2015年5月云南双柏ML4.1地震活动机理初探

2015年5月19日9时58分，云南双柏、峨山一带出现密集的小震活动，并于6月14日发生M_L4.1地震，形成显著的震群事件。本文利用双差定位法对该震群进行精定位，结果显示，重新定位后震中分布优势方向更明显，主要沿绿汁江南段断裂呈NNE向分布，震源深度为6～12km，其中，又以8～10km最具优势。与原始数据相比，重定位后残差明显减小，走时残差平方和由原来的0.303s降为0.034s，震源位置估算误差在EW方向平均为0.21km，在NS方向平均为0.173km，在垂直方向平均为0.175km；利用广义极性振幅技术（GPAT）方法，计算得到了24个M_L≥2.5地震的震源机制解，结果显示，此次震群的震源机制类型绝大部分表现为正断性质，与该区历史地震多为走滑的结果有所差异。结合震区附近的龙门水库蓄水水位和该震群的频度-震级关系分析认为，双柏震群活动与水库蓄水过程密切相关，该震群的发生是龙门水库水体载荷加载而引起正断层产生错动的结果。

A Preliminary Study of Mechanism for the 2015 Shuangbai ML4.1 Earthquake

A notable earthquake swarm occurred near the Shuangbai and Eshan area and its activities continue since 09:58,May 19,2015.The largest magnitude is M_L4.1 which occurred on June 14. We relocated this swarm by using hypoDD method. It has been shown that:the earthquake swarm distributed along the Lvzhijiang fault,and the dominant direction of relocated aftershocks is more obvious than before. The focal depths are at 6~12km,and 8~10km underground is the dominate layer where earthquakes can occur more easily. In comparison with original data,the RMS average value of arrival residual decreases form 0.303s to 0.034s,and the average error is 0.21km in EW direction,0.13km in NS direction and 0.175km in vertical direction. We calculated the focal mechanisms for earthquakes with magnitudes larger than M_L2.5 by using an inversion technique for focal mechanisms of local and regional earthquake,named Generalized Polarity and Amplitude Technique(GPAT),and 24 results were obtained in the end. The results show that the main type of focal mechanisms for this swarm is normal,and this is not the same as before. On the basis of the water level of the Longmen reservoir dam and the frequency and magnitude of the earthquakes,we suggest that the activities of this swarm are closely related with the process of reservoir inpounding. This swarm is the result of a normal fault movement which was caused by the reservoir inpounding.