Crustal Anisotropy Structure And Crustal Deformation Characteristics In Sichuan Basin And Its Adjacent Area

The strong deformation and uplift of the Qinghai-Tibet Plateau is caused by the collision and subduction between the plates,which makes the crustal deformation of the adjacent plateau area severe and causes frequent earthquakes.The southeastern margin of the QinghaiTibet Plateau is an important channel for the eastward flow of the plateau material.It is of great significance to study the deep underground structure in the southeastern margin of Qinghai-Tibet Plateau to understand the deformation mechanism and dynamic process of Qinghai-Tibet Plateau.In this paper,the Sichuan Basin and its adjacent areas on the southeastern edge of the Qinghai-Tibet Plateau are used as the research area.The teleseismic data recorded by 174 stations in the study area in the Western Sichuan Array from 2006 to2009 are selected and the azimuth superimposed receiver function method is used.And layer stripping technology to obtain the anisotropy parameters of the upper and lower crust,and then obtain the Moho depth and wave velocity ratio by the anisotropic Hk superposition method.In this paper,according to the method of azimuth superposition receiving function,the anisotropy parameters of Pms phase under 174 stations are obtained.The results show that the slow wave delay time of Pms wave is between 0.04 s and 1.39 s,with an average of 0.54 s.The fast wave polarization direction of the stations in the study area and the GPS velocity field direction are roughly the same as the NW-SE direction.The slow wave delay time observed inside the Sichuan Basin is smaller than that observed at the southwestern edge of the basin.The anisotropy of the Longmenshan fault zone has the characteristics of zoning.The polarization direction of the fast wave under the stations in the northern and southern sections of the Longmenshan fault zone is the NW-SE direction,which is consistent with the direction of the principal compressive stress.Below the station in the middle of the Longmenshan fault zone The polarization direction of the fast wave is parallel to the fracture zone.The delay time of stations on the west side of the Longmenshan fault zone is generally longer than that on the east side of the fault.In the southern Sichuan Basin,comparing the Pms wave splitting results in this paper with the SKS splitting parameters of Chang Lijun,it is found that the fast wave polarization direction in this area is close to the NW-SE direction,and the fast wave polarization direction is consistent with the surface motion field and the direction of the surface structure.The surface deformation characteristics of this area are consistent with those of the deep upper mantle,indicating that the lithospheric deformation conforms to the vertical coherent deformation model.The fast polarization direction of the Pms wave in the south of this paper is also the NW-SE direction,which is basically consistent with the SKS splitting result.In this paper,the two-layer anisotropic structure obtained by layer peel technology shows that the upper crust slow wave delay time is between 0.07s?1.11 s,the average delay time is0.42 s,and the polarization direction of fast wave is mainly NEE direction.The slow wave delay time of the lower crust is between 0.03 s and 1.17 s,the average delay time is 0.43 s,and the fast wave polarization direction is NW-SE.Below the stations in the Sichuan Basin,the fast-wave polarization directions of the upper and lower crusts differ greatly,indicating that the upper and lower crusts are decoupled in the study area.This result is consistent with the results of previous studies.In the Longmenshan fault zone,the polarization directions of the upper and lower crusts are basically the same,indicating that the upper and lower crusts are coupled in this area.Below the station of the Songpan-Garze block,the fast wave polarization directions of the upper and lower crusts are basically orthogonal,and the splitting time of the lower crust is generally longer than that of the upper crust,indicating that the upper and lower crusts are decoupled and the anisotropy of the crust mainly comes from the lower crust.Crust.In order to study the deep crust-mantle structure of The Sichuan Basin and its adjacent areas,the H-K superposition method was used to find that the Moho surface depth decreased from more than 60 kilometers in the Songpan-Ganzi block to about 40 kilometers in the Sichuan basin from north to south.At the junction of the Sichuan Basin and the Longmenshan fault zone,a gradient zone with drastic changes in the depth of the Moho is formed,indicating that the Moho may be affected by faults and its structure is relatively complex.The Poisson's ratio in the study area generally shows a characteristic of gradually increasing from south to north.The Songpan-Garze terrane has a low Poisson's ratio in the north,a medium-high Poisson's ratio in the south,and a Poisson's ratio in the middle of the Sichuan Basin.Medium-high Poisson's ratio.The Poisson's ratio between the Longmenshan fault zone and the Xiaojiang fault zone is extremely high,which may indicate that there is a weak zone or molten material in the lower crust of the fault zone.