Seismic anisotropy of the Northeastern Tibetan Plateau from shear wave splitting analysis

We present shear wave splitting results obtained from the analysis of teleseismic SKS, SKKS and PKS phases recorded by 70 permanent seismographic stations located in the Northeastern Tibetan Plateau. We identify a contrast in the splitting pattern complexity beneath different parts of NE Tibet. In the western and northern part, anisotropy observations are well explained by a single layer of anisotropy with a fast anisotropic direction trending NWW–SEE or NW–SE. In Xining and its adjacent area, the anisotropy shows strong azimuthal dependence of splitting parameters that can be modeled by two anisotropic layers. The fast direction for the upper layer lies in the N75–95°E range, which is consistent with the surface movement direction determined from GPS, and could be associated with middle to lower crustal flow. The fast direction in the lower layer is in the N105–125°E range and similar to the direction observed in the western and northern part where only a single layer is required. These NWW–SEE or NW–SE fast feature could be related to the current orogenesis induced from the India–Eurasia collision, or flow in the asthenosphere related to the absolute motion of Eurasia. Comparison between the anisotropy patterns expected from proposed models with our shear wave splitting observation suggests that no unique geodynamic model can reconcile all splitting measurements for such a complex region.