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

摘要： 利用国内外短周期P波初动符号及全球数字地震台网(GDSN)的宽频带数字记录资料,研究了1997年1月23日至1997年11月4日伽师强震群的震源机制解和伽师强震群中的5个强震、后续地震及其周边地震的震源破裂过程.从震源机制解来看,伽师强震群主要有走滑和正倾两种破裂类型,主压应力轴方向主要为NNE向和近垂直,而主张应力轴为NW向并近水平,与区域构造应力场方向存在差异,具有明显的局部特征.从破裂过程看,伽师强震群的破裂过程相对简单,破裂面积不大,上升时间较短,是由一点向四周快速扩散的脆性破裂,无明显的伸展方向,与阿图什地震完全不同.研究结果表明,伽师强震群与震源附近的地壳结构在垂向和横向上的非均匀变化有着密切关系,而阿图什地震与塔里木盆地的现今构造运动关系密切.伽师强震群是在震源区附近地壳上部垂向和水平向力共同作用下发生的多次沿NNE向的快速脆性破裂,从而形成了以张性破裂和左旋走滑为主的震源特征.

关键词: 伽师强震群, 震源机制, 破裂过程