Tomographic image of shear wave structure of NE India based on analysis of love wave data

Abstract

Crust and uppermost mantle shear-wave velocity in northeast India is elaborated based on the non-linear inversion of the Love wave group velocity data. This is the first time Love wave and SH wave tomography have been carried out in this region. We investigated the vertical and lateral variation of the SH wave velocity and the crustal thickness. We used the data from 26 broadband seismic stations that recorded the earthquakes from 2001 to 2015. Group velocity dispersion curves of 228 moderate and higher magnitude earthquakes are inverted to obtain tomography images of group velocities at periods 6–60 s. Low velocity is confined to the Bengal Basin (BB) and the Indo-Burma Ranges (IBR) at short periods. In contrast, high velocity is present in the Shillong Plateau (SP), Mikir Hills (MH), and Assam syntaxis. Higher velocity indicates an upward buckled crustal structure. The Tibetan plateau has a low velocity, indicating a thick crust and partial melt in the middle to lower crust. The inverted SH wave velocity models also show similar patterns. Based on such analysis, variation in the Moho depth below the study area could be estimated. Moho depth below SP and MH varies from 35 to 45 km. Moho depth below BB varies between ~ 28 to ~ 32 km. The uppermost crust in BB indicates a thick sediment deposit of ~ 15- ~ 20 km. The Moho depth of NE India gradually increases towards the Eastern Himalayas, Lhasa block, and IBR. Steeply dipping Moho below IBR indicates the subduction of the Indian plate below the Burma plate. The Tibet and Lhasa blocks show a crustal thickness of ~ 85 km, which is the maximum compared to other parts of the study area. Highly variable shear wave crustal structures, added with low/high-velocity anomalies at different depths, give new insights into the heterogeneous and complex geotectonics.