Seismic Shear Wave Splitting And Medium Anisotropy In Shaanxi Region

The study of seismic anisotropy has been paid much attention by many scholars from various countries.The shear wave splitting method is one of the most common methods for studying the anisotropy of the earth medium.The shear waves passing through anisotropic media of the crust and mantle will split into fast shear wave and slow shear wave with different polarization directions and propagation speeds,so the shear wave splitting phenomenon can be used to study the direction and intensity of the anisotropy of the earth's crust and mantle medium.Shaanxi and its surrounding areas are located in the intersection area between the northeastern margin of the Qinghai-Tibet Plateau and the North China block,including the Alashan block,the Ordos block,the Fenwei fault zone,and the Qinling orogenic belt,At the same time,it is affected by the northward push of the Indian plate and the westward subduction of the Pacific plate.The study of geodynamic processes in this area has attracted much attention.In this paper,the anisotropy of underground media in Shaanxi is studied by the shear wave splitting method.In this paper,the area of study is between 31 ° N-40 ° N and 105 ° E-112 ° E.Select the SKS wave data with the magnitude greater than 5.5 and the epicentral distance distribution within the range of 90-110 ° and S wave data of local earthquake with the incident angle ?45 °.The correlation analysis method,the tangential energy minimization method,and the eigenvalue method were used to calculate and analyze the teleseismic SKS wave data of 60 stations in Shaanxi and its surrounding areas.The results show that the fast wave direction in the western edge of Ordos is the NW-SE direction,the direction of the fast wave is NWW-SEE and near EW in the south and east margins of the block and the Weihe Basin area,which may be squeezed and flowed near NWW-SEE due to the compression of the Qinghai-Tibet Plateau near the NEE-SWW.Thus,this area produces near-NWW-SEE fast wave,which is basically consistent with the direction of the GPS velocity field and the direction of the Qinling orogenic belt.The Fenhe fault on the eastern edge of the Ordos block shows the EW direction,and the northeast margin of the Ordos block is the NWW-SEE direction,which is basically consistent with the direction of the GPS velocity field.From the calculation of the slow wave average time delay,the thickness of the anisotropic layer in shaanxi area is 119 km.Combined with the crustal thickness of the study area,it is judged that the anisotropic layer is distributed in the range of the crust and upper mantle.The correlation analysis method was used to perform S-wave splitting calculation on near-earthquake S-wave data of 27 stations in Shaanxi and its surrounding areas,and it was found that the dominant direction of the fast shear wave polarization on both sides of the Qinling orogenic belt is the NWW-SEE direction.This direction is consistent with the GPS velocity field in the area and the direction of the Qinling orogenic belt,indicating that the polarization direction of fast waves is affected by crustal movement.The directions of SKS wave splitting fast wave and S wave splitting fast wave in the area near the Qinling orogenic belt are in good agreement with the direction of the GPS velocity field.It can be preliminarily inferred that there is consistency between the movement of crustal material and the movement of upper mantle material in the study area.There may be a certain coupling relationship in them.