The Study Of Tectonic Stress Field In Yunnan And Its Adjacent Areas

The study area is located at southeast margin of Tibetan Plateau and belongs to the southern segment of the north-south seismic belt in China.It is a key zone of turning movement of materials and cuts across several blocks.The complex stress environment and strong tectonic deformation made the area suffered frequent earthquakes,lesding huge casualties and property damages.Earthquakes are the products of tectonic activity under the regional stress fields.The study of the focal mechanism and stress field of Yunnan and its adjacent area is important for understanding the seismogenic background,seismic activities and regional tectonic activities,and provides basic data for subsequent research.In this paper,the seismic waveform data provided by the National Earthquake Data Center(http://www.seisdmc.ac.cn/)in the study area(21°N-30°N,96°E-106°E)from January 2014 to July 2019 is used,428 focal mechanisms are determined by using gCAP method and 874 focal mechanism solutions are collected from GCMT and published data,adding up to 1302 earthquake focal mechanism solutions.The results show that strike-slip earthquakes in the study area are dominant,and also with normal and thrust-faulting mechanisms.Strike-slip events are distributed throughout the study area.Thrust-faulting events are distributed in northeastern Yunnan(east of the Sichuan-Yunnan junction)with the compression in more obvious.Normal events are distributed in western Yunnan(the Dayingjiang fault and Longling fault area)and northwestern Yunnan(western border of Sichuan-Yunnan rhombic block).The earthquakes occurred at the depth of approximately 20 km.The regional stress fields are inverted by the damped stress tensor method with the focal mechanism solutions from our studies and collected from GCMT and published data,we divide the study area into 1°×1° grids and obtain 49 grid stress fields.The results show that the study area is dominated by strike-slip stress fields,the plunge of compressive and tensile stress axis is close to horizontal.Normal stress field is distributed in the western boundary of the Sichuan-Yunnan block(near the Deqin-Zhongdian-Daju fault),showing tensile stress state.Thrust stress field is distributed in the northern South China block,showing compression stress state.The Deqin-Zhongdian area is a normal stress field with the compressive stress of NW.The Tengchong area is a strike-slip stress field with the compressive stress of NS-NNE.From north to south,the compressive stress axis has a clockwise rotation trend.Stress field is different in block.The direction of the compressive stress in the western boundary of the South China block and the Sichuan-Yunnan block is NW-NNW.Compressive stress in the southern Yunnan block changed greatly,from NNW to near NS and then to NNE.The direction of the compressive stress in the western Yunnan block is NNE.The overall stress field in Yunnan and its adjacent area are controlled by southeastward movement of the Sichuan-Yunnan block that pushed by the Tibetan Plateau,the lateral compression of the Indian plate,and the South China block NW force.Local stress field is also affected by block and regional structure.Compressive stress axis is consistent of the polarization direction of the fast shear-wave exhibits divergence and the direction of the principal compressive strain by GPS data.