The Crust and Upper Mantle Structure Beneath the Bangladesh and Its Effects on Seismic Hazard

Abstract

Bangladesh is one of the most densely populated and seismically active region of the world, which is situated on the easternmost part of the India-Eurasia collision zone. It lies at the junction of the three plates—the Indian plate, the Eurasian plate and the Burma plate. With a population over 160 million, the nation Bangladesh is located on a seismically active fold and thrust belt that forms the updip point of an active, oblique subduction zone plunging to the east beneath Myanmar on the eastern edge of the India-Eurasia collision zone. Approximately 15–20 km of sediments have been deposited here during past 55 million years. The process of sedimentation continues till today as the rivers Brahmaputra, Ganges and Meghna flow across the nation Bangladesh and formed the Ganges–Brahmaputra Delta (GBD), the largest delta in the world. The region is mainly an accretionary prism made up mostly of sediments from the cretaceous to the Eocene. Its location on the largest river delta in the world and proximity to the sea puts it at risk for tsunamis and the potential for rivers to overflow their banks in the case of an earthquake. Due to a scarcity of seismic data from Bangladesh, it is still difficult to comprehend the structure of the Bengal Basin and the tectonic forces, which are main sources of its deformation. Understanding the structure beneath Bangladesh is one of the important aspects in order to evaluate its vulnerability towards the earthquake hazard scenario. This study will present an improved 3-D shear wave velocity image of the crust and uppermost mantle beneath Bangladesh region. The shear velocities are obtained from the joint inversion of the receiver function and fundamental mode Rayleigh wave group velocities, which are calculated from the cross-correlation of ambient noise as well as earthquake seismogram. Our findings show that the crust and the upper mantle structure beneath Bangladesh region is highly varying. The shear wave velocities of the sediments have been observed up to a depth of ~15 km. We can clearly see the subduction pattern in terms of the velocity along the east–west profile from the Indian craton across the Ganga Brahmaputra Delta (GBD) underthrusting the Burma arc of south eastern Asian plate. The findings help us to understand the particular episode of subduction by placing important restrictions on the sedimentary and lithospheric features throughout the system.