| When the seismic wave propagates in anisotropic media,its propagation velocity and the polarization direction of the particle are changed with the direction of wave propagation,which is called seismic anisotropy.At present,common methods to study seismic anisotropy include shear wave splitting and azimuthal anisotropy inversion.The eastern part of the Qinghai-Tibet Plateau is an "ideal natural test site" for studying the continental-continental plate collision,plateau evolution and material escape.As the northeast and southeast corners of the progressive deformation of the Qinghai-Tibet Plateau,they provide important constraints for the uplift and expansion of the plateau caused by the collision of the Indo-Eurasian plates.The study of anisotropy can improve our knowledge of the coupling relation between the crust and mantle,therefore it is important for us to better understand the deep structure and geodynamic process in this region.In this study,based on teleseismic shear waves and ambient noise data,we investigate the anisotropy in the northeastern and southeastern regions of Qinghai-Tibet Plateau.First,by collecting teleseismic data recorded by 238 stations in northeastern Tibet and 350 stations of the Chin Array Phase I in southeastern Tibet,we used the shear wave splitting method to measure splitting parameters at each station—the fast-wave polarization direction and delay times.Then based on the splitting features,we analyzed the deformation pattern of the upper mantle beneath NE and SE Tibet.The results can be summarized as follows.In the Qilian block,the Alsxan block and the northern part of the Ordos block,the fast wave direction is NNW-SSE which is significantly different from the NE-SW direction of the displacement field,and the average delay time is 0.85 s.In the western part of the Qiangtang block and the Songpan-Ganzi fold belt,the fast wave direction rotates clockwise in the same way as the ground displacement field direction,and the average delay time in this region is ?1.24 s.In the junction of the eastern part of the Songpan-Ganzi fold belt,the Qinling orogenic belt and the southern part of the Ordos block,the fast-wave direction shows a complicated pattern and is inconsistent with the ground displacement field direction,and the average delay time is 1.08 s.In the northern part of the Sichuan-Yunnan,the fast wave direction is in N-S direction and similar to the direction of the ground displacement field,and the average delay time is 0.925 s.In the southern part of the Sichuan-Yunnan block located south of 26°,the fast wave direction is generally distributed in the E-W direction,which is obviously distinct from the direction of the surface displacement field,and the average delay time is 1.065 s.In generally,a complicated anisotropy pattern is observed in this study area.At the same time,we collected surface wave dispersion data in parts of the northeastern and southeastern margins of the Qinghai-Tibet Plateau for azimuthal anisotropy inversion,and obtained the Three-dimensional shear wave speed model and corresponding azimuthal anisotropy in the crust and uppermost mantle beneath the Qinghai-Tibet Plateau.The results show that the direction of azimuth anisotropic fast wave at 5-40 km is basically the same as that of GPS,while the direction of azimuth anisotropic fast wave at 45-60 km is more consistent with the upper mantle anisotropy obtained by shear wave splitting.By combining the results of previous SKS splitting study and the GPS velocity field,we suggest that in the western part of the Qiangtang block and the Songpan-Ganzi fold belt,and the northern part of the Sichuan-Yunnan,the crust and upper mantle are coupled;in the Qilian block,the Alsxan block and the northern part of the Ordos block,and the junction of the eastern part of the Songpan-Ganzi fold belt,the Qinling orogenic belt and the southern part of the Ordos block,as well as the southern part of the Sichuan-Yunnan block,the crust and mantle are likely decoupled. |
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