Abstract:An earthquake with a magnitude of ML5.1 occurred in Aruhorqin Banner,Inner Mongolia Autonomous Region,on October 15,2021. To enhance the understanding and analysis of the seismotectonic structure of this event,this study employs a comprehensive methodology. The hypoDD method is utilized to accurately relocate 34 earthquakes,while the CAP method is applied to determine the focal mechanism of the main shock. The Focmec method is used for aftershocks with magnitudes above ML 2.0. Subsequently,the stress field is inverted based on the focal mechanisms of the earthquake sequence,and fault fitting is performed on the relocated events. The relocation results indicate that the earthquake source depth is 19.89km. The distribution of 31 relocated earthquakes is more concentrated,exhibiting a trend from southeast to northwest,with the rupture sequence propagating from deeper to shallower depths. The focal mechanism results for the main shock reveal that the strike,dip,and rake angles of nodal plane I are 134°,86°,and-8°,respectively,while those of nodal plane II are 224°,82°,and-177°,respectively. The moment magnitude (MW) is determined to be 4.51,indicating a predominantly strike-slip event. Given the northwest distribution of aftershocks,nodal plane I is inferred to correspond to the seismogenic structure. Fault fitting results show that the fault strike is 142.54° and the dip angle is 85.49°,which aligns closely with the direction and inclination of focal mechanism plane I,both trending northwest (NW) . Regional stress field analysis indicates that the maximum principal stress axis (σ1) trends at 93.51° with an elevation angle of 8.69°,while the minimum principal stress axis (σ3) trends at 186.49° with an elevation angle of 18.81°. These results suggest that the fault stress in this region is predominantly characterized by strike-slip motion. Based on the above findings and the seismic geological structural characteristics of the region,it is inferred that the seismogenic structure of this earthquake is a northwest-trending concealed fault.