Passive Seismic Experiment in the Xingjiang‐Jiashi Strong Earthquake Region and Discussions on its Seismic Genesis

AbstractFrom January 21 to April 16 of 1997, seven strong earthquakes took place in the Jiashi region of Xinjiang, China. This phenomenon had never happened in the Chinese continent, since earthquakes were recorded by instruments. To investigate the deep structural conditions of the Jiashi strong earthquake sequence, a portable broadband seismic array consisted of 30 stations was deployed in the Jiashi region of about 11,000 km2. This portable array crosses through the Tarim basin and the Tian Shan fold belt. The distance between stations is about 5–10 km. The frequency bandwidth of the observation system is 0.05–20 Hz.The 3‐D S‐wave velocity structure down to 100 km beneath the portable array was obtained by using the teleseismic receiver function migration and its nonlinear inversion techniques (Liu et al. 1996). From our observations the following conclusions are drawn: (1) The Moho depth on the side of the Tarim basin is about 40–52 km and it becomes 60–76 km on the side close to the Tian Shan. The crustal thickness beneath the fold belt nearby the south Tian Shan is much thicker than that of the Tarim basin and obvious structural deformation exists in the crust. (2) The north border of the Tarim basin is clearly laterally inhomogeneous. The contact deformation between Tarim and Tian Shan reveals the compressional movement in NNW direction. The topography of the Moho discontinuity beneath the array has an 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 above the gradient zone of the crust‐mantle boundary. The genesis of the Jiashi earthquake sequence is probably related to the activity of the conceal faults nearby the sources. This may be a result of the inhomogeneous deformation at the north border and the current tectonic movement of the Tarim basin. (4) The Jiashi strong earthquake sequence occurred mainly within the gradient zone from sediments to the crystalline 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 that of general cases. 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 feature of the deep structures. This should be studied further by numerical stress modeling.