2023年12月18日甘肃积石山6.2级地震序列震源机制解与发震构造分析

Focal mechanism solutions and seismogenic structure of the M_(S)6.2 earthquake sequence in Jishishan,Gansu,on December 18,2023

2023年12月18日积石山发生了6.2级地震,地震序列丰富且跟踪结果显示为主-余震型。基于甘肃区域数字地震台网提供的震相报告,采用双差定位方法对积石山地震序列重新定位,再通过CAP方法反演主震和16次MS≥3.0余震震源机制,初步分析积石山6.2级地震序列的发震构造。结果显示:(1)重定位后获得的832次地震事件总体呈NNW—SSE向展布,长轴约15 km,宽2~6 km;西北端分布较窄,东南端较宽;震源深度集中在5~13 km,主震震源深度为13.95 km。(2)采用CAP方法得到积石山6.2级地震的震源机制解,节面Ⅰ:走向161°/倾角40°/滑动角120°;节面Ⅱ:走向304°/倾角56°/滑动角67°;矩心深度为9 km,矩震级MW为5.94;节面Ⅱ走向与震源区附近余震展布方向一致,可判定为发震断层面。16次MS≥3.0余震活动中12次为逆冲型地震,4次为逆走滑特征,矩心深度主要集中在6~11 km范围,与重定位结果接近。(3)应力张量反演结果显示最大主应力σ1方位角10.8°、俯角0.2°,与其震源区所处区域构造应力场基本一致,显示出震源区受NNE向近水平力的作用,反映区域内构造活动主要受区域构造应力场控制。根据地震序列重定位、震源机制以及震源区应力结果,结合区域构造活动和应力场特征,初步分析认为,积石山6.2级地震的发震构造可能是拉脊山断裂带中一条NE倾向的次级逆冲活动断裂。

An M_(S)6.2 earthquake occurred in Jishishan County,Gansu Province,on December 18,2023.The mainshock-aftershock type is the earthquake sequence.Based on the seismic phase report provided by the Gansu Digital Seismic Network,the double-difference positioning method was employed to relocate the Jishishan earthquake sequence,and the focal mechanisms of the mainshock and 16 aftershocks with M S≥3.0 were then inverted using the CAP method.A preliminary analysis of the seismogenic structure of the Jishishan earthquake sequence was performed.Results indicate the following:(1)The relocated 832 events are distributed in the NNE-SSE trending,with a long axis of approximately 15 km and a width of 2-6 km.The focal depths of the sequence are concentrated at 5-13 km,and the focal depth of the mainshock is 13.95 km.(2)Based on the focal mechanism solution of the M_(S)6.2 mainshock obtained by the CAP method,the strike,dip,and rake angles of the nodal planesⅠandⅡare 161°/40°/120°and 304°/56°/67°,respectively.The centroid depth is 9 km,and the moment magnitude is 5.94.The seismogenic fault plane is identified as planeⅡbecause its strike is consistent with the distribution direction of aftershocks.Among 16 M S≥3.0 aftershocks,12 are thrust earthquakes,and 4 are reverse strike-slip earthquakes.The centroid depths of aftershocks are mainly concentrated in the range of 6-11 km,which is close to the relocation results.(3)The inversion result of the stress tensor shows that the azimuth angle of maximum principal stress is 10.8°and the depression angle is 0.2°,which is consistent with the tectonic stress field in the source area.This finding indicates that the NNE-trending near-horizontal force affects the source area,and the regional tectonic stress field mainly controls the tectonic activities in the region.Based on the relocation results,focal mechanism solutions,and stress in the source area,combined with the characteristics of tectonic activity and stress field,the seismogenic structure of the Jishishan M_(S)6.2 earthquake may be a secondary thrust active fault with an NE trend in the Lajishan fault zone.