基于小震经验格林函数研究中等地震破裂方向性:以2008年攀枝花M_(S)6.1地震为例

The rupture directivity study of moderate earthquakes using empirical Green′s function approach:A case study of Panzhihua earthquake

中小地震破裂方向性研究在确定实际发震面以及地震灾害评估中具有非常重要的意义.本文以2008年8月30日攀枝花M_(S)6.1地震为研究实例,以较小余震作为经验格林函数地震,利用表观震源时间函数随台站方位角变化特性,确定了主震的破裂方向、破裂尺度以及破裂速度,验证了该方法的适用性和可行性.研究结果表明,主震的破裂方向为几乎正南的184°,破裂长度8.05 km,破裂速度3.35 km·s^(-1),进一步确认了主震震源机制解其中沿南北向的节面是地震实际发震面,主震在水平方向沿着南北向元谋断裂从北往南破裂.攀枝花地震震源破裂速度接近地壳剪切波速度,这可能是造成此次地震虽然震级不大但破坏性严重的重要原因.本文综合分析主震的矩心深度、震源机制解、破裂方向性以及构造应力背景等信息,探讨了地震发生的破裂过程以及灾害严重的原因.

Rupture directivity is an important factor in determining the actual rupture planes of moderate earthquakes and related seismic hazard assessments.In this study,we take the 2008 M_S6.1 Panzhihua earthquake as a study case to demonstrate the effectiveness and practicality of the Empirical Green′s Function(EGF)approach to estimating the rupture directivities of moderate earthquakes in China.Using the records of a smaller event of similar location and mechanism to the mainshock,we obtain the apparent source time functions at stations of various azimuths and use them to determine the propagation direction,length and speed of the mainshock rupture.Our results show that the rupture propagation direction of the mainshock is 184°(S4°W),consistent with the strike of the NS-trending Yuanmou fault.The relatively high rupture speed of 3.35 km·s~(-1)close to the shear-wave speed may be the primary reason for the moderate Panzhihua earthquake to have caused disproportionately heavy damages.We also combine the centroid depth,focal mechanism,and rupture directivity of the mainshock as well as regional background stress to further analyze the seismogenic environment of the Panzhihua earthquake to understand its implications to future seismic hazards.