基于相对质心震中的地震破裂方向性测定方法研究:以2008年云南盈江MS6.0地震为例

地震破裂方向性参数包含了断层几何形态、破裂长度等信息,对于地震灾害评估、孕震机理研究具有重要意义.但是基于点源近似的震源矩中心张量机制解(CMT)或断层面解(fault plane solution)只能给出两个节面,无法确定破裂断层面.本文提出一种基于质心与起始震中的差异确定发震断层的方法,该方法利用P波到时来测定主震与参考地震间相对起始震中,同时利用波形反演震源机制过程中的主震时移与参考地震时移之差测定相对质心震中,在假定单侧破裂的情形下,根据时移之差随方位角的变化推断破裂断层面.本文使用该方法研究了2008年云南盈江M_S6.0走滑型地震,发现其破裂方向与其他方法结果一致.由于该方法基于相对起始震中和质心震中,可有效削弱速度结构模型不准确以及地震绝对位置误差带来的影响,应可适用于其他类似类型6级左右地震的破裂方向性研究,但仍需进一步工作对其进行验证.

A method of resolving earthquake rupture directivity with relative centroid location and it's application to the 2008 Yingjiang MS6.0 earthquake

Rupture directivity of an earthquake is essential for studying seismic hazard and understanding seismogenic processes. However, point source approximation with centroid moment tensor (CMT) or fault plane solution only provides two nodal planes instead of specifying the physical rupture plane. We developed a technique to determine the rupture plane via measuring the spatial difference between centroid location and hypocenter. The technique involves the waveform time shift difference of main shock and smaller events with similar focal mechanism, which calibrates time shift of main shock for velocity heterogeneity and location error. We use relative location method to acquire exact relative hypocenter locations of smaller events and main shock with onset arrival of P waves and use waveform inversion to acquire waveform time shift to resolve relative centroid locations. Azimuthal variations of waveform time shift differences reveal the main shock rupture directivity. We apply the approach to the 2008 Yingjiang M_S6.0 earthquake, the result is consistent with spatial distribution of relocated aftershocks using hypoDD and analysis result derived from an empirical Green's function approach. It might be an effective technique to study rupture directivity of strike slip earthquake with magnitude ~6, yet further studies are necessary to verify its applicability.