A comparative study of ground motion parameters at bedrock and surface level in Kathmandu Basin

Nepal, a landlocked country in the Himalayan region, was struck by a devastating earthquake of magnitude Mw 7.8 on 25th April, 2015. The major earthquake destroyed millions of structures and caused immense loss of life. Unfortunately, only a few seismic stations recorded the earthquake, presenting a challenge for understanding the observed non-uniform structural damage in the region. In this study, synthetic ground motions are generated at the bedrock level using the stochastic finite fault method. The ground motions are later estimated at the surface level using the equivalent linear site response analysis program, using soil profiles from 9 borehole locations from the Kathmandu basin. The key characteristics of the synthetic strong ground motions are tabulated and analyzed. Peak ground accelerations (PGA) at bedrock in the region range from 0.064 g to 0.09 g. Remarkably, the Kankali site (BH6) exhibits the highest outcrop acceleration response, with bedrock and outcrop PGAs measuring 0.083 g and 0.170 g, respectively. Observations indicate that soil profiles experience their greatest amplification ratio within the frequency range of 1.2 Hz–7.3 Hz. Plots of response spectra for the synthetic ground motions are derived and compared with the provisions of the Nepal’s seismic design code. The key characteristics of strong ground motions and observations from the derived response spectra correlate well with the available reports of structural damage in the earthquake. These observations provide valuable insights into seismic vulnerability and soil behavior that is crucial for seismic hazard assessment and engineering design considerations.