伽师强震群震源破裂特征的初步分析

为深入研究1997年新疆伽师地区连续发生的强震群的震源破裂特征,利用全球数字地震台网(GDSN)宽频带数字资料及区域台网资料,较详细地研究了伽师强震群的震源机制及震源破裂特征.结果表明:伽师强震群的震源机制解主要有走滑和正倾两种破裂类型,其共同特点是主压应力轴方向沿北北东向,主张应力轴沿北西向,与区域构造应力场方向存在差异,具有明显局部特征;从震源破裂特征来看,伽师强震的滑动尺度、上升时间和持续时间均较小,震源破裂面积不大,是由一点向四周快速扩散的脆性破裂,无明显伸展方向;伽师强震群的破裂断层面为北东向,与震源深度梯度变化带、地壳接触变形梯度变化带、等烈度线以及地震扩展方向吻合;伽师强震群是在近南北向挤压环境下,在震源区附近剪切和张扭应力作用下发生的多次沿北东向的脆性快速破裂,从而造成了伽师强震群以张性破裂和左旋走滑为主的震源特征.

A PRELIMINARY ANALYSIS ON RUPTURE CHARACTERISTICS OF JIASHI STRONG EARTHQUAKE SWARM

A strong earthquake swarm, including 7 events with M s≥6.0, occurred in Jiashi, Xinjiang in 1997. It is exceptional for the intraplate seismicity. Using GDSN broadband P wave records and the adaptive hybrid global search algorithm, we have investigated the rupture process of the strong swarm. The results show that Jiashi strong swarm shows mainly strike slip and normal focal mechanisms. The common features of the earthquakes in the swarm are that the source stress field is characterized by a pressure stress in NNE, and a tension stress in NW. The directions of principal stresses are different from directions of the regional modern pressure stress field, showing out the local characteristics. The focus repture characteristics show that slip scales, risetimes and the rupture times of finite fault are smaller, similar with focal rupture area. The rupture growth trajectories are from one point to all sides without obvious extended direction. The rupture surface is to NNE, similar to gradient belt of focal depth, gradient belt of crustal deformation, isoseisms and extended direction of earthquakes. Jiashi strong earthquake swarm occurred several times with brittle failure to NNE under tension shear stress which is formed under nonhomogeneous deformation in the upper crust. The characteristics of focus mechanisms are formed with sinistral strike slip and normal rupture.