Seismic properties of the central Indian shield

Abstract

We use broadband seismic data at Hyderabad to investigate average crustal properties of the central Indian shield. Crustal receiver-functions (P to SV conversions) based on data of excellent quality and azimuthal coverage show essentially no signal on the transverse component, indicating laterally homogeneous structures near this station. A joint analysis of receiver-functions and the dispersion of fundamental mode Rayleigh waves result in a simple model for the shear wave speed (Vs) with a moderate gradient of 0.010-0.028 km/sec/km in the crust. The Moho is sharp, with a contrast of approximately 0.7 km/sec in Vs across a small thickness of only 0-4 km. Measured from the middle of the Moho region, the crust is 30-34 km thick, with an average Vs of 3.58 ± 0.10 km/sec. By modeling the timing and waveforms of Sp and SsPmp phases (SV to P conversions), we estimate the average crustal Poisson's ratio to be 0.26 ± 0.01. This value is comparable to that found in the Tanzanian craton which is also of Archean age, both values being somewhat lower than the global average of 0.29 ± 0.02 for shield regions that include both Proterozoic and Archean crust. A low Poisson's ratio and a slow Vs in the crust indicate that, on average, Archean crust beneath the central Indian shield is intermediate to felsic in composition, somewhat less mafic than globally averaged Precambrian crust.