Probabilistic seismic hazard analysis and construction of design spectra for Pokhara valley, Nepal

Abstract

Pokhara is one of the most naturally beautiful cities in the world with a unique geological setting. This largest metropolitan city in Nepal, having experienced a number of destructive earthquakes over the past centuries, is under intense pressure from rapid urbanization and population growth. The latest Gorkha earthquake (epicenter 70 km from Pokhara) occurred on 25th April 2015 of Magnitude 7.8 (Mw) killed nearly 9000 people and injured more than 22,000 people in Nepal. It was the worst natural disaster to strike Nepal since the 1934 great Nepal-Bihar earthquake (Mw = 8.0). With these recent seismic events flourishing new data, the heightened possibilities of a larger earthquake in this region have come forth. To this end, the probabilistic techniques were used to predict the expected future strong ground motion characteristics in the study area. For this, a probabilistic seismic hazard assessment (PSHA) is done by considering all the possible seismic events of historical earthquakes by dividing the whole area of interest into seven areal source zones. Based on the available data from each source zone, the frequency–magnitude relationship has been established after the elimination of dependent events followed by the completeness check of the data. Peak ground acceleration and spectral acceleration for 2% and 10% probabilities of exceedance in 50 years are estimated for predefined bedrock conditions with rock and soil sites, respectively. The resulting uniform hazard spectra (UHS) represent the approximate seismic loading on the structures. The estimated ground motion parameters in the present study have higher values compared with the values suggested in the earthquake-resistant design codes in standard practice. The study also proposed the design response spectra for the study area, which is much more defensible in engineering decision-making for reducing seismic risk.