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

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.