Hybrid broadband ground motion simulations in the Indo-Gangetic basin for great Himalayan earthquake scenarios

This study presents broadband ground motions for the Indo-Gangetic basin, a large sedimentary basin in India, for potential future great (Mw 8.5) Himalayan earthquakes. We use a recently developed 3D earth structure model of the basin as an input to simulate low-frequency ground motion (0-0.5 Hz). These ground motions are further combined with high-frequency scattering waveforms by using a hybrid approach, thus yielding broadband ground motions (0-10 Hz). We calibrate the 3D model and scattering parameters by comparing the simulated ground motions against available recorded data for two past earthquakes in Himalaya. Our approach accounts for the physics of interaction between the scattered seismic waves with deep basin sediments. Our results indicate that the ground motion intensities exhibit frequency-dependent amplification at various basin depths. We also observe that in the event of a great earthquake, the ground motion intensities are larger at deep basin sites near the source and exhibit an attenuating trend over distance similar to the ground motion models. The extreme ground motion simulations performed in our study reveal that the national building codes may not provide safe recommendations at deep basin sites, especially in the near field region. The period-dependent vertical-to-horizontal spectral ratio deviates from the code-recommended constant 2/3 at least up to 6 s at these sites.