Radially anisotropic shear-wave velocity structure of northern India, Himalaya and Tibet

<p><strong>3-component regional waveforms for ~14700 raypaths sampling India, Himalaya and Tibet, have been used for multi-taper polarization analysis of surface waves between periods of 10 and 120 s. Rayleigh (LR) and Love (LQ) wave energy arriving at the station within +/- 10 degree of the theoretical back azimuth have been used to compute 1D path average fundamental mode group velocity dispersion. Theoretical dispersion of fundamental and first two higher modes have been computed using each 1D path average velocity structure constructed from CRUST1.0 over IASP91 mantle model. These are compared with the observed dispersion dataset to identify and remove those periods with higher mode overlap with the observed fundamental mode picks. The shortlisted dispersion datasets consist of ~90% of the original dataset. To ascertain the lateral variation in the group velocity, the observed dispersion has been used to compute 2D tomography maps of LR and LQ group velocities at discrete periods between 10 and 120 s. From these maps, seven 2D profiles across northern India, Himalaya and Tibet have been extracted for modeling the radially anisotropic shear-wave velocity structure of the lithosphere. Haskell-Thompson (H-T) matrix method is used to calculate synthetic LQ dispersion. For the LR dispersion, the H-T method with reduced delta matrix has been used, considering Vph&#8800;Vpv and Eta&#8800;1. The inversion scheme uses genetic algorithms (GA) to search the model space parameterized using Vsh, Vph, Xi and thickness for 3 crustal layers and 2 mantle layers underlain by a mantle half-space. Synthetic tests have been performed using theoretical LR and LQ dispersion curves, computed from global models with 5% and 10% anisotropy, introduced in the mantle layers. The fit to the synthetic LR and LQ dispersion data and the model recovery using GA inversion is satisfactory for such tests. This inversion scheme is being applied to the observed LR and LQ dispersion data from the seven profiles and the results will be presented.</strong></p>