Preliminary Analysis Of Multi-Layered Lithospheric Anisotropic Characteristics In The North Part And The South Part Within The North-South Seismic Zone

Located in the central and western part of China,the North-South Seismic Zone is the intersection of the Tibetan Plateau Block,the Ordos Block,the South China Block and the Myanmar-Yunnan Block.There are several large faults,strong earthquakes occur frequently.In this paper,the study areas include the north and south part of South-North Seismic Zone(from Haiyuan Fault to Yinchuan graben and Sichuan-Yunnan Rhombic Block).They are two main channels of the eastward flow of material in the Tibetan Plateau Block,have complex structures,different press,different material flow and strong anisotropy by blocks compressing.XKS(SKS,PKS,SKKS)wave splitting is one of the important methods to anisotropy.The direction of fast wave can reflect the crustal stress and material flow direction below the station,and the splitting time can be used to calculate the thickness or intensity of anisotropic layer.By XKS wave splitting,the relationship between crustal and mantle anisotropy in this region can be discussed,which reveals the basis for the form and distribution of material flow eastward in this region and the model of lithospheric deformation.Considering part of the study area may be mulit-layered,if the directions of fast wave change periodically,we try to get both layers of anisotropic parameters by using double layers model,and discuss the distribution of the anisotropic layers according to the layers of fast wave directions and splitting times.The crustal stress and the direction of deep material flow in the study area are analyzed based on the data of the single layer model stations,double layers stations and other researches.The data were obtained from XKS seismic waves of fixed stations in Ningxia,Shanxi,Sichuan and Yunnan.The data in the north part of South-North Seismic Zoneis from 2010 to 2019,the data in the south part of South-North Seismic Zone is from2007 to 2010.The results show that in the northern part of the South-North Seismic Zone,fast wave direction of the upper layer of Haiyuan Fault is 133oN and the splitting time is 1.3 s.The lower fast wave direction is 57oN and the splitting time is0.7 s.The upper layer of Yinchuan graben has a fast wave direction of 74oN and a splitting time of 0.3 s.The lower fast wave direction is 11oN and the splitting time is1.1 s,which is different from the traditional single layer XKS wave splitting results.According to the formula,the accuracy of the fast wave direction and splitting time obtained by the double layers model inversion method is estimated.It shows that the method has higher accuracy in calculating the fast wave direction and lower accuracy in calculating the splitting time.The anisotropy direction of the upper layer of both regions is close to the strike of the fault.Combining with the imaging results of the upper mantle region,there may be an upper mantle flow roughly in the NE direction under the northeast margin of The Tibetan Plateau Block,which turns to flow in the near N-S direction due to the obstruction of the Ordos Block.At the Sichuan-Yunnan Rhombic Block in the southern part of the South-North Seismic Zone,layered anisotropy is observed only by KMI station.The upper fast wave direction is 23oN and the splitting time is 0.3 s,which is close to the fault strike nearby and the s-wave splitting results in the near field.The lower fast wave direction is 69 o N and the splitting time is 0.6 s,which is close to the results of the nearby station XKS single layer station.It is found in the study that the boundary in the Sichuan-Yunnan Rhombic Block is 26o20' N,the northern fast wave direction is roughly N-S direction,and the southern is E-W direction.The data are similar to those obtained by others in this region.Combined with the study of local crustal thickness,Poisson's ratio and tectonic setting,the northern crust and lithosphere are thicker and harder than the southern,mantle material in the southern region may flows eastward around 26oN due to the obstruction of the northern crust.The amount of data used in the study is small,and the seismic azimuth distribution is not well,so the constraints in the application of the double layers modelare insufficient,the result is preliminary.Especially,in the Sichuan-Yunnan Rhombic block,only a few stations and data were used.The result is a preliminary attempt,which need more data to carry out further research in the future.