2021年5月21日漾濞M_(S)6.4地震震源区三维P和S波速度结构与地震重定位研究

Three-dimensional P and S wave velocity structure and earthquake relocation of the May 21,2021 Yangbi M_(S)6.4 source region

2021年5月21日漾濞M_(S)6.4地震震源区位于川滇块体西边界的维西—乔后—巍山断裂西侧,该地区近些年来发生了多次中强地震,地震活动较为活跃.对漾濞地震序列重定位和漾濞震源区及邻区的地壳精细结构研究,有助于深入理解漾濞地震的孕震环境、发震机理和破裂过程.本文基于2008年1月1日到2021年6月3日区域固定台站接收到的36938条Pg和32111条Sg波到时数据,采用新发展的三重差地震层析成像算法(tomoTD)开展了漾濞M_(S)6.4地震震源区三维速度结构成像与地震重定位研究.结果显示:(1)余震活动主要集中在维西—乔后—巍山断裂的西侧,整体呈现沿北北西向的条带状分布,结合已有走滑型震源机制解特征,揭示了北西向隐伏断裂是发震断层,其北西段表现为倾角较陡、结构相对简单的走滑断裂,南东段由两条分支断裂组成.(2)主震的发生及地震序列分布与地壳速度结构不均匀性有着密切的关系.主震及4级以上的地震发生在高速边界上或高低速过渡区域,余震主要发生在低速、高V_(P)/V_(S)区,主震上方与下方均显示高V_(P)/V_(S)异常,推测在区域构造应力场的作用下,应力在孕震区的刚性介质中积累,中下地壳流体(或者部分熔融地壳物质)侵入发震断层区,弱化了漾濞6.4级地震的主震区.另外,余震东南侧的低V_(P)/V_(S)区可能代表介质刚性强,可能阻碍了余震向南东方向继续扩展.(3)结合2013洱源5.5级地震研究结果,推测维西—乔后—巍山断裂西侧可能存在着较大的北北西向隐伏断层.

The May 21,2021 M_(S)6.4 Yangbi earthquake source area is located west side of Weixi-Qiaohou-Weishan fault,belonging to Chuandian block west boundary.In recent years,there have been some moderate and strong earthquakes in this area,and the seismic activity is relatively active.Studying on relocation of the Yangbi earthquake sequence and imaging fine velocity structure of the Yangbi M_(S)6.4 earthquake source region and its adjacent area is important to understand the characterization of the seismogenic zone,the mechanism and rupture process of the Yangbi M_(S)6.4 earthquake.In this paper,we collected 36938 Pg and 32111 Sg arrival times received by permanent stations from January 1,2008 to June 3,2021 and conducted a study of 3-D seismic velocity structure of source region and relocation of the Yangbi earthquake sequence with a new triple-difference tomography method(tomoTD).The result shows that:(1)The aftershock activity mainly occurred in the west side of Weixi-Qiaohou-Weishan fault and the distribution of the aftershocks exhibit a strip along the NNW direction.Combining with focal mechanism solutions of strike-slip faults suggests that the NNW direction potential fault is the main seismogenic structure.The north-western section of the NNW direction fault is a strike-slip fault with a steep dip and a relatively single structure,and the south-eastern section is composed of two branch faults.(2)The occurrence of mainshock and the distribution of the Yangbi earthquake sequence are coincided well with the crustal structure of velocity heterogeneity.The mainshock and aftershocks with a magnitude larger than 4 seem to occur at the transition areas between high and low velocity anomalies or at the boundary of the high velocity zone and the aftershocks mainly occurred in the low velocity and high V_(P)/V_(S) zones and some high V_(P)/V_(S) anomalies are below and above the mainshock,which suggests that regional stress field is accumulated in the rigid medium of the seismogenic zones,and fluids from middle-lower crust(or the partial melting of crust materials)intrude into the seismogenic zones,which weakens Yangbi M_(S)6.4 mainshock zone.In addition,the low V_(P)/V_(S) zone,located southeastern side of aftershocks,may have strong rigidity and prevent aftershocks spreading towards the southeast.(3)Combined with the study result of 2013 M_(S)5.5 Eryuan,it is speculated that there may be a large NNW potential fault on the west side of Weixi-Qiaohou-Weishan fault.