A radially anisotropic Shear wave velocity structure of the Himalaya

Abstract:

A precise shear wave velocity (Vs) structure is a first-order proxy to understand the structural, thermal and deformational configurations of the lithosphere. We use surface wave data from the Dolakha Nepal earthquake recorded by 8 stations in Jammu and Kashmir Himalaya (JAKSNET) to compute fundamental mode Rayleigh and Love wave dispersion curves. We perform joint inversion of the two datasets to obtain radially anisotropic shear-wave velocity model (V_SH and V_SV) for the Himalayan lithosphere. Dispersion curves have been computed between 15 and 90 s period using the multiple filter analysis. The individual source-receiver dispersion curves have been averaged by error weighting to obtain the mean and standard deviation at discrete periods. The starting model for the joint inversion is constructed by combining CRUST1.0 and PREM, for the crust and upper mantle structure, respectively. In the inversion, the model space is exhaustively explored using Genetic Algorithms (GA), and the synthetic dispersion curves are computed using Thomson-Haskell method with reduced delta matrix, where depth decay factors (r₁ and r₂) are complex conjugates. The free parameters in the inversion are horizontally polarized compressional and shear wave velocities (V_PH and V_SH), layer thickness (h) and Vs anisotropy represented by Xi (ξ=V_SH²/V_SV²). The misfit between the calculated and observed datasets are iteratively minimised to obtain the best fitting model. Results reveal variation of ξ as a function of depth. The lower crust has ξ<1, which results from V_SV being faster than V_SH. However, in the upper mantle ξ >1, and hence V_SH is greater than V_SV. This change of ξ from the lower crust to the upper mantle points to deformational decoupling.