Shear-wave Splitting In The Crust In Qinling Orogen And Southeast Of The Yunnan-guizhou Plateau

Fluid-saturated Parallel aligned cracks(EDA cracks) widely exist in the earth’ crust, which is the main cause of crustal anisotropy. Shear wave is a useful way to research the anisotropy of crust. Splitting will occur while shear wave transmits in anisotropy media and we will obtain two important parameters(The polarization of fast shear-wave and the time delay of slow shear wave), which can be used for describing the earthquake anisotropy. The polarization of fast shear-wave is parell el to the strike of the fault and consistent with the direction of principal compressive stress; Time delay of slow shear wave reflects the extent of anisotropic media. The anisotropy in the crust is closely related to regional structure, Shear wave splitting parameter can be used to research the relationship between regional stress and structure, and the degree of regional structure.Yangtze block and north China block are separated by Qinling Orogen. The region is affected by Liupanshan fault, Fenwei fault, Longmen Shan fault. In this study, the polarization of fast shear waves(PFS) and time delay of slow shear-wave beneath the 17 stations in this area were computed by shear-wave Splitting Analysis Method(SAM) based on the seismic data during January 2006 to march 2014, recorded by Shannxi Regional Digital Seismic Network. The result show that: the polarization of fast shear waves(PFS) has a striking regional division character. In east of Ordos basin and Weihe Basin, the PFS exhibits the North-East and East-Northeast direction, which agree with the direction of pressure stress in North China block. West of Ordos basin and Weihe Basin is located in the interchange areas of the Northwestern faults and Northeastern faults. The dominant direction of PFS shows a complex direction, indicating the complicacy of regional structure. The south Qinling orogen is separated by Mianlue belt, north areas is Qinling tiny plate and south areas is located in north of Yangtze plate. The PFS in Qinling tiny is North-West and West-northwest direction, which is according with the direction of pressure stress in South China block. North of Yangtze plate, The dominant direction of PFS is North-East, consistent with strike of faults. Regional time delay is coincident in North of Yangtze plate, bigger than other areas, indicating the bigger anisotropy.The Southeastern Yunnan-Guizhou Plateau is a transition areas, connectting the western and eastern area of Chinese mainland tectonic. The local seismic waveform data are recorded by temporary stations in southwestern Guizhou province and northwestern Guangxi province, and the results from SAM analysis show that: there is an apparent difference between northern and southern areas. The PFS in Northwestern Guangxi is in North-West direction, which is consistent with principal compressive stress of South China block. Southwestern Guizhou is located in transition zone of Youjiang fold belt between South China fold system and Yangtze plate. The PFS in this area exhibits North-East direction, which is perpendicular to principal compressive stress of South China block and probably affected by a series of fault of North-East direction in this place. It also shows that Southwestern Guizhou have a complex regional principal compressive stress and local geological structure. By comparing the time delay of slow shear-wave in Southwestern Yunnan-Guizhou Plateau with Sichuan-Yunnan block and Southeastern Cathaysia block, the crustal seismic anisotropy in Southwestern the Yunnan-Guizhou Plateau is similar to the South China block.