Surface Wave Group Velocity Dispersion Study of the Eastern Himalayan Foreland Basin

Kumar, Utpal (2014) Surface Wave Group Velocity Dispersion Study of the Eastern Himalayan Foreland Basin. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Seismic surface waves, recorded at regional distances, sample the 1-Dimensional structure between the earthquake source and receiver. Dispersed surface waves are sensitive to the vertical average of shear wave velocities in a layered medium. Using both these properties, we model the crust and upper mantle shear wave velocity structure of the eastern Himalayan foreland basin by modeling multi-mode surface wave group velocity dispersion data. We use earthquakes originating in Sikkim-Bhutan Himalaya and the Bengal basin recorded at the permanent broadband seismological observatory at Bakreshwar (BAKR). Source-receiver raypaths sample the foreland basin adjacent to the Sikkim-Bhutan Himalaya and the Bengal Basin. We measure Rayleigh and Love waves dispersion for both fundamental and the first higher mode, in the frequency-time domain, using the multiple filter analysis (MFA) technique. Observed dispersion curves are clustered into 2 groups based on the raypath sampled. Rayleigh and Love wave dispersion data for Cluster 1 ranges from 6 to 30 s and 5 to 19 s, respectively, for the fundamental mode. Similarly, the fundamental mode Rayleigh wave dispersion data for Cluster 2 ranges from 9 to 36 s. Clustered multi-mode dispersion data has been jointly inverted to model the crust and upper mantle structure beneath the sampled regions. The Eastern foreland basin has 4 km thick sedimentary layer as compared to approximately 7 km thick sedimentary layer of Western and Central Foreland Basin. The crustal thickness of Bengal Basin, which is overlain by 18 km thick sediments / sedimentary rocks, is just 20 km. This shows that it is a continental margin crust which has been overlain by the sediments brought down by the rivers like Ganga and Brahmaputra from the orogeny.

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