Broadband Ground Motion in Indo-Gangetic Basin for Hypothetical Earthquakes in Himalaya

Abstract

Indo-Gangetic (IG) Basin, formed between the Indian shield and Himalayas, is the largest sedimentary basin in India. The region also constitutes many metropolitan cities, including the capital city New Delhi. The seismic risk in the region is attributed due to the proximity to seismically active Himalayan faults, the possible seismic wave amplification due to huge sedimentary layers, and the vulnerability due to urban agglomerations. However, the region lacks a dense set of recorded data curbing the direct assessment of ground motion intensities. Hence, seismic hazard needs to be estimated based on synthetic ground motions for possible scenario earthquakes. These ground motion simulations require a proper understanding of the spatial variation of material properties, viz., density and wave velocities in the region of interest. Hence, the present work focuses on developing the 3D regional velocity model for ground motion simulations in IG Basin spanning between longitude 74.5–82.5E and latitude 24.5–32.5N. The spatially varying material properties of the region are derived by suitably interpreting the available velocity models constrained according to geological features reported in the literature. The 3D velocity model derived from the study is first employed in a finite element platform to obtain low-frequency ground motion. These ground motions rich in low-frequency content are further combined with the high-frequency ground motion simulated using the Zeng-scattering method on a hybrid broadband ground motion generation platform. Hence, the simulated time histories comprise of energy in the period between 0 and 10 s, thus complying to engineering interest. The 3D velocity model developed from the study is validated using the strong motion data available for an event in the Main Boundary Thrust. The simulated time histories are observed to match the phase and energy content of recorded data. The model is further employed to simulate time histories for Mw 8.5 hypothetical earthquake in the Himalayas. The obtained response spectra are compared with the IS1893 code recommendations.