Upper Mantle Anisotropy beneath the Western Segment, NW Indian Himalaya, Using Shear Wave Splitting

Shubhasmita Biswal,Sushil Kumar,M. Ravi Kumar,W. K. Mohanty,Ajay Paul,Sunil K. Roy,Mahesh Prasad Parija,Sarah M. Roeske

doi:10.2113/2020/8856812

Abstract

Abstract This study investigates the upper mantle deformation pattern beneath the Indo-Eurasia collision zone utilizing the core-refracted (S(K)KS) phases from 167 earthquakes recorded by 20 broadband seismic stations deployed in the Western Himalaya. The 76 new shear wave splitting measurements reveal that the fast polarization azimuths (FPAs) are mainly oriented in the ENE-WSW direction, with the delay times varying between 0.2 and 1.7 s. The FPAs at most of the stations tend to be orthogonal to the major geological boundaries in the Western Himalaya. The average trend of the FPAs at each station indicates that the seismic anisotropy is primarily caused due to strain-induced deformation in the top ~200 km of the upper mantle as a result of the ongoing Indo-Eurasian collision. A contribution from the mantle flow in the direction of the Indian plate motion is possible. The mantle strain revealed in the present study may be due to a combination of basal shear resulting from plate motion and ductile flow along the collision front due to compression.

Highlights

The Himalaya-Tibet orogen is a result of a collision between the Indian and Eurasian plates that has been ongoing since 50 Ma
The fast polarization azimuths (FPAs) at station KHI in the western part is along the plate motion direction, while in the eastern part, station GTU shows an orientation along the ENE-WSW direction
This study presents results of shear wave splitting analysis of core-refracted phases (SK(K)S) at 20 broadband seismological stations across the Western Himalaya

Summary

Introduction

The Himalaya-Tibet orogen is a result of a collision between the Indian and Eurasian plates that has been ongoing since 50 Ma. Seismic anisotropy is an intrinsic property associated with the earth’s medium that characterizes a change in the velocity of a seismic wave with direction [3, 4]. This causes the shear wave to split into two components while passing through an anisotropic medium. These two components are orthogonally polarized and travel with different velocities, with the component that travels parallel to the fast direction moving faster than the other.

Results

Discussion

Conclusion