Seismic wave propagation simulations in Indo-Gangetic basin using spectral element method

Abstract

SUMMARY This study focuses on developing a 3-D computational model of the Indo-Gangetic basin (IG basin) using the spectral element method. The region includes the subcontinent's most densely populated areas. The basin is unique as it consists of geologically younger sedimentary layers along with several ridges and depressions in its domain. However, the proximity of great Himalayan earthquakes and the presence of thick sedimentary layers of the basin results in higher seismic hazards. The limited instrumentation of the domain poses challenges in understanding the response of the basin due to a seismic event. This motivated us to develop a computational model of the IG basin by incorporating the best-known geometry, material properties and fine resolution topography. In the lateral direction, the modelled part of IG basin spans over ∼6° × 4° (between longitude 80.5°–86.5°E and latitude 25°–29°N). The validation of the developed basin model is performed by simulating the ground motions for the 2015 Mw 7.9 Nepal main shock and five of its aftershocks. Both qualitative and quantitative comparison of the simulated time histories suggests that the developed model could accurately simulate ground motions over a frequency range of 0.02–0.5 Hz. The developed basin model is then used to understand the seismic wavefield characteristics during the 2015 Mw 7.9 Nepal main shock. The spatial variation of peak ground velocity (PGV), as well as amplification, are investigated at a 0.2° × 0.2° grid and selected cities in the basin. The contours of PGV amplification indicate a higher value of ∼8–10 in the horizontal direction and ∼2.5–3.5 in the vertical direction for sediment depth >4 km. A comprehensive comparison of the simulated PGVs and the ground motion prediction equations shows that, while the simulations agree with the prediction, they also show heterogeneity of ground-motion distribution that cannot be fully described by empirical prediction relations. Hence the results from this study are more reliable and find applications in seismic hazard assessment of the cities in the basin. Besides, the results can be used to guide the installation of future seismic stations in the region.