Characteristics Of Deep-shallow Crustal Deformation In The Southeast Margin Of Tibet Plateau Constrained By GNSS And Seismic Data

The southeastern margin of the Tibet Plateau has the characteristics of complex topography and frequent earthquakes.Its surface deformation mechanism and stressstrain state have always been hotspots in geoscience research,but the deformation mechanism of the southeastern margin of the Tibet Plateau has always been a controversial topic.Is the deformation of the earth's crust continuous? Is the surface coupled with the crust? Are the crustal deformation characteristics consistent at different depths? Scientific research on the above issues is of great significance for understanding the deformation mechanism of the southeastern margin of the Tibet Plateau.Starting from the three research directions of strain rate field,tectonic stress field and seismic anisotropy,this paper jointly analyzes the structural deformation mechanism at different depths on the southeastern margin of the Tibet Plateau.Based on the GNSS data from 1999 to 2016,this paper analyzes the displacement field and deformation field of the southeastern margin of the Tibet Plateau.The GNSS velocity field under the Eurasian framework shows that the movement velocity of the block inside the Tibet block is 21.08mm/a,the movement velocity of the block around the Xianshuihe fault is 10.96mm/a,and the movement velocity on the north side of the Lijiang-Xiaojin River is about 17.33mm/a.a,the south side is about 14.53mm/a,the movement rate of the southwestern Yunnan block is 4.13mm/a,and the movement rate of the South China block is 9.04mm/a.It is found that the Lijiang-Xiaojinhe fault,the Red River fault and the Xiaojiang fault all affect the block movement and material migration play a regulatory and control role,and the regulation of the Lijiang-Xiaojinhe fault is relatively weak.Based on the measured velocity field and uniform velocity field,the least squares collocation method,multi-scale spherical wavelet method,and Gaussian weighted interpolation method are used to solve the problem.It is found that the multi-scale spherical wavelet method is more applicable in the southeastern margin of the Tibet Plateau.Taking into account the uneven distribution of stations,the spatial resolution is higher.The multi-scale spherical wavelet method is used to solve the strain rate field on the southeastern margin of the Tibet Plateau.The principal strain rate extremes are concentrated on the eastern boundary of the Sichuan-Yunnan block,and there is a clockwise rotation trend from south to north.The east boundary of the Sichuan-Yunnan block is also the high value area of the maximum shear strain rate,which is distributed parallel to the fault strike.The area strain rate study shows that the expansion and compression of the southeastern margin of the Tibet Plateau are alternating characteristics.Rotational strain rate research shows that the eastern boundary of the Sichuan-Yunnan block has a counterclockwise rotation,and the west side of the Red River fault has a clockwise rotation,which is consistent with the rotation trend of the eastern Himalayan structure.The tectonic stress field in the southeastern margin of the Tibet Plateau is calculated based on the 1205 sets of seismic focal mechanism solution data.The Pressure and Tension axes obtained by the calculation show that there are zoning characteristics at both ends of the Sichuan-Yunnan block.The tectonic stress field has good continuity in the spatial distribution,and the optimal distribution direction of the maximum principal stress axis is similar to that of the GNSS velocity field,presenting a clockwise rotation around the Himalayan tectonic knot.The axis of maximum principal stress is near EW in the north of Lijiang-Xiaojinhe fault,and changes to near NS in the south of LijiangXiaojinhe fault,showing divergence characteristics consistent with the GNSS displacement direction.The results of depth inversion of the tectonic stress field show that the tectonic stress field changes continuously within 0-40 km.It is inferred that the crust is coupled within 0-40 km.The Lijiang-Xiaojinhe fault is a transition zone for material escape on the Tibet Plateau.Combined with the analysis of shear strain,shear strain gradient and historical earthquake catalog,the Longmenshan fault zone is an area with low values of shear strain and shear strain gradient.It is inferred that the Wenchuan earthquake and aftershocks will basically release the strain;the east of Sichuan-Yunnan diamond block The boundary and the middle-south section of the Red River fault are areas of high shear strain,and are the transition areas of high and low shear strain gradients.There have been many earthquakes of magnitude 5 or higher around the area,and the strain has not been fully released.It is inferred that the above fracture areas have a certain degree of Earthquake risk.The GNSS strain rate can reflect the characteristics of surface deformation,the tectonic stress field can reflect the deformation characteristics of earthquakes at different depths,and the seismic anisotropy can reflect the deformation characteristics of the crust and upper mantle at different depths.The combined analysis of the three can explore the deep and shallow structural deformation of the Tibet Plateau.feature.According to the analysis results of strain rate field,tectonic stress field,and nearearthquake S-wave anisotropy,the surface of the southeast margin of the Tibet Plateau is basically coupled with the upper and middle crust,and the deformation of the two is consistent.According to the results of strain rate field and Pms anisotropy analysis,the crustal deformation is coupled as a whole,roughly bounded by Lijiang-Xiaojin River,and there are different deformation modes on the north and south sides.According to the results of strain rate field and XKS anisotropy analysis,the principal compressive strain and seismic anisotropy distributed along the strike of the Lijiang-Xiaojinhe fault maintain good consistency in the crust and upper mantle,while the rest of the region is quite different It is speculated that the crust and upper mantle are decoupled in the region,and the Lijiang-Xiaojinhe fault extends from the surface to the upper mantle,blocking the tectonic movement on the north and south sides of the Sichuan-Yunnan block,leading to different deformation modes.