Basement topography of the Kathmandu Basin using microtremor observation

Kathmandu Valley, an intermontane basin of the Himalaya, has experienced many destructive earthquakes in the past. The observations of the damage pattern during the 1934 Earthquake (Mw = 8.1), in particular, suggest that the spectral ground amplification due to fluvio-lacustrine sediments plays a major role in intensifying the ground motion in the basin. It is, therefore, imperative to conduct a detailed study about the floor variation of sediments in the basin. In this paper, a preliminary attempt was made to estimate the thickness of soft sediment in the Kathmandu Basin using microtremor observations. The measurements of microtremors were carried out at 172 sites spaced at a grid interval of 1 km. The results showed that the predominant frequency varies from 0.488 Hz to 8.9 Hz. A non-linear regression relationship between resonance frequency and sediment depth was proposed for the Kathmandu Basin. The thickness of lacustrine sediments at various points in the basin was estimated using the proposed equation, and then the estimated thickness was used to plot a digital elevation model of the basement topography and cross profiles of the sediment distribution in the basin. The results were validated by correlating the estimated sediment thickness with geology and geomorphology of the study area.

► Study of basement topography in the Kathmandu Basin using microtremor observation.
► Large variation of predominant frequency is found within a short distance.
► Resonance frequency versus thickness relationship is proposed for Kathmandu Basin.
► Sediment thickness increases towards the center of the valley.
► Two depressions identified, which are separated by buried ridges along NW to SE.