Crustal Anisotropic Structure And Deformation Characteristics Beneath The Western Sichuan And Its Adjacent Areas

The southeastern margin of the Qinghai-Tibet Plateau,as a necessary place for lower crustal flow and tectonic escape,has always been a hotspot in geoscience research.Sichuan-Yunnan area is located in the southeastern margin of the Qinghai-Tibet Plateau and is one of the areas with the strongest crustal deformation and seismic activity.In order to explore the anisotropic characteristics and crustal deformation structure in the Sichuan-Yunnan region,18375 effective P-wave receiver functions are obtained by using the data of the teleseismic waveform recorded by 149 seismic stations located in the Western Sichuan Array.In the case of anisotropy,the arrival time of the converted wave depends on the incident azimuth angle,and the PS wave of the receiving function only changes with the Azimuth angle after reflection by the MOHO surface.Therefore,the anisotropy of the crust is reflected by the variation of PS phase with Azimuth Angle.The apparent splitting parameters of Pms wave reflect the anisotropy of the crust.The results show that the crust splitting time is between 0.04 s and 1.48 s,and the average splitting time is 0.64 s in the study area.In some areas,there are obvious transition phases in the crust,the anisotropic parameters of the lower crust are obtained by delay time correction of the transformation phase.In this paper,43 stations were studied for upper and lower crustal stratification.The results show that the average delay time of the upper crust is 0.46 s,and the average delay time of the lower crust is 0.58 s.We believe that the anisotropy of the crust may be mainly affected by the anisotropy of the lower crust.Comparison with the SKS splitting results,a comprehensive analysis shows that there is a coupling relationship between the upper and lower crust on both sides of the Xianshui River fault,the crust and mantle are coupled;In the Songpan-Garze block,the crust and mantle are coupled,and the upper and lower crust are decoupled to some extent;as same as the Sichuan-Yunnan diamond block.While the upper and lower crust may be decoupled near the Xiaojiang fault and the Longmenshan fault.Due to the influence of the anisotropic layer in the study area,the conversion of PpPs,PsPs,and PpSs will affect the results of the H-k scanning method,so we added the anisotropic information to the H-k methods.The results show that the crustal thickness varies from 36 km to 69 km in the study area.The thickest area is located inSongpan-Garze block,the crustal thickness about 69.40 km,and the thinnest area is near the Longmenshan fault,the crustal thickness about 36.30 km,Poisson's ratio results are ranging from 1.75 to 2.03,and the Poisson's ratio is more than 2 in some stations.We roughly infer that the rocks below these areas is melting.