A study of local amplification effect of soil layers on ground motion in the Kathmandu Valley using microtremor analysis

Abstract

Past researchers have anticipated the occurrence of a great earthquake in the central Himalayas in the near future. This may cause serious damage in the Kathmandu Valley, which sits on an ancient lake bed zone, with lacustrine sediments of more than 500 m depth. In this study, the predominant frequency of ground motion is evaluated using the Horizontal-to-Vertical (H/V) spectral ratio technique and recordings of ambient noise. The results of the H/V ratio show two peaks in about 20 percent of the locations, which are distributed mainly in and around the center and northern part of the Kathmandu Valley. The predominant frequencies vary from 0.5 Hz to 8.9 Hz in the study area, whereas the second resonance frequency varies from 4 Hz to 6 Hz in the center and northern part of the valley. This indicates that the center and northern part of the valley have a wide range of resonance frequency due to two levels of impedance contrast — one may be from the surface layer and the other may be from the layer underneath. These two levels of resonance indicate the importance of considering the effects of surface and lower layers during the planning and designing of infrastructures in the Kathmandu Valley.