Revisiting the 1934 Mw 8.2 Bihar Nepal earthquake – Simulation of Broadband ground motions

Summary The 1934 Mw 8.2 Bihar-Nepal earthquake was one of the devastating earthquakes, which made seismologists realize the importance of proper seismic hazard analysis and design aspects in India. The event occurred way before proper seismic networks were implemented and hence there are no recorded ground motions available for this event. The present study, thus aims to generate possible ground motions for the 1934 Mw 8.2 Bihar-Nepal event. The complex geographical features, ambiguous source information, and lack of ground motion data make the simulation and validation of ground motions very difficult. In this regard, the broadband (BB) ground motions are simulated and validated for the most recent well-documented Himalayan event, i.e., the 2015 Mw 7.9 Nepal earthquake in order to calibrate the model and simulation methodology. For this purpose, the computational model presented by Sreejaya et al. (2023) is extended up to a region of 1000 km × 670 km (longitude 80-89 °E and latitude 23-30 °N) in the Indo-Gangetic Basin to simulate the low-frequency (LF) ground motions using spectral element method (Komatitsch and Tromp 1999). These deterministically simulated LF ground motions are combined with stochastically simulated high-frequency (HF) ground motions based on an improved seismological model following Otarola and Ruiz (2016). The seismic moment and dimensions of the rupture plane presented by Pettanati et al. (2017) are used to generate ten samples for the finite fault source model having different slip distribution along the rupture plane as a random field (Mai and Beroza 2000; 2002). The BB ground motions (0.01–25 Hz) are obtained by merging LF and HF ground motions in the time domain by matching them at a frequency of ∼0.3 Hz. Such BB results are simulated at a grid of stations and at locations where Modified Mercalli Intensity (MMI) intensity values are available. The estimated MMI values and the observed MMI values are compared to emphasize the efficacy of the model. The maximum PGA estimated from the simulated ground motions in horizontal and vertical directions are observed to be 0.48 g and 0.4 g. Further, 5% damped response spectra and spectral amplification are analyzed concerning the sediment depth of the Indo-Gangetic Basin. The results from the study can serve as inputs for dynamic analysis and the design of earthquake-resistant structures across different locations in the Indo-Gangetic Basin.