Crust and Upper Mantle Shear Velocity Structure and Azimuthal Anisotropy of Northeastern Tibet from Rayleigh-wave Two-station Analysis

Wednesday, 17 December 2014
Yuan Yang, USTC University of Science and Technology of China, Hefei, China and Huajian Yao, University of Science and Technology of China, Laboratory of Seismology and Physics of Earth’s Interior, Hefei, China
Surface wave tomography is an important tool to study crust and upper mantle structure as well as seismic anisotropy, therefore providing insights into patterns of deformation in tectonically active regions. As we known, depth sensitivity of surface waves to shear wave speeds varies with period; therefore, we can probe seismic anisotropy at different depths from the inversion of surface wave dispersion data. In this study we use the earthquake surface-wave two-station method to determine the inter-station Rayleigh wave phase velocity dispersion curves in NE Tibet. We use 107 stations in NE Tibet, which are mainly from the INDEPTH-IV project and some regional networks. Rayleigh waves from 628 teleseismic events are analyzed to obtain 16771 dispersion curves in the period band 20-150 s, which are finally used to produce 2619 average inter-station dispersion curves. Then we perform surface wave tomography at various periods to simultaneously obtain phase speed variations and azimuthal anisotropy. The fast directions at different periods and the magnitudes of anisotropy are not alike in different regions. In some region the variation of fast axes with periods is small, possibly implying a vertically coherent deformation pattern there. There are regions with large changes of fast directions with periods, indicating complex deformation patterns in NE Tibet. We observe clear fault-parallel fast axes at intermediate periods, implying that the lithospheric deformation may be controlled by big strike-slip faults regionally in NE Tibet, for instance, the Kunlun fault. In the next step, we are planning to invert for 3-D depth-dependent shear wavespeeds and azimuthal anisotropy from the azimuthally anisotropic dispersion curve at each grid point using a Neighborhood Algorithm. This will provide more direct constraints on depth-dependent deformation patterns in the crust and upper mantle in NE Tibet.