1989—1999大同地震序列的隐伏断层研究:库仑应力分析和余震JHD重定位

Blind-faults of Datong earthquake sequence:JHD and Coulomb stress analyses

1989年到1999年,大同—阳高地区发生了一系列MS≥5的中强地震.本文基于前人对1989年三次MS≥5地震的震源机制反演的结果,通过建立不同断层模型,利用库仑应力方法,计算前震对于主震,以及前震和主震对于余震的库仑应力触发关系,提出了一种可能的破裂模型,即1989年前震沿北西西方向发生左旋破裂,之后主震和余震沿北北东方向发生右旋破裂.根据这种破裂模式计算得出,前震发生后,主震震源处的库仑应力增加了约2×105 Pa,余震震源处的库仑应力出现下降;主震发生后,余震处的库仑应力出现回升,最后余震处的库仑应力几乎没有变化.基于大同地震台网的近场观测数据,用JHD(Joint Hypocentral determination)定位方法,对1999年11月1日MS=5.6地震后一个月的余震进行重定位,得到一条走向118°,倾角85°的左旋走滑断层,余震的深度分布在5km至20km范围内,显示该断层是隐伏断层.另外提出对主震震中位置约10km的修正.本文对1989年三次MS≥5地震序列和1999年MS=5.6地震余震空间分布的研究揭示该地区存在两条活跃的共轭隐伏走滑断层(1989年主震的北北东方向和1999年地震的北西西方向),并且推断已知的大王村断裂和团堡断裂是地下这两条共轭的隐伏走滑断层构造/地震活动在地表的响应.

A series of MS≥5earthquakes struck Datong area from 1989 to 1999.In this study we calculated the Coulomb stress triggering relationship between the foreshock and main shock,and between the foreshock,main shock and the aftershock to test several faulting models from previous studies.We propose one viable faulting model for the three events of 1989 which should be very useful for studying the seismic hazard in this region.Based on the focal mechanisms of the1989′s 3earthquakes（foreshock,main shock,aftershock）from previous studies,several faulting models from previous studies were constructed for testing here.We then calculated the Coulomb stress triggering relationship between the foreshock and main shock,and between the foreshock,main shock and the aftershock,and the results had led to a viable faulting model for the three events of 1989 as the following.The 1989 foreshock fault stroke left-laterally along the NWW direction.Several hours later,the main shock and the aftershock struck right-laterally along the NNE direction.After the foreshock,the Coulomb stress in the main shock area increased about2×105 Pa,while the Coulomb stress in the aftershock area underwent a fall.After the main shock,however,the Coulomb stress in the aftershock area increased.With the access to the data from the Datong earthquake array,one month′s aftershocks after the 1999-11-1 MS=5.6earthquake were relocated using JHD（Joint hypocentral determination）method.The distribution of these relocated aftershocks reveals a buried left-lateral fault with the strike of 118°and the dip of 85°.The location of the main event（MS=5.6）should be adjusted according to the fault location.The results in this study have suggested that two buried conjugate strike-slip faults control the faulting system of the Datong-Yanggao area,consisting of the NNE fault,corresponding to the main shock in 1989,and the NWW fault,corresponding to the main shock in 1999.We further propose that the shallow NE Dawangcun fault and NW Tuanbao fault are the manifestations of the two buried conjugate faults at the shallow crust.