Seismogenic structure analysis of earthquake surface rupture zone in Maisu fault

Based on the continuous seismic waveform data of the dense seismic array set up around the surface rupture zone of the Maishu fault earthquake from December 2020 to July 2022, 578 seismic events in the area of 31.55°−31.85°N and 98.31°−98.98°E were obtained by using the double difference location method. Using HSAH method based on P-wave initial motion, 37 sets of focal mechanism solutions are obtained. The stress environment of the study area is analyzed based on the inversion method of stress field in damped region with focal mechanism solution. The relocation results show that there is a WNW-ESE seismic dense strip about 40 km long along the seismic surface rupture zone in the study area. The focus is densely distributed in the depth range of 3−10 km, the depth profile slopes sharply to the north, and there is also a NNW-SSE seismic dense strip about 30 km long. The focus is densely distributed in the depth range of 3−11 km, and the depth profile slopes sharply to the west. The results of focal mechanism analysis show that strike-slip type is the main source mechanism of earthquakes in the study area, accounting for 51.4% of the total, and there are also a few reverse fault types and positive fault types. The dominant azimuth of the focal mechanism solution P axis is near NW-SE trending, which is similar to the direction of the maximum principal compressive stress axis σ₁ 323°; the P and T axes, as well as the maximum compressive stress σ₁ and minimum compressive stress σ₃ axes have small dip angles in genera, which indicates that the horizontal compressive stress state in the study area is nearly NW-SE trending. The type of stress structure is strike slip, which is the same with the location and focal mechanism solution. R＝0.57, indicating that the maximum compressive stress axis σ₁, the intermediate compressive stress axis σ₂ and the minimum compressive stress axis σ₃ are basically arranged in an isometric manner, in which σ₁ and σ₃ are clear. The WNW-ESE trending seismic dense strip has dextral strike-slip with both normal and reverse fault components, while the NNW-SSE trending seismic dense strip has sinistral strike-slip characteristics with conjugate fault characteristics. It is concluded that an earthquake of magnitude 7 or above has occurred in the study area. The stress state of the nearly NW-SE trending horizontal extrusion is related to the strong sinistral strike slip of the NW-SE trending Ganzi-Yushu-Xianshuihe fault.