Seismic anisotropy beneath the eastern margin of the Tibetan Plateau from SKS splitting observations

Abstract

The eastern margin of the Tibetan Plateau is one of the areas of the entire region with the most complex deformation. Seismic anisotropy reveals the imprints of past and present deformation in the lithosphere and asthenosphere, which could help elucidate the deformation mechanism. This study investigated the characteristics of seismic anisotropy beneath 512 seismic stations based on SKS phases from ~200 earthquake. The measured splitting delay time varied from 0.2–1.4 s (average: 0.7 s). In the central Sichuan basin (SCB), the average delay time is only ~0.35 s, consistent with the characteristics of the relatively thick and rigid lithosphere of the stable SCB. However, the northeastern SCB has relatively stronger anisotropy with a fast polarization direction (FPD) with NE-SW orientation, which is markedly different from the weak anisotropy with NW–SE orientation in the central SCB. We speculate that owing to E–W compressive stress caused by eastward escape of material from the Tibetan Plateau, the lithosphere in this area has undergone considerable deformation along the right-lateral maximum shear direction. In the Chuandian diamond block, the FPD of several adjacent stations was approximately E-W, i.e., almost perpendicular to the FPD in the northern and southern regions, which might be related to partial melting in the lithosphere and to compressive stress. The FPD in the west Qinling orogenic belt is approximately parallel to the strike of the surface faults and the average delay time is approximately 1.3 s. As the lithosphere is insufficient to produce such strong anisotropy in this region, we infer that the lithosphere and asthenosphere both contribute to the seismic anisotropy. Delay time in the southern Songpan-Ganzi block is much smaller than in the north, which might be attributable to low-velocity mantle upwelling and complex tectonic stress in this area.