Tomographic Image of S-Wave Structure of NE India Based on Analysis of Rayleigh Wave

Tomographic images of crust and upper mantle of NE India and its surroundings were obtained by inversion of Rayleigh wave data recorded at local and regional distances by a dense seismic network. The images reveal thick layer of sediments in Bengal Basin, thickness increases from 10 km at its western end to 21 km at its eastern end. The basement of sediments suddenly becomes steeper on the eastern side of Eocene Hinge zone. Indo-Gangetic Plains and Brahmaputra River valley also comprise of low velocity medium of 5-6 km thick sediments. Crustal thickness in the higher Himalayas and southern part of Tibet is 70 km whereas its value varies between 30 km to 40 km in the remaining part of the study area. Crust and upper mantle below Shillong plateau and Mikir hills bend upwards. This may represent a pop up structure or the crest of buckled up Indian plate. Patches of low velocity medium is present in the lower crust of southern Tibet which indicates presence of either partially molten material or aqueous fluid. These patches of low velocity occur between the Kung Co and Yadong Gulu rift zones and to the east of Yadong Gulu rift zone. Also, the Moho depth changes from 75 km to the west of Yadong Gulu rift zone to 60 km to its east indicating the complicated effect of underthrusting of the Indian plate below the Eurasian plate.