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PASSIVE SEISMIC EXPERIMENT IN XINGJIANGJIASHI STRONG EARTHQUAKE REGION AND DISCUSSION ON ITS SEISMIC GENESIS

LIU QI-Yuan;CHEN JIUHUI;LI SHUN-CHENG;GUO BIAO (Institute of Geology, China Seismological Bureau, Beijing 100029, China)  
From January 21 to April 16 of 1997, seven strong earthquakes took place in Jiashi region of Xinjiang, China. This Phenomenon was never happened in Chinese continent since earthquakes were able to be recorded by instruments. To investigate the deep structure environment of the Jiashi strong earthquake sequence, a portable broadband seismic array of 30 stations was set up in Jiashi region over about 11,000 km~2. The portable array crossed the Tarim basin and the Tian Shan fold belt. The distance between stations is about 5-10 km. The frequency bandwidth of our observation system is 0.05--20 Hz. The 3--D S-wave velocity structure down to 100 km beneath the army was obtained by using the teleseismic receiver function migration and its nonlinear inversion techniques (Liu etc. 1996). The following conclusions can be obtained from our observations: l) The Moho depth is about 4052 kin on the side of the Tarim basin and becomes 60-76 km deep on the side close to the Tien Shan. The crustal thickness beneath the fold belt nearby the south Tien Shan is much thicker than that of the Tarim basin and the obvious tectonic deformation exists in the crust. 2) The north boundary of the Tarim basin is clearly laterally inhomogeneous. The relationship of the contact deformation between Tarim and Tien Shan reveals the compressional movement to the NNW direction. The topography of the Moho discontinuity beneath our array has obvious correlation with the arctuate tectonics on the surface. The arctuate fold formation is closely related with the deep structure and the compression by the Tarim basin. 3) The Jiashi strong earthquake sequence mainly occurred over the gradient zone of the crust--mantle boundary. The genesis of the Jiashi earthquake sequence probably related to the activity of the conceal faults nearby the sources. This may result from the inhomogeneous deformation at the north boundary and the current tectonic movement of the Tarim basin. 4) The Jiashi strong earthquake sequence mainly occurred in the upper crust beneath the fold belt with a lower shear module. This could be a reason why the stress drop of these events is much smaller than common. All of these demonstrate that the Jiashi earthquake sequence genesis is related not only to the current tectonic movement of the Tarim basin, but also to the deep structure. This will need to be investigated further by the numerical stress modeling.
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