BODY WAVE TRAVEL TIME STUDIES AND INFERENCES

A variety of techniques are well developed for body wave travel times analysis and study of crustal and upper mantle velocity structure in India. P wave travel time curves determined from shallow earthquakes data in various directions from India reveal significant variations while S wave travel times show comparatively better agreement with respect to the world average JB travel time tables. Both P and S wave velocities in the uppermost mantle beneath the Indian subcontinent are relatively high (P velocity 8.2–8.4 km/s and S velocity 4.6–4.8 km/s) as compared to other regions of the Earth. Deep earthquakes travel times analysis in the Hindukush region also confirmed the relatively high P and S wave velocities in the upper mantle in this region. Velocity-depth models for the mantle beneath the Indian subcontinent inferred from P wave travel time curves reveal prominent discontinuities at average depths ( below the crust) of 400, 650, 950, 1350 and 1900 km. These inferred velocity discontinuities in the mantle are also consistent with the discontinuous nature of P wave amplitude decay curves observed out to 98° distance. The prominent velocity discontinuities at 400 km and 650 km depths in the upper mantle transition zone are also well brought out by dT/dΔ measurements at the GBA seismic array as well as deep earthquakes travel times analyses and velocity determinations at the depths of foci. These two discontinuities are probably associated with both compositional and phase changes in the transition zone. Modeling results of some anomalous travel time observations from deep focus earthquakes suggest that the phase transformations associated with the 400 km and 650 km velocity discontinuities may be relatively elevated by 80–150 km and 30–80 km respectively in the subducting lithospheric slabs. New models of the lithospheric velocity structure in the Indian shield obtained from explosion and earthquake seismological data consistently reveal prominent low velocity layers, with large velocity contrasts at their boundaries, both in the continental crust and the subcrustal lithosphere. These models suggest a rheological stratification of the continental lithosphère. Anomalously high uppermost mantle P wave velocities of up to 8.6–8.9 km/s and a higher than normal velocity gradient are also found in some regions, especially in the southeastern part, of the Indian shield. These high velocities and high velocity gradient are indicative of presence and/or continuati