Research On Focal Mechanism Solutions And Stress Field Characteristics Of Small And Moderate Earthquakes In Southern Sichuan Basin Area

In general,the seismicity of southern Sichuan is lower than that of western Sichuan.However,after the Wenchuan earthquake in 2008 and Lushan earthquake in 2013,the stress field in sichuan Basin changed,and the frequency and magnitude of earthquakes in southern Sichuan increased.In addition,in recent years,with the frequent industrial exploitation activities,the frequency of small and moderate earthquakes in southern Sichuan increased rapidly.Therefore,this paper collected the ML2.0-4.5 small and medium seismic events recorded by dense mobile stations from December 1,2019 to October 30,2021 to study the seismic location,focal mechanism solution and regional stress field in Weiyuan and Changning regions,the key regions in southern Sichuan.Firstly,the Hypo2000 method was used for precise location,and then the HASH(Hardebeck&Shearer)method was used to retrieve the focal mechanism solution of the earthquake.On this basis,the stress field parameters in the study area were calculated by using the damping region stress inversion method.The results show that:1)The small and medium earthquakes in Weiyuan area are mainly distributed on both sides of the Molin fault on the SSE strike of the southern wing of Weiyuan anticline,and the focal depth is concentrated within 10km,all of which are shallow earthquakes.The medium and small earthquakes in Changning area are mainly distributed near Didiwan fault,and the focal depth is concentrated within 10km.So the medium and small earthquakes in Changning area are also shallow source earthquakes.The earthquakes in Weiyuan area and Changning area are centered near the shale gas well water injection layer,so it is speculated that the occurrence of these earthquakes is related to the shale gas hydraulic fracturing operation in this area.2)The focal mechanism solution type in Weiyuan area is mainly thrust type,and the strike dominant distribution is NNE-Ne and SSE-S,and the fault dislocation type is relatively complex.According to the focal mechanism solution results,it is speculated that there are a series of blind thrust faults on both sides of the Molin fault,and there may also be other small hidden faults or ruptures in the shallow strata of the study area.The focal mechanism solutions of Changning region are mainly thrust type and strike-slip type,and the reverse fault type and strike-slip type earthquakes have the characteristics of cluster occurrence,which can be divided into two cluster regions.The dominant direction of focal plane strike is NNE-NE,and the morphology of fault plane is mainly high Angle inclination.The fault dislocation types in the study area have good consistency.3)The type of regional stress field in Weiyuan area is thrust and strike-slip stress field,which is consistent with the main type of focal mechanism solution,indicating that the small and medium earthquakes in Weiyuan area are mainly controlled by stress field.The R value is 0.35,indicating that the region as a whole is in a compressive tectonic environment,and the intermediate principal stressσ2 has a certain tension component.The type of regional stress field in Changning area is thrust stress field,which is mainly thrust earthquake stress field,with a small amount of strike-slip component.R value is 0.38-0.43,the region is in extrusion state as a whole,and the intermediate principal stress is closer to the maximum principal stress,which is mainly in extrusion state,and there is a certain tension component.