2018年9月4日新疆伽师5.5级地震序列震源机制特征分析

Analysis of the Focal Mechanism of Jiashi,Xinjiang,M_(s)5.5 Earthquake Sequences on September 4,2018

利用新疆区域数字地震台网的波形资料,采用CAP方法和P波初动方法计算了2018年9月4日新疆伽师5.5级地震的前震,主震及15次M_(s)≥3.0余震序列的震源机制解.整理了各研究机构给出的伽师5.5级主震的震源机制解,求出了与所有测定的震源机制的标准差最小的解作为中心震源机制解.伽师5.5级主震中心解为节面Ⅰ:走向144.22°/倾角87.13°/滑动角-175.78°;节面Ⅱ:走向54.01°/倾角85.79°/滑动角-2.88°,最佳矩心深度18 km,表明该地震是一次走滑型事件.统计所有计算得到的震源机制解结果,走滑型地震震源机制占比例最大,同时还存在其他震源机制类型的地震,表明该地区有复杂的应力场环境.结合震源区的地质构造和震源机制解类型,推测伽师5.5级为下苏洪隐伏断层一次新的破裂事件,判定节面Ⅰ为伽师5.5级主震的发震断层面.初步分析了此次伽师5.5级地震序列与1997-1998年和2003年伽师强震群的异同.震源区应力场呈走滑应力体系,该区域最大主应力σ_(1)方位为NEE—SWW向,得到应力型因子R为0.1,说明张应力轴确定,震源区主要受EW向拉张为主的张应力主导作用.结合伽师地区1997-2003年的几次强震群反演的应力场结果,本次地震造成最大主应力方位发生顺时针旋转,对后续强震有一定的指示意义.

Based on the broadband waveform recorded by Xinjiang Digital Seismic Network,we inversed the focal mechanism solutions of foreshock,mainshock,and 15 times M_(s)≥3.0 aftershock sequences of the Jiashi M_(s)5.5 on the September 4,2018 by using CAP mothod and P-wave intial motion method.With the results collected from different research institutions,determination of central of focal mechanism solution is determined with the smallest square difference from several focal mechanisms of the same earthquake.The central focal mechanism of Jiashi M_(s)5.5 mainshock had the Nodal planeⅠwith strike 144.22°/dip 87.13°/rake-175.78°;Nodal planeⅡwith strike 54.01°/dip 85.79°/rake-2.88°,the best centroid depth of 18 km.Those indicate that the earthquake is a strike-slip event.Calculating the results of all focal mechanism solutions,strike-slip earthquakes account for the largest proportion of all earthquakes.At the same time,other focal mechanism types of earthquakes also exist,indicating that the area has a complex stress field environment.Combined with the geological structure and focal mechanism solution type of the focal area,it is inferred that Jiashi M_(s)5.5 is a new rupture event of the Xia Suhong blinded fault,and the node plane I is determined to be its seismogenic fault plane.A preliminary analysis is made on the similarities and differences between the sequence of this Jiashi earthquake with other Jiashi strong seismic groups in 1997—1998 and 2003.Inversion of the stress field in the focal area shows that the stress field is a strike-slip stress system.The azimuth of the maximum principal stressσ_(1)in the region is NEE-SWW,and the stress shape ratio(R)is 0.1,indicating that the tensile stress axis is determined,and the focal area is mainly affected by the tension in the EW direction.Combined with the stress field inversion results of several strong earthquakes from 1997 to 2003,the direction of maximum principal stress caused by this earthquake rotates clockwise,which has certain indication significance for the subsequent strong earthquakes.