2021年西藏比如MS6.1地震序列发震构造分析

利用双差定位方法对西藏比如M_S6.1地震序列141次M_L≥2.0地震进行重新定位,采用CAP波形反演方法获得主震的震源机制解,并运用最小空间旋转角方法比较不同机构发布的震源机制解的差异。重新定位后主震震中位置为(31.924°N,92.824°E),靠近余震区中心,震源深度为12.8 km;余震分布沿NE向展布,长约18 km。沿NE向深度剖面结果显示,在主震右上方存在5 km×10 km的近椭圆形地震破裂空区。主震的震源机制解为正断兼走滑型,最佳矩心深度为9.3 km,矩震级为5.98。结合重新定位后余震分布、主震与历史地震震源机制解及地质构造背景等分析,认为具有左旋运动性质的安多南缘断裂可能是该次地震序列的主要发震构造。

Preliminary Study of the Seismogenic Structure of the Biru,Tibet MS6.1 Earthquake

The Double-difference location method is used to relocate the Biru,Tibet M_S6.1 earthquake sequence,and the optimized focal parameters of the mainshock and the distribution characteristics of the aftershocks are obtained;the epicenter of the mainshock is located at 31.924°N,92.824°E,and the focal depth is 12.8 km.The aftershocks are mainly distributed along the NE direction,with a length of 18 km or so.The depth profile in NE direction shows that there is an obvious seismic gap on the upper right part of the mainshock's sourece,and the earthquakes outside the gap evenly distribute along the depth profile.This may be due to the obstacles in the seismic gap,which block the propagation of seismic ruptures in this direction.By using the CAP(Cut and Paste)waveform inversion method,the following parameters of the Biru eartqhuake are got:the depth of the optimal moment center of the main shock is 9.3 km.The moment magnitude M_W=5.98,and the focal mechanism solutions of the main shock—Nodal plane Ⅰ has a strike of 347°,an inclination of 60°,a sliding angle of-135°.Nodal plane Ⅱ has a strike of 230°,a dip of 52°,a sliding angle of -39°.P-axis has a strike of 203°,a dip of 52°.T-axis:strike=107°,inclination=5°.The difference between the focal depths obtained by the relocation method and the CAP waveform inversion method may reflect that the rupture of the mainshock starts in the deep and then develops to the shallow.The relocated aftershocks'distribution,the focal mechanism solutions of mainshock,the historical earthquakes and geotectonic settings reveal that the normal,left-lateral,and strike-slip fault along the southern margin of the Anduo Basin may be the main seismogenic structure of the Biru M_S6.1 earthquake,and the seismogenic fault plane is nodal plane Ⅱ.