiaojiang fault zone, and analyzed their individual fault pattern and tectonic geomorphology. They presented that these strong earthquake areas were located in some special parts of the fault zone, where the major branch-fault of the fault zone formed left stepping, parallel, and fork-like patterns, and in strong earthquake areas structurally complicated basins were developed. Diao, Zhang and Wang et al. (1999) studied 3-D characteristics of focal fault of 1995 Ms6.5 Wuding earthquake of Yunnan.
Wang and Li (1999) presented a method for inversing the kinematics parameters of a fault zone and then used this method to study the present kinematics characteristics of the Altun fault zone. The results show that the Altun fault zone was generally compressive in the direction of S14°E, its compressive rate was 1.13 mm/a, and the rate of its left lateral shear movement was 0.17 mm/a.
Shen, Chen and Xu et al. (2000) analyzed the characteristics and amount of displacement of Liangshan active tectonic zone in Late Cenozoic Era. Geological mapping indicates that the fault motion was dominated by left-lateral slip in Late Cenozoic Era, and the total amount of displacement was 13.5-15.5 km. In the Quaternary, the Liangshan fault zone offset a series of geological and geomorphologic bodies and the average left-lateral slip rate was about 2 mm/a.
III. CRUSTAL DEFORMATION
The intraplate deformation in Chinese mainland is strongly controlled and affected by the relative motion between the Eurasia and Indian plates. It is characterized partly by the frequently intracontinental earthquakes. Geodetic monitoring across main active faults has been carried out for the last 20 years. The recent crustal deformation is obviously controlled by the regional tectonics.
Yang, Wang and Han et al. (1999) analyzed the present state of crustal horizontal movement along Shanxi fault by use of the repeated GPS observation data (1996-1997-1998). Yang, Zhao and Han et al. (2000) reported the monitoring of the horizontal movement along the Shanxi fault zone. Jiang, Zhang and Chen et al. (2000) used the data from North China GPS monitoring network in 1992, 1995 and 1996, and obtained the distribution patterns of relative horizontal displacement field and strain field in North China area. Combining with Discontinuous Deformation Analysis (DDA) method, they simulated preliminarily the relative horizontal displacement field obtained from GPS data in 1992 to 1995, and analyzed the state of tectonic stress field in North China. Zhang, Wang and Niu (1999) analyzed the station deformation data measured from 1990 to 1998 in North China by applying the wavelet transform method. Yang, Zhao and Han et al. (2000) analyzed the current horizontal movement of the Shanxi fault zone and its relationship to the Yangyuan-Hunyuan earthquake (M5.6) occurring at the north part of the monitoring network on the basis of the repeated measurements (1996-1999) of GPS monitoring network.
Qiao, Wang and Wang et al. (2000) densified the GPS monitoring network and repeatedly observed in the Jiashi and the northeastern area to the Pamir, and acquired the observation results by use of the high-accuracy GPS data processing software. The relative accuracy of base line reaches 10-8-10-9. From these results the map of present crustal deformation rate and the time sequence of each base-line vector were obtained primarily. Zhao, Qin and Qiao et al. (2000) simulated the crustal movement and deformation in Jiashi area before and after the 1998 strong earthquake swarm by using Discontinuous Deformation Analysis method. By using GPS geodesy in 1994 and 1998, Wang, Ding and Qiao et al. (2000) presented the geodetic evidence of rapid convergence of about 19 mm/a across the western Tianshan, which was about 50% greater than the seismic moment solution (13 mm/a) by the estimation of major earthquakes in the 20th century, and discussed tectonic implication of the deformation discrepancy.
Based on the Chinese mainland GPS network (1994-1996), Fujian GPS network (1995-1997), cross fault deformation network (1982