2008年8月30日攀枝花-会理6．1级地震序列ML≥4．0事件的震源机制解

Focal mechanism solutions of M_L≥4.0 events in the M_S6.1 Panzhihua-Huili earthquake sequence of Aug 30, 2008

针对2008年8月30日在四川攀枝花—会理发生的M_s6.1地震序列,本研究基于四川和云南两省数字地震台网的宽频带波形记录,采用CAP方法反演了该序列主震及M_L≥4.0余震的震源机制解.结果显示:主震震源机制解的两个主应力轴仰角小于10°,其中,主压力轴方位为140°;节面之一走向185°、西倾83°、滑动角5°,显示左旋走滑略兼逆冲分量的断层作用性质.结合余震、烈度分布以及震区的活动构造,判定该节面代表了主震的发震断层面,相应的发震断层应是穿越震区的近南北向红格断裂(南段).本研究还获得主震震源机制解的最佳拟合误差深度为10km,与该事件的定位结果相一致.该序列中6次M_L≥4.0余震也具有与主震类似的震源机制解.分析初步表明:空间上,2008年攀枝花—会理M_s6.1地震序列的震源机制解与研究区内更早地震的震源机制解具有良好的协调性,反映了该序列是在川滇地块SE-SSE向水平运动的背景下、沿近S-N向红格断裂发生左旋走滑略兼逆冲运动的结果.

For the Ms6.1 Panzhihua-Huili, Sichuan, earthquake sequence of Aug 30, 2008, this study determines the focal mechanism solutions of the mainshock event and the ML≥4.0 aftershock events in the sequence using the CAP method and the broadband waveform records from the digital seismic networks of Sichuan and Yunnan. The result shows that the focal mechanism solution of the mainshock has two principal stress axes with low dip angles of 〈10°, and the principal compressive stress axis trends 140°. One of the two nodal planes of the focal mechanism solution of the mainshock strikes in 185°, dips westward at 83°, has a rake of 5°, and shows left-lateral strike-slip faulting with a small thrust component. This nodal plane has been identified to be representative of the seismogenic fault plane of the mainshock by considering the distributions of the aftershocks and the seismic intensities, as well as the local active geologic structure. The corresponding seismogenic fault has been determined to be the nearly N-S- trending Hongge fault （its southern segment） running through the epicentral area of the sequence. The best misfit depth of the mainshock＇s focal mechanism solution is estimated to be 10 km, being consistent with the located depth of this event. It is also found that the other 6 ML≥4.0 aftershocks have similar focal mechanism solutions with that of the mainshock. A preliminary analysis suggests that spatially the focal mechanism solutions of the Ms6.1 Panzhihua-Huili earthquake sequence of 2008 are coordinated with those of the previous events in the study region. This reflects that the 2008 sequence has been resulted from the left-lateral strike-slip faulting with a thrusting component along the nearly N-S-trending Hongge fault, under the SE- to SSE-ward horizontal motion of the Sichuan-Yunnan massif.