A preliminary study on fine structures of Jiashi earthquake region and earthquake generating fault

It is very unusual that nine large earthquakes of similar magnitudes (M = 6.1-6.8) occured within a very small area and a very short period of time (1997-1998) in Jiashi of Xinjiang Province, Northwest China. This paper analyzes the observed data of the aftershocks in the Jiashi earthquake region for studying the generating mechanism and deep structural background of the Jiashi strong earthquake swarm. The simultaneous inversion method was used to calculate the hypocenter positions and 3-D velocity structure of this region. The results show that the aftershocks concentrated in a NNE-trending belt, which roughly coincides with the NE-trending belt of the two strong earthquake belts. Velocity structure obtained from 3-D inversion suggests that there exist two low velocity zones, one trending NNE and the other NNW, below the depth of 12 km. These low velocity zones are almost consistent with the two strong earthquake belts. The preliminary results lead to the conclusion that the low velocity zones correspond to two deep crustal faults. At the time of our observation, microearthquakes took place on the NNE-trending fault, while the NNW-trending fault was relatively quiescent.