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

Wu Yifeng; Tang Fangtou; Jiang Xudong

doi:10.11939/jass.20220225

Abstract：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 solu-tions 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 loca-tion 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 mech-anism 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)indic-ates 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 re-verse 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.