Seismic anisotropy of the northeastern margin of the Tibetan Plateau derived from analysis of SKS and Pms seismic phases

Abstract

Rock deformation may produce anisotropy, thus, seismic anisotropy contains deformation of the crust and upper mantle. Based on teleseismic SKS and Pms phases recorded by permanent broadband stations in Gansu and Qinghai provinces in the northeastern margin of the Tibetan Plateau, shear-wave splitting analysis is made to investigate the seismic anisotropy of crust and upper mantle in this region, which permits to infer the related deformation mechanism. This study obtains 230 pairs of splitting parameters at 56 stations using the minimum transverse energy method and rotation correlation method jointly from teleseismic SKS phase. The splitting parameters of crustal anisotropy were also revealed by the data from 24 stations using the sinusoidal moveout of the P-to-S converted phase from the Moho (Pms) in receiver functions. The matches and mismatches between observations at the surface and those from deep structures provide critical clues regarding the nature of the geodynamic process. Our shear-wave splitting results show that the average fast polarization directions of SKS and Pms are 123 degrees and 132 degrees, respectively. The NW-SE directed fast directions of crust and upper mantle are generally consistent with the main active structures on the surface, suggesting a vertically coherent deformation within the lithosphere. The average splitting times of SKS and Pms are 1. 0 s and 0.6 s, respectively, indicating that crustal anisotropy appears to play an important role in explaining the amount of SKS splitting times. The fast directions of SKS and Pms near the Kunlun fault are approximately parallel to its strike, which suggests that the Kunlun fault cut through the lithospheric mantle. However, the angle difference between the fast directions of SKS and Pms at the eastern Altyn Tagh fault implies the fault is a crustal-scale structure at its eastern end.