3D TOMOGRAPHIC MODELLING FOR NE INDIA USING SURFACE WAVES

Abstract

This thesis aims to explore the variation of the shear wave velocity structure for Northeast India and its surrounding region by using surface waves. Surface wave group velocities are obtained from recorded earthquakes on regional distances in NE India. I carried out three different types of studies for the study area using the available earthquake data. A brief description of the results and interpretation are given below. Surface wave group velocity is used to infer the shear wave velocity structure variation in the northeast Himalayas and Indo-Burmese arc regions. The motivation of the present study is to find out the possible results of shear wave velocity variation in the study area to resolve the geotectonic complexity in NE India. NE India is one of the most seismically active regions of the world which lies in the highest seismic hazard zone of India. The region is tectonically very complex and lies between the thrust tectonics in the Himalayas to the north, subduction tectonics along the Indo-Burmese ranges in the east, and indention tectonics in the north-east edge of the Indian plate. The region has experienced two great earthquakes i.e. Mw 8.5 the Assam earthquake of 1950 and Mw 8.4 Shillong Plateau earthquake of 1897. The region also frequently experiences large and strong magnitude earthquakes. These all make it very interesting to investigate the shear wave sub-surface crustal structure to understand the geodynamic process of the region. I used the data of moderate and higher magnitude recent earthquakes which frequently occur in NE India and adjoining parts. The data is available on the regional broadband seismological network. However, there are very few stations in the region located near the borders of different countries in remote locations with variable topography. I started my research career by learning very simple intricacies of earthquake waveform data with a main focus on surface wave data and its applications. In a first attempt, I used earthquake records from the Shillong station to investigate the surface wave dispersion characteristics and inverted them to obtain shear wave velocities from different parts of tectonic plates around the Shillong Plateau. I used 25 earthquake data from four groups with a magnitude range of 5.0–6.7, epicentral distance range of 368–800 km, and focal depth of less than 50 km recorded at Shillong station. Ray paths transversely pass through different geotectonic units of the NE Himalayas, Indo-Gangetic Plains, and Indo-Burma collision zones. The weighted average dispersion curve and the path-averaged shear wave velocity models are computed at different azimuths around Shillong Plateau. Non-linear least-square inversion is performed to obtain the shear wave velocity structure of the crust and uppermost mantle from individual and joint inversion of Rayleigh and Love waves group velocities. Next, using these shear wave velocities anisotropy coefficient values at different depths for each path are estimated. A high variation of group velocity and the shear wave velocity models from one group to another indicates that the region is geotectonically very complex. Group velocity for both Love and Rayleigh waves could be estimated for the period range of ~4 s to ~50 s to obtain S-wave velocity for ~80 km thick zones. Inferred velocities for all the directions are mostly lower than that of PREM and AK135 global models with a much thicker crust in some parts of the study region. The path passing through the Bengal Basin and the Indo-Burma Ranges have low shear wave velocity in the upper ~10 km, indicating the effect of the presence of low-velocity sediments. The overall trend for the data of the four groups has nearly increasing velocities for the upper 45 km depth with very close structure for ~30–45 km. Deeper regions have a low-velocity zone for the paths toward the Indo-Eurasia collision zone and the northern part of the Indo-Burma Ranges. This may indicate the presence of fluid-filled fractures or partial melting in the lower crust or uppermost mantle. 1D Vs structure beneath the Eastern Himalayas has an average crustal thickness of ~ 65 km varying from ~45 km beneath the Eastern Himalayas Syntaxis and the southern part of the Indo-Burma Ranges. Radial anisotropy varies even within the northern part from the Indo-Eurasian collision zone to EHS and northern to southern Indo-Burma ranges.