abstract

Surface wave group velocities across central India are measured from computer plots of frequency-time analysis using multiple-filter technique. Periods range from 5 to 56 sec for fundamental mode Rayleigh wave, 5 to 16 sec for first higher mode Rayleigh wave, and 5 to 49 sec for fundamental mode Love wave. The wave paths from Koyna earthquakes to Bokaro cross purely through the Peninsular Plateau. The group velocities for these paths are found to be similar to those of surface waves recorded at Poona from earthquake which lie to the northeast of Bokaro in sub-Himalayan regions. From this, it is concluded that the shield structure of the peninsula extends to the sub-Himalayan regions to northeast of the Peninsular Plateu with a negligible part of sedimentary layer. Group velocities of fundamental mode Rayleigh and Love waves are also obtained for the paths from Koyna earthquakes to Shillong and are found to be similar to the above-mentioned group velocities except at very low periods where group velocities are affected by the small part of each path through thick sedimentary layer of Bengal Basin. Northern limit of this basin sedimentary layer has been identified.

Mean and standard deviation of observed group velocity at each period are obtained for each of Rayleigh fundamental, Rayleigh first higher, and Love fundamental modes. The group velocities closely agree with those obtained earlier for the path between New Delhi and Kodaikanal and are also found to be similar to those observed in some shield regions.

Monte Carlo inversion method is applied and crust and upper mantle structure of the Indian Shield across central India is obtained. Crustal structure obtained shows a marked resemblance to the results obtained earlier through body wave study.

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