Significant seismic anisotropy beneath the southern Lhasa Terrane, Tibetan Plateau

Shear wave splitting measurements using teleseismic PKS, SKKS, and SKS phases recorded by station LSA on the southern part of the Lhasa Terrane of the Tibetan Plateau reveal significant azimuthal anisotropy with a splitting time of up to 1.5 s, a conclusion that is contradictory to previous studies which suggested isotropy or weak anisotropy. In addition, systematic variations of the splitting parameters (fast polarization direction and splitting time) with the arriving azimuth of the seismic ray path are observed, suggesting a model of anisotropy that is more complicated than a single layer with horizontal axis of symmetry. The measurements are consistent with a model with two layers of anisotropy. The top layer has a NE-SW fast direction, which is consistent with GPS-revealed direction of surface movement, and can be associated with lattice preferred orientation of middle-lower crustal minerals such as amphibole. The lower layer has a nearly E-W fast direction and can be the consequence of either the N-S directed compressional stress originated from the India-Eurasia collision or flow in the asthenosphere related to the absolute motion of Eurasia. This study underlines the importance of a long duration of deployment of seismic stations in resolving complex anisotropy.