Seismic Microzonation of Densely Populated Area of Kathmandu Valley of Nepal using Microtremor Observations

Abstract

The Kathmandu Valley with an estimated current urban population of 5 million has been shaken many times in the past by damaging earthquakes. The valley was formerly a lake and although it is clear that much of the historical damage in the valley arose from seismic amplification associated with thick lake sediments, which in places exceed 500 m, few studies have been undertaken to understand the variability of seismic site response throughout the valley. In this study, we map the spatial variability of the dominant period of amplification using a horizontal-to-vertical spectral ratio technique derived from recordings of ambient noise. The resulting map is a first step towards seismic microzonation. We find that the area near the center of the valley, corresponding to the thickest sedimentary deposits, is associated with dominant periods that approach 2 s. Long-period shaking during future earthquakes is expected to be especially severe in this part of the valley since it corresponds to the urban center where numerous tall structures and long-span bridges with relatively long natural periods are to be found.