Seismogenic Structure Analysis Of Seismic Surface Rupture Zone In Maisu Fault

The Sichuan-Yunnan rhomboid block and block boundary fault zone is one of the most intense seismic regions in China.It is bounded by the Jinshajiang fault zone near NS,the Ganzi-Yushu-Xianshuihe fault zone near NW-SE,the Anning HeZemuhe-Xiaojiang fault zone near NS,and the Honghe fault zone near NW-SE.Compared with the other three boundaries of the Sichuan-Yunnan rhombobod block,the Jinshajiang fault zone on the western boundary has fewer earthquakes of magnitude 6 or above,while the northern part of the western boundary has fewer earthquakes of magnitude 6 or above.Due to the harsh geographical environment and sparse population in this area,the current research degree is very low.With the continuous improvement of satellite image accuracy,more and more seismic surface rupture zones are gradually discovered.During the interpretation of satellite images of the northern section of the Jinshajiang fault zone,an obvious surface rupture phenomenon is found.Field geological surveys and aerial drone photography confirmed the latest seismic rupture zone.The rupture zone is a WNW-ESE zone with a total length of about 50 km and can be divided into four right-order oblique seismic surface rupture zones.The maximum vertical displacement of 2 m and horizontal displacement of 4 m were found along the earthquake surface rupture zone.Because the location of the seismic surface rupture zone is less than 2 km away from the regional Maiku fault,it is considered that the seismic surface rupture zone is the Maiku fault earthquake surface rupture zone.In order to study the geophysical characteristics of seismogenic structures in the surface rupture zone of Maisu fault earthquake,15 seismometers were set up around the seismic surface rupture zone to form a small intensive array for seismic observation,and the continuous seismic waveform data from December 2020 to May 2023 were obtained.A combination of PhaseNet phase picking algorithm based on deep neural network and double difference location method was used to locate the earthquake,and 871 seismic events were obtained in 31.55°—31.85°N,98.31°—98.98°E region.According to the seismic precise location list obtained,37 focal mechanism solutions were obtained by selecting appropriate seismic events and using HSAH method based on P wave initial motion.Then the stress field in the damped region is obtained by using the MSATSI program based on the source mechanism solution.The fine location results show that there is a WNW-ESE intensive seismic strip about 60 km long along the seismic surface rupture zone in the study area,and the focal depth intensive distribution range is 3 to 10 km,showing the strike-slip property of steep dip to the north,and there is also a NNW-SSE intensive seismic strip about 30 km long.The focal depth ranges from 3 to 11 km,showing the strikeslip property of steep dip to the west.The results of focal mechanism analysis show that strike-slip earthquakes are predominant in the study area,accounting for 51.4%,and there are also reverse fault type and positive fault type.The results of stress field inversion show that the maximum principal compressive stress axis σ1 is 323° in the near NW-SE direction and the dip Angle is 15°;the intermediate principal compressive stress axis σ2 is 194° in the near SN direction and the dip Angle is 67°;the minimum principal compressive stress axis σ3 is 58° in the near SN direction and the dip Angle is 17° in the WSW-ENE direction.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 dominant orientation of the focal mechanism solution P axis is near NW-SE direction,which is similar to the maximum principal compressive stress axis σ1 direction of 323°.The P and T axes,as well as the maximum compressive stress σ1 and the minimum compressive stressσ3,have small dip angles on the whole,which reveals that the horizontal compressive stress in the study area is near the NW-SE direction.The stress structure type is strike-slip type,which is consistent with the results of location and source mechanism.The seismic dense strip along the WNW-ESE direction has dextral strike-slip with both positive and reverse fault components,while the seismic dense strip along the NNW-SSE direction has dextral strike-slip with conjugate fault characteristics.Based on the field survey data,satellite interpretation data and seismic location results,it is concluded that an earthquake of more than 7 magnitude has occurred in the study area,indicating that an earthquake of more than 7 magnitude has occurred in the northern section of the western boundary of SichuanYunnan block.The stress state of the horizontal extrusion near NW-SE is related to the strong sinistral strike-slip of the Ganzi-Yushu-Xianshuihe fault in the north NWSE direction.