Focal mechanism solutions and stress field characteristics of small and moderate earthquakes in Weiyuan area， southern Sichuan basin

Based on the 42 small and moderate events of M_L3.0−4.5 recorded by the dense portable stations in Weiyuan area of southern Sichuan basin during the period from December 1, 2019 to June 30, 2020, we used Hypo2000 relocation method to precisely relocate the events. Then we performed the focal mechanism inversion for 31 events among them by HASH （Hardebeck & Shearer） method. And then we used damping spatial and temporal stress inversion method to carry out the inversion for the stress field parameters of the study area. The results indicate that the small and moderate earthquakes are mainly distributed on the SSE-striking Molin fault on the southern flank of the Weiyuan anticline with focal depths concentrating within 10 km, which suggests they are shallow earthquakes. The main type of focal mechanism solutions is thrust, and the type of fault-slip is relatively complex. According to the results of focal mechanism solutions, it is inferred that there are a series of blind thrust faults and normal faults on both sides of the Molin fault. Meanwhile, there may be other small hidden faults or fractures in the shallow strata of the study area. The azimuth of the maximum principal stress σ₁ is 103° and the dip is 1°, and the azimuth of the minimum principal stress σ₃ is 192° and the dip is 51° in the area. The type of the stress field is thrusting with strike slip, which is consistent with the main type of focal mechanism solutions in the area.