Complex seismic anisotropic structure beneath the central Tien Shan revealed by shear wave splitting analyses

Shear wave splitting parameters (fast polarization directions and splitting times) obtained at the permanent broad-band station Wushi (WUS), which is situated in central Tien Shan, China, demonstrate systematic azimuthal variations. The fast polarization directions range from 26° to 85° and the delay times range from 0.4 to 1.6 s, suggesting a complex anisotropic structure that cannot be adequately explained by a single anisotropic layer with horizontal axis of symmetry. The azimuthal variations have an apparent 90° periodicity and thus can be best explained by a two-layer anisotropic model with horizontal symmetry axes. The optimal fast direction for the upper layer is in the 60°–80° range that is subparallel to the strike of the Tien Shan orogenic belt, and can consequently be attributed to the ongoing N–S directed shortening. The fast direction for the lower layer ranges from 85° to 105° and can be explained by lattice-preferred orientation of olivine crystals in the asthenosphere associated with present-day absolute plate motion of the Eurasian plate.