Crustal Structure of the Western Bengal Basin from Joint Analysis of Teleseismic Receiver Functions and Rayleigh-Wave Dispersion

Abstract

Teleseismic data recorded at the broadband seismological observatory of the Indian Institute of Technology (IIT) Kharagpur are analyzed to determine the seismic characteristics of the crust beneath the western Bengal basin. Receiver functions calculated from the teleseismic P waveform for a range of back azimuths show little variation in the Moho Ps arrival time, indicating a nearly one-dimensional crustal structure beneath Kharagpur. Transforming the time domain receiver functions into the H - V p / V s domain for a stack of all receiver functions reveals a 38±2 km thick crust with a V p / V s of 1.73±0.01. Receiver functions stacked in narrow bins of back azimuth and great-circle-arc distance were inverted jointly with group velocity dispersion data for periods from 15 to 45 sec. Joint inversion results reveal a near surface sedimentary layer of ∼1.5 km thickness, a 13 km thick upper crust, and a 23 km thick lower crust with the Moho at a depth of ∼37.5 km. The average crustal S -wave velocity is 3.7 km/sec. The top 0.5 km of the sedimentary layer has very low S -wave velocity and a large impedance contrast with the underlying layer. This represents the thin layer of unconsolidated sediments and weathered laterites near the surface as seen from deep seismic sounding (DSS) studies across the region. The S -wave velocity structure of the crust beneath Kharagpur is the first such result for the western Bengal basin and is broadly similar to that of the Indian Shield crust. A comparison of the crustal thickness and lower crustal velocities between Kharagpur and Agartala, seperated by the Eocene hinge zone, shows remarkable differences. Crustal models beneath Agartala reveal ∼36 km thick crust with the presence of a high-velocity lower crust (∼6 km thick), possibly oceanic in nature, which is absent beneath Kharagpur. The hinge zone, therefore, separates the continental crust of the western Bengal basin from the eastern deep basinal oceanic crust, both being overlain by sediments derived from the Himalayas.