Study On Crustal Deformation Characteristics And Fault Activities Of The Eastern Boundary Of The Sichuan-Yunnan Block

The Sichuan-Yunnan block is located on the southeastern boundary of the Qinghai-Tibet Plateau,with complex geological structures and frequent seismic activities.The eastward extrusion of the material on the Qinghai-Tibet Plateau and the continuous wedging of the NEE direction in the Assam Angle cause lateral extrusion and slippage of the Sichuan-Yunnan block,which makes the eastern boundary of the Sichuan-Yunnan block one of the areas with active Late Quaternary tectonic movement and frequent occurrence of strong earthquakes Therefore,it is of great significance to study the tectonic activity,stress accumulation and release,degree of fault blockage and seismic periodicity of the eastern boundary of the Sichuan-Yunnan blockBased on the geological tectonic background,main fault activities and seismic activities of the Sichuan-Yunnan block and the eastern boundary,this paper uses the GPS velocity field data from 2009 to 2017 and the cross-fault short-level data from 1985 to 2018 on the eastern boundary of the Sichuan-Yunnan block,researches the crustal movement mode,deformation characteristics and fault activity characteristics of the eastern boundary of the Sichuan-Yunnan block,combined with the Ms?3.0 seismic activity data in the region and surounding areas from 1970 to 2018,and comprehensively analyses the relationship between the crustal deformation characteristics and seismic activities of the eastern boundarv of the Sichuan-Yunnan block.The following preliminary conclusions are obtained(1)Based on the GPS horizontal velocity field data relative to the Eurasian plate from 2009 to 2017,the horizontal deformation characteristics of the study area are analyzed,the results show that the size and direction of the GPS horizontal motion field and strain field in the study area change.Taking the east boundary as the boundary,the horizontal movement rate is reduced from 26.55mm/a on the west side to 2.48mm/a on the east side,and the direction is gradually deflecting from E and SEE in the north to SE and SSW in the south.The horizontal strain rate results show that the Wenchuan earthquake has a certain control effect on the deformation of the eastern boundary of the Sichuan-Yunnan block,the main strain is enhanced by NW-SE compression and SW-NE tensile deformation,and the maximum shear strain range is distributed in the form of strips on the eastern boundary,and reflects relatively high strain accumulation background and seismic hazard in the Daofu-Shimian section of the Xianshuihe fault,Anninghe-Zemuhe-Daliangshan fault structure area and the Yiliang-Jianshui section of the Xiaojiang fault.For further research in the area of horizontal deformation mode,with South China as a benchmark for horizontal deformation field,the results show that the Sichuan-Yunnan block is affected by dynamic factors and the lower crustal material flow of the eastern boundary,showing a pattern of the north-to-north movement and the south-to-south movement,and with the eastern boundary as the boundary the difference of left-lateral movement rate between the eastern and western sides is about(1.12~3.54)mm/a.(2)Based on the cross-fault level data from 1980 to 2018,the dynamic characteristics of fracture vertical deformation in the study area are analyzed,the results show that there is a big difference in the vertical activity of the fault along the eastern boundary of the Sichuan-Yunnan block.The Xianshuihe fault has severe vertical activity from 1980 to 2018,and the stress continues to accumulate;the vertical activity of the Anninghe-Zemuhe-Xiaojiang fault may be in a state of "lock up" between 1990 and 2010,then with breaking the trend of stagnation and creep of faults after Wenchuan earthquake,the new activity signs began from 2010 to 2018,and the stress accumulation continues to increase,meanwhile the Xiaojiang fault may have a "blocking" feature from 2014 to 2018,which should be paid attention to.(3)Based on the Ms>3.0 seismic activity data from 1970 to 2018,the relationship between the active structure and seismic activity in the study area is analyzed.The results show that after the Wenchuan Ms8.0 earthquake,it may cause the moderate-strong earthquakes above Ms4.0 in the eastern boundary of the Sichuan-Yunnan from 2009 to 2018,and the deep media tectonic setting shows that the eastern boundary of the Sichuan-Yunnan block has significant segmentation characteristics of seismic activity,with complex interactions between faults at the intersection of active faults,which is consistent with the results of three-dimensional crustal movement from 2009 to 2018 obtained in this paper.Further analysis of the fracture strain accumulation and seismic hazard after the Wenchuan earthquake and comparing with the GPS horizontal field results from 1999 to 2007,it is concluded that the Anninghe-Zemuhe fault has the largest deformation width and maximum lockup degree,as well higher seismic hazard while the middle-south part of Xianshuihe fault and the southern part of the Xiaojiang fault are relatively low.(4)Comprehensive analysis of the relationship between crustal deformation characteristics and seismic activities indicates that the seismic risk of the middle-south section of the Xianshuihe fault,the Anninghe-Zemuhe-Daliangshan fault structure area and the southern section of the Xiaojiang fault are further increased,which should be paid attention to.