Spatial Distribution Characteristics Of Focal Mechanisms And Stress Field In Southwestern Yunnan And Its Adjacent Areas

Yunnan region,as the overlap of two very important tectonic divisions of"north-south seismic belt"and"Sichuan-Yunnan lozenges",is one of the regions with the strongest seismic activities in mainland China and it is a hotspot of global dynamics research.Yunnan region is located in the eastern part of the collision zone between the Eurasian plate and Indian plate,also it is located in the southeastern edge of the Tibetan plateau.It has been used to explore the dynamics of the Tibetan plateau.Therefore,there are many researches on geological structure and geophysical significance of Yunnan,in which includes many researches on focal mechanism solutions and stress field.But just a few researches on southwest of Yunnan and its adjacent areas.This article selects the southwest of Yunnan and its adjacent regions(97°E-102°E,21°N-26°N)as the studied area,discusses the characteristics of stress field and focal mechanism solutions.It can provide the contestant to tectonic activities and dynamics in the region.Based on CAP method,this study inversed partial focal mechanism solutions of M_L3.5 and above from January 2014 to January 2018 in the studied area,there are 74focal mechanisms in total.In addition,511 historical source mechanism solutions were collected before 2014.So we obtained 585 focal mechanism solutions in the studied area.Tectonic stress field in the studied area is estimated using damping stress tensor inversion method,and obtained preliminary characteristics of stress field.The focal mechanisms in the studied area is dominated by strike-slip focal mechanisms,accounting for 74%,which is consist with the strike-slip tectonic background in Yunnan.normal and thrust focal mechanisms account for a small proportion(The focal mechanisms in the studied area is dominated by strike-slip focal mechanisms,accounting for 74%,which is consist with the strike-slip tectonic background in Yunnan.Normal and thrust focal mechanisms account for a small proportion(13%and 10%respectively).Normal focal mechanisms are concentrated near Yingjiang and Tengchong areas,while distribution of strike-slip and thrust focal mechanisms are uniform.In addition,the earthquakes in Southwestern Yunnan region are mainly in the upper and middle crust,concentrated in 5~15km,which is consistent with the dominant source depth of Yunnan region.According to the inversed results of the stress field,the plunge angles of both the compressive stress axis and tensile stress axis are small in the studied area,the stress state of horizontal compressive stress is obviously,and the whole area shows obvious strike-slip stress state.The dominant direction of compressive axis is NS direction and the dominant direction of tensile stress is WE direction in the studied region.According to the result of stress field inversion of 0.5°×0.5°,the compressive stress axis rotates from NE to NW and the tensile axis rotates from NW to NE in the area of24°N~26°N.Among 22°N~24°N,the maximum principal compressive stress axis rotates from NNE to SN and maximum advocate stress axis rotates from NWW to EW from west to east.Among 21°N~22°N,the maximum principal compressive stress axis is extruded in NNE direction and the minimum principal compressive stress axis is stretched in NWW direction.The R value in the whole study area increases from northwest to southeast,roughly from 0.2 to 0.8.Taking the NanTingHe fault and its extension line to the northeast as boundary,the R value near the boundary is about 0.5,At this time,the principal compressive stress axis,the intermediate stress axis and the principal stress axis are equally arranged,the deviatoric stress of the intermediate stress axis is 0.The R value gradually decreases to the northwest direction from the boundary.That means the intermediate stress axis act as compressive stress.There is a low R value region(the lowest value is less than 0.2)in the northwest of the studied area.In this case,the intermediate stress axis act as compressive stress,showing the biaxial compression and uniaxial tension state.From NanTingHe to southeast direction,the R value gradually increase.That means the tensile effect of the intermediate stress axis becomes more and more obvious.In the southeast of the studied area,a high R value region(the highest value is higher than 0.8)is formed.In this case,the intermediate stress axis act as tensile effect.This shows the state of biaxial tension and uniaxial compression.The line is also the boundary of the sub-block of baoshan and jinggu.Jinggu sub-block is subjected to the tensile effect of the baoshan sub-block and the baoshan sub-block is subjected to the tensile effect of the myanmar region.Such interaction between plates may be responsible for the variation of the stress field.In addition,we discuss the problem of whether the homogeneity assumption is satisfied on the stress field inversion grid in this study.This paper has conducted in-depth verification and analysis by using two inversion methods to invert stress field of single grid basing on different types of focal mechanism solution data,using artificial synthetic focal mechanism data to confirm and residual analysis of each focal mechanism solution relative to the reference stress tensor.The analysis shows that the phenomenon may be caused by the inhomogeneous stress field in a single grid,and the stress field inversion result is the neutralization of multiple stress fields.The results of this study can provide reference for the rational division of the grid and the evaluation of stress field results in the subsequent stress field study.