基于区域地震波形的2017年新疆精河MS6.6地震破裂方向性及发震构造研究

2017年8月9日的新疆精河MS6.6地震是近年来天山北缘发生的最大地震,震中位于由多条逆冲断层组成的库松木契克断裂带内.由于震源较深、构造形变复杂、区域地震台站相对稀疏,仅根据震源机制解、余震分布和InSAR观测结果等难以直接判定发震构造.本文针对倾滑型地震发展了一种基于区域地震波形的破裂方向性测定方法,利用余震作为参考地震进行路径校正,根据主震和参考地震的波形时移差和Pn-Pg到时差分别确定主震在水平方向和深度方向的破裂尺度,进而推断同震破裂的延展方向和延伸尺度.本文在反演了主震的点源参数后,应用新发展的方法测定了地震的破裂方向性.点源反演结果显示,精河地震是一个发生在中地壳的高角度逆冲地震,矩震级约6.2,质心深度21km,震源持续时间5.5s,两个双力偶节面分别为102°/45°/106°(NP1)和259°/47°/74°(NP2).破裂方向性分析结果显示,地震的破裂面为南倾的NP1节面,地震沿着破裂起始点向西南方向、向下破裂,总破裂长度约11.5km,其中,沿深度的破裂范围约7km,沿水平的破裂范围约9km,平均破裂速度约2.1km·s-1.综合区域地质资料、卫星影像等判定本次地震的发震断层为精河南断层,地震可能只破裂了断层的下段(17~25km),并未破出地表.

Resolving the rupture directivity and seismogenic structure of the 2017 Jinghe MS6.6 earthquake with regional seismic waveforms

The Jinghe M_S6.6 earthquake on August 9, 2017 is the largest earthquake in northwest Tianshan in recent years. The earthquake is located within the Kusongmuqike fault zone, and there are several active thrust faults near the epicenter region, including Chahantu fault, Kusongmuqike Frontal fault and Jinghenan fault. Due to the deep source, complicated tectonic deformation and sparse seismic network, it is difficult to directly determine the seismogenic structure of the earthquake based on focal mechanism, aftershock distribution and InSAR observation. In this paper, a method for determining the rupture directivity based on regional seismic waveforms is developed for dip-slip earthquakes. Aftershocks are chosen as reference events, which can be applied as path calibration, and the rupture lengths in the horizontal and depth directions are determined based on the waveform time-shift and Pn-Pg arrival time difference. Then, the rupture plane and rupture length can be inferred. We first invert the point source parameters of the mainshock, then apply the new algorithm to determine the rupture directivity. The results show that the moment magnitude of the mainshock is about 6.2, the centroid depth is 21 km, the source duration is 5.5 s, and the two double-coupled planes are 102°/45°/106° (NP1) and 259°/47°/74° (NP2), respectively. The rupture plane is the south-dipping NP1, and the earthquake ruptured towards southwest and downdip for about 11.5 km. The rupture range along depth is about 8 km, the horizontal rupture range is about 9 km, and the average rupture speed is about 2.1 km·s^-1. Furthermore, we combine the seismological results with regional geological data, satellite imagery, etc., and infer the seismogenic fault of this earthquake is the Jinghenan fault, and the earthquake may only rupture the deeper section of the fault (17~25 km) without breaking the surface.