Analysis of crustal seismic anisotropy of the Eastern Himalayan collision zone and its adjoining regions

Abstract

The crustal anisotropy of the Eastern Himalaya collision zone, Brahmaputra Valley and Shillong Plateau of Northeast India Region, is investigated by measuring the shear wave splitting of the PmS phase of teleseismic earthquakes recorded by 17 seismic stations. The anisotropic parameters are nearly stable for each tectonic regime. The mean value of delay time for the Eastern Himalaya stations is 0.45 s which indicates significant crustal anisotropy originated mostly from mid to lower crustal depth. Some stations near deep-seated fault zones exhibit fault-parallel fast polarization directions, indicating structure-induced crustal anisotropy. The fast polarization directions are mostly parallel or sub-parallel to the Himalayan arc suggesting the effect of extension tectonics predominant in the lower crustal depth owing to NW-SE compression tectonics. The maximum shearing of anisotropic minerals presents in the lower crust along the extension direction is the primary causing factor of observed anisotropy. Comparision of the PmS phase splitting with fast polarization directions from shear wave splitting of local earthquakes and stress field analysis demonstrates large angle differences at most stations, imply contrasting anisotropy properties between upper and mid-lower crust.