Regional Stress Field In Yunnan Area

The collision between the Indian plate and the Eurasian plate leads to the rapid uplift of the Tibet Plateau and the eastward flow of the plateau material,which is blocked by the stable South China block and the plateau material flows southeast.Yunnan is located in the southeast margin of Tibet Plateau.The geotectonic position is very special and the structure is very typical,which straddles many geological tectonic units.The eastward movement and southward deflection of materials in the Tibet Plateau have resulted in complex tectonic movement,strong tectonic deformation and frequent strong earthquake activity in Yunnan.A large number of seismic events provide a data basis for exploring the fine regional tectonic stress field in Yunnan.By studying the focal mechanism and tectonic stress background of the earthquake in this area,It is of great significance to understand the occurrence process and dynamic background of strong earthquakes in this area.In this paper,the broadband waveform records of the M ? 3.0 earthquake from January 2008 to June 2018 recorded by 87 fixed stations and 200 mobile stations in Yunnan and Sichuan networks are used,and the CAP(Cut And Paste)method(Zhao et al.,1994;Zhu et al.,1996)is used to calculate the focal mechanism solutions of 627 earthquakes in Yunnan(21o~29oN,97o~107oE),and the spatial distribution characteristics of focal mechanism solutions and P,T axis spatial distribution characteristics of earthquakes in Yunnan are analyzed.Among them,370 strike-slip earthquakes(59%),135 normal fault earthquakes(21%),64 inverse fault earthquakes(10%),the spatial distribution is basically consistent with the local active structures,and most of Yunnan is dominated by strike-slip earthquakes.In particular,the central Yunnan block mainly shows strike-slip movement.The border area between northeast Yunnan and Sichuan basin(for example,)is dominated by reverse faults,showing strong compression characteristics,Most of the normal faults are in the northwest and west of Yunnan(such as Deqin-Zhongdian,Lijiang et al.),and the tensile effect is obvious.Then,using the focal mechanism solutions of the 627 earthquakes,the g CMT(global Centroid Moment Tensor)solutions of 80 M > 4.5 earthquakes from 1976 to 2018 are collected,and the regional tectonic stress damping linear inversion method(DRSSI,Hardebeck et al.,2006)is used.The study area is divided into 0.3°× 0.3°grid,and the stress tensor in each grid is calculated.The direction and corresponding force value of principal stress in each grid are solved by calculating the eigenvalues and eigenvectors of stress tensor,and the maximum is obtained.The relative magnitude and direction of the minimum and intermediate principal stresses are obtained,and a total of 364 grid points of stress tensor.The stress field in most areas of Yunnan is mainly strike-slip type,which is the largest and the minimum principal stress axis is near the horizontal.In Tengchong volcanic area,Huize area,Lijiang-Ninglang area,Deqin-Zhongdian area,the stress properties are mainly normal fault,showing tensile stress state,the maximum principal stress axis is nearly vertical,and the minimum principal stress axis is close to horizontal.In the border area between northeast Yunnan and Sichuan Basin,the maximum principal stress axis is nearly horizontal and the minimum principal stress axis is nearly vertical,showing a state of compression stress.It is considered that the stress field in Yunnan is controlled by the northward and northeastward movement of the Indian plate and the power source of the collision material flow between the Indian plate and the Eurasian plate,and the local area is also affected by the interaction within the block.The regional dynamic action caused by the interaction between blocks and fault zones.Comparing the maximum principal stress direction with the GPS velocity field and the upper crust shear wave fast wave polarization direction,it is found that the maximum principal stress direction is in good agreement with the GPS velocity field direction,and is consistent with the upper crust shear wave fast wave polarization direction in some regions.However,there is a large angle between the two directions in some regions.