Crust and upper mantle velocity structure beneath north and central India from the phase and group velocity of Rayleigh and Love waves

Abstract

Crust and upper mantle velocity structure is determined using the fundamental mode Rayleigh and Love waves for 23 different paths across north and central India. The earthquakes occurring in Kashmir, Nepal Himalaya, Burma and nearby regions and recorded at Hyderabad, New Delhi and Kodaikanal seismic stations are used for dispersion studies. Frequency Time Analysis (FTAN) using the multiple filter technique is done for the fundamental mode surface waves to estimate group velocities from the time periods of 13–71 s. The single station technique is used to estimate Rayleigh and Love wave phase velocities at time periods of 15–75 s. Velocity structure down to a depth of 120 km is determined using both Rayleigh and Love wave phase and group velocities. The crustal thickness is found to be 50 km, with 4 km sedimentary thickness, for the northern India across Burma to New Delhi path, and 44 km for the central India across Kashmir, Nepal Himalaya and northeast India to Hyderabad and Kodaikanal path. A low-velocity zone ( V p = 8.4 km/s, V s = 4.15 km/s ) is found from 100 km below the surface. Velocity structure shows that the shield structure of the peninsula extends up to northernmost India below sub-Himalayan regions with an overlying sedimentary thickness of 4 km. The lower value of observed group velocity and higher phase velocity across northern and central India may be caused by the presence of varying sedimentary thickness in the northernmost part of India (the Indo-Gangetic Plain) below the sub-Himalayan region. The observed lower value of Rayleigh wave group velocities below 40 s time periods can be explained by assuming a thin layer of a high contrast material with a P-wave velocity of 4.6 km/s, an S-wave velocity of 2.5 km/s, a density of 3.24 g/cm 3 and a thickness of 3 km, centred above the Moho.