Seismic Hazard in the Himalayan Intermontane Basins: An Example from Kathmandu Valley, Nepal

Abstract

Because of its location in the active plate boundary zone, in the last century, Himalaya witnessed eight lethal earthquakes that killed more than 46,845 causing hefty environmental costs in the form of loss of property and rehabilitation works. The first decade of twenty-first century became most unfortunate as the earthquake of magnitude 7.4 hit the Kashmir region of Pakistan killing at least 73,338 people leaving 51,28,309 affected. This single event caused economic loss of US$5.2 billion. These data have clearly pointed out the vulnerability level of the region and inadequacy of preparedness programme to mitigate earthquake disaster risk. Recurrence of such event in the area having soft sediment geology (e.g. fluvio-lacustrine deposits) would have even catastrophic devastation in the form of human causalities, structural damage and environmental degradation because soft sediments usually amplify the energy of the seismic waves. In this context, good amount of works has been carried out in the Himalayan region to understand the level of seismic hazard. However, there are limited works on seismic site response analysis of the soft sediments, which is a key to assess the intensity of ground deformation and structural damages. Therefore, in this contribution, first, a brief review on seismo-tectonics, paleoseismoloy, earthquake genesis, and active tectonics is presented; second, one-dimensional seismic site response analysis in the southern part of Kathmandu valley is presented to understand the seismic behaviour of the fluvio-lacustrine deposits in the intermontane basin. The results show variation of peak spectral acceleration from 1.27 to 1.28 g, which is usually a high value for the study area. On the other hand, amplification factor ranges from 1.908 to 7.788, which is similar to the Mexico earthquake (1985) that caused massive destruction in similar sediments of the Mexico City. The high amplification values are estimated mainly in densely populated urban areas of the valley. The obtained results show good correlation with the damage pattern of 1934 Bihar-Nepal earthquake indicating that the amplification of the ground motion would be the main culprit during the impending great earthquake in an already identified “Central Seismic Gap”. An integrated approach comprising of paleoseismological studies, seismic microzonation, deployment of earthquake early warning system, development and enforcement of site specific building code, insurance policy along with preparedness directed awareness programs could be key measures in reducing earthquake risk in rapidly urbanizing intermontane basins.