Seismogenic structure of the earthquake surface rupture zone along the Maisu fault

Field geological survey revealed the presence of an earthquake surface rupture zone 50 km long near the Maisu fault in the northern segment of the Jinshajiang fault zone, with a maximum vertical offset of 2 m. Based on the continuous waveform data of a dense seismic array set up around the rupture zone from December 2020 to July 2022, 578 seismic events in the area of 31.55°N−31.85°N and 98.31°E−98.98°E were located by using the double-difference location method. The obtained seismic events were then analyzed using the HASH method based on P-wave initial motion to invert the focal mechanisms, and 37 focal mechanism solutions were acquired. The stress state of the study area was further analyzed by using the damped region-scale stress inversions method based on the focal mechanism solutions. The precise location results show that within the study area, there is a seismic belt 40 km long oriented WNW−ESE along the earthquake surface rupture zone, with focal depths clustered in the range of 3−10 km, and the depth profile steeply dips to the north. Moreover, there is another belt 30 km long oriented NNW−SSE, with focal depths concentrated at 3−11 km, dipping to the west. The results of focal mechanism solutions indicate that source mechanisms in this area are mostly strike-sip, accounting for 51.4% of the total. In addition, there are also a small number of reverse and normal fault mechanisms. The dominant azimuth of the P-axis of the focal mechanism solution is nearly NW−SE, which is similar to the direction of the maximum principal compressive stress σ₁ at 323°. The overall dips of the P-axes, T-axes, maximum compressive stress σ₁, and minimum compressive stress σ₃ are small, suggesting that it is under an NW-SE oriented horizontal compressive stress regime. The stress structure is strike-slip, consistent with the earthquake location and focal mechanism solution results. The stress factor （R=0.57） indicates that the maximum compressive stress σ₁, the intermediate compressive stress σ₂ and the minimum compressive stress σ₃ are basically in arithmetic progression, where σ₁ and σ₃ are determined. The WNW−ESE seismic belt exhibits dextral strike-slip with both normal and reverse components, while the NNW−SSE belt shows sinistral strike-slip, displaying conjugate fault features. It is concluded that this area has experienced at least one earthquake with M＞7.0. The near NW-SE horizontal compressive stress state is related to the intense sinistral movement of the NW−SE oriented Garze-Yushu-Xianshuihe fault zone.