Seismic anisotropy in the crust in northeast margin of Tibetan Plateau and tectonic implication

The northeastern margin of Tibetan plateau is surrounded by the Alashan block to the north, the Tarim basin to the northwest, the Ordos block to the east, and Songpan-Garze terrane to the south. As the northern terminus of contiguous deformation, it records far field effects of collision and convergence occurring between the Indian and Eurasian, it is still in the stages of uplift and deformation, so it is an ideal region for studying the far field effects of collision and convergence between the India and Eurasian plate. Seismic anisotropy is one of the main specialities of the Earth, which can be interpreted with the Lattice Preferred Orientation (LPO) of minerals and shape Preferred Orientation of cracks or melts caused by tectonic stress. In this study, we use the seismic data recorded by the Gansu and Ningxia Province Seismic Network from Jan. 2001 to Oct. 2010. And seismic anisotropy is obtained by using the SAM technique. The average time delay and polarization are caused by the regional structure and stress field, whereas the residual values of the splitting parameters are considered to be related to local structures such as local faults. The residual polarization of fast shear-waves suggests the strike direction of local fault. The residual delay of slow shear-waves suggests the contribution extent induced by local fault. During our data process, we take the posterior choice on earthquakes with small epicenters that is a makeshift for the regions lacking earthquakes, but is useful to increase the credibility of the analysis for fast-wave polarization. The results show that the crustal seismic anisotropy in the northeastern margin of Tibetan vary in different tectonic blocks. There are the two average polarization of fast shear-waves, which are individually at NE47. 72 degrees +/- 21.83 degrees and 121.65 degrees +/- 22.07 degrees, and the average time delay of slow shear-waves is 2.63 +/- 1.03 ms . km(-1). The first dominant polarization of fast shear-waves is almost consistent with the regional principal compressive stress due to collision between Indian and Asian plate. The second predominant polarization direction of fast shear-waves is in the NWW direction which indicates the influence by the deep fault. The delay time of slow shearwaves increases from southweastern to north eastern in the study area. The residual polarization of fast shear-waves are almost consistent with the strike of local fault, and the residual delay indicates the contribution extent induced by the local faults that means which part has a higher effect. Compared with the upper mantle anisotropy results, crust-mantle anisotropy are different in Qilian-Hexi Corrider, while they have some consistence in the southeast of Gansu.