Crustal Shear-wave Splitting In The Mid-south Part Of Tibetan Plateau And In The Sangjiang Area Of The Southeast Margin Of Tibetan Plateau

The Tibetan Plateau generally uplifts with strong tectonic movement and medium deformation.The lateral large-scale structural faults inside the plateau are interlaced with the active structures of the longitudinal tensile properties.The tectonic stress is complex and variable,the surface velocity field has strong non-uniformity,and the deep anisotropy of crust and mantle are particularly significant.Due to insufficient near-field seismic observation data,the study of seismic anisotropy in the upper crust in the Tibetan Plateau has been keeping a blank.In this study,we use seismic waveform data from May 2009 to May 2017 recorded by Xizang(Tibetan)seismic networks to investigate seismic anisotropy in the upper crust in the Tibetan Plateau by shear-wave splitting.Obtaining the anisotropic parameters of the crustal medium by shear-wave splitting of near-field small earthquakes,it can be used to analyze the principal compressive stress state,dielectric deformation characteristics and the degree of anisotropy of the subsurface medium in the study area,so as to better understand spatial distribution characteristics of anisotropy in the upper crust of the Tibetan Plateau.This study used seismic observation data recorded by 16 fixed stations stored on the Tibetan Plateau.Due to sparse distribution of seismic stations in the Tibetan Plateau,the near-field seismic events applicable to shear-wave splitting are rare.This study uses single-station location technology to constrain hypocentral location on those events without focal depth information in the published earthquake catalog.In addition,we apply Match and Locate(M&L)method to retrieve the continuous seismic waveform and identify the new microseismic events(small earthquakes)missed in the catalog.The number of new microseismic events obtained by microseismic identification is about six times more than those in the earthquake catalogue,and new data are adopted to analyze shear wave splitting in the study area.Comparing the polarization directions of fast shear-waves(PFS),it is confirmed that new data obtained by microseismic identification greatly increase the number of effective data and improve the reliability of the results.The results show that the PFS in the eastern part of the Lhasa block between the Yarlung Zangbo suture zone and the Bangong-Nujiang suture zone are mainly affected by regional stress field,and the PFS are mainly in NS or NNE direction,indicating the direction of the maximum regional principal compressive stress.However,the PFS of the individual seismic station(Dangxiong station)is neither parallel to the fault strike nor parallel to the direction of the regional principal compressive stress,revealing the local deformation of the crustal medium.In Gaize and Pulan in the western part of the study area,and in Shuanghu in the northern part of the study area,the PFS are consistent with the strikes of the faults.In Changdu and Chayu in the eastern part of the study area,the PFS are consistent not only with the strikes of the faults(or tectonic lines),but also with directions of the surface movements,revealing the possible migration direction of the deep matter in the eastern part of the Tibetan Plateau.Although this phenomenon still needs verification by more research,it suggests that seismic anisotropy in the upper crust combining with surface deformation data possibly are applicable to explore the movement of deep crustal material.Sangjiang area,located in the southeast margin of the Tibetan Plateau,is a typical tectonic area to study the medium deformation caused by the lateral plate collision and “eastward material flow of the Tibetan Plateau”.Under the squeezing effect of the Indian plate in the Sanjiang research area,the driving force of the orogenic movement mainly comes from the NNW direction.The earthquakes recorded by the 28 seismic stations of the Sanjiang mobile station and the surrounding 7 fixed stations and the microseismic identification method are scanned.The M&L method obtained the missing seismic records in the Sanjiang area is 7.6 times the number of seismic records in the earthquake catalog.The results of crustal shear-wave splitting obtained by using the missing seismic events satisfying the research conditions are highly similar to those of the earthquake catalogue,effectively complements valid research data for the study area.Preliminary results indicate that: The PFS of the seismic stations in the study area are in good agreement with the direction of the regional principal compressive stress,but it also exhibits a clockwise rotation.From west to east,mainly NNW,near NS and NNE,revealing common constraints on nearby structural faults.