Crustal structure in the Himalayan foothills area of north India, from ‘P’ wave data of shallow earthquakes

Travel times of P_g, P* and P_n waves of twenty shallow earthquakes with epicenters in the Himalayas and recorded at New Delhi, Chatra, Dehradun, Bokaro and Shillong Observatories of India were analyzed statistically to determine the crustal structure. A short seismic refraction profile about 15 kms long shot by the ONGC in the Punjab foothills enabled us for the first time to take into account, in the evaluation of crustal structure, the effect of the sedimentary layers which had been invariably neglected by the earlier workers. From the present study, the most probable crustal structure in the Himalayan foothills area of northern India, appears to consist of sedimentary layer 6 ± 1 kms (velocity 2.7 km/sec average), granitic layer 8 ± 5 kms (velocity 6.2 ± 0.1 km/sec) and basaltic layer 14 ± 7 kms (velocity 6.9 ± 0.1 km/sec), giving a total crustal thickness of 28 ± 8 kms. The upper mantle velocity in this area was found to be 8.2 ± 0.1 km/sec.