On the seismic structure of Indian upper-mantle regions

Abstract

Several velocity models on upper mantle regions of the world have been postulated during the last two decades. There has been a broad agreement amongst seismologists that upper mantle has got two transition zones, though the models differ in detail. These zones have been found to occur around ‘400 km’ and ‘650 km’ depth ranges with varying thicknesses of the zones. A limited number of such studies have been made on the upper mantle structure of the Indian subcontinent. High positive velocity gradients were reported to exist around the above depth range. Evidence for lateral heterogeneities has also been found. We address some problems like refinement of Indian upper mantle velocity models specially after considering the effect of scattering and attenuation on the short periodP-waves. The study of proper positioning of the cusps of the travel-time branches and their extension is essential as well. In our opinion, analysis of such problems would help in the better understanding of the nature of propagation of seismic waves and mechanism of earthquakes. Complexity of seismic signatures observed is another major problem and may also be taken into consideration.