关键词: 伽师强震群, 震源机制, 破裂过程

Abstract: A series of strong earthquake swarm including 7 earthquakes with M_S≥6.0 occurred in Jiashi, Xinjiang during January-November 1997-an exceptional case for the intraplate seismicity. Using GDSN broadband P wave records and first motion direction of primary wave in short period records from regional seismic network of China, we have investigated the focal mechanism solutions of earthquakes occurred during January 23 to November 4, 1997 in Jiashi, as well as the rupture process of 5 strong earthquakes of Jiashi swarm, including subsequent and adjacent earthquakes. The focal mechanism solutions of the Jiashi strong earthquake swarm indicate that the earthquakes are characterized mainly by strike-slip and normal dip-slip rupturing. The axis of principal compressive stress is oriented mainly to NNE, and is nearly vertical. The axis of principal extensional stress is oriented to NW, and is nearly horizontal. The directions of principal stresses are different from those of the regional modern stress field, which is dominated by a NW-oriented compressive stress. It is clear that the Jiashi seismogenic zone has a distinct localized character. The processes of focal rupturing of these strong earthquakes are relatively simple, and characterized by relatively small area of rupture and shorter rise-time. They are a brittle rupturing that is generated from one point and propagates rapidly to all directions without a specific extensional direction, and differ significantly from the rupturing process of the Artux earthquake. The results of this study show that the Jiashi strong earthquake swarm bears a close relation with the vertical and lateral inhomogeneity of the crustal structure near the source region. In contrast, the Artux earthquake bears a close relation with modern regional tectonic movement. It is concluded, therefore, that the Jiashi strong earthquake swarm is the products of multiple rapid brittle rupturing along NNE direction in the source area due to the action of both vertical and horizontal stresses, and hence the focal mechanism of these earthquakes is dominated mainly by left-lateral strike-slip and normal dip-slip faulting.

Key words: Jiashi strong earthquake swarm, Focal mechanism, Focal rupture