A Study On The Azimuthal Anisotropy Of Upper Mantle Beneath Yunnan Area By XKS Splitting

Yunnan area(21°N-29°N,97°E-106°E),located in the southeastern margin of the Qinghai-Tibet Plateau and the southern section of the north-south seismic belt,is affected by the combined effects of the Indian plate subduction to the north,the Myanmar microplate subduction to the east,and the blocking of the Sichuan Basin.Seismic anisotropy provides unique possibilities for observing the nature of the earth at depth and the geodynamic processes,seismic activity,plate motion processes.The study of seismic anisotropy in Yunnan area is of great help to probe seismic mechanism and seismicity in Yunnan,and the study of upper mantle anisotropy is also helpful for the study of ancient and modern mantle flow patterns.For the wider tectonic background,we can further probe the deformation patterns about crust and mantle beneath the southeastern margin of the Tibetan Plateau by studing the anisotropy beneath Yunnan area,which is helpful to explore the dynamic process of uplift evolution of the Tibetan Plateau.Different seismic anisotropy research methods can better understand the deformation process in the depth of the earth.Shear wave splitting analysis is utilized increasingly as an intuitive and reliable method for delineating mantle deformation and structure.The polarization direction of the fast wave after shear wave splitting through the anisotropic medium indicates the deformation information and the delay time carries the information about accumulated stress and varying time.The anisotropic splitting parameters can be obtained directly by shear wave splitting,so as to infer the anisotropic information and deformation process beneath the study area.We collected the three-component waveform data of 17 stations in the study area during 1999 and 2019,this paper studies the XKS splitting of 979 teleseismic events with a epicenter distance of 80°~145°,a magnitude Mw greater than 5.0 and less than10.0,and finally 550 splitting measurement results were obtained to analysis.The average splitting parameters obtained from the splitting measurement are compared with the existing research results of other scholars,the motion direction of GPS velocity field,the research results of local seismic S wave and the splittingmeasurements of the Pms phase from the Moho.It is found that the anisotropy in Yunnan area is more difference and complex between north and south,with 26°N as the boundary,on the north side of 26°N,the fast wave polarization near NS direction and change into nearly EW in the south.We inferred that the deformation patterns of the crust and mantle are coupled on the north side of 26°N,consistent with the deformation mechanisms of the interior Tibetan plateau;decoupled between the crust and mantle in the south.After further analysis of the parameters of double-layer anisotropic splitting,the polarization direction of fast wave in the upper layer is nearly NS,it is believed that the anisotropy may originate from the crust or lithosphere mantle;the anisotropy in the lower layer is closer to the average anisotropic splitting parameter,near EW,and the anisotropy may be caused by the differential motion between the bottom of the lithosphere and at the top of the asthenosphere.