3D EARTHQUAKE MODELLING OF BASIN EDGE EFFECT IN KATHMANDU VALLEY

Abstract

Kathmandu is particularly vulnerable to earthquakes due to its location near major fault lines. In addition to this, its basin shaped morphology, characterized by deep sedimentary deposits, can amplify seismic waves, increasing the intensity of ground shaking. Various types of basin effects such as: (1) basin-edge effect; (2) focusing effect; and (3) resonance effect can be observed during the ground shaking. The present work is the study of basin edge effects in Kathmandu by FDM modelling of earthquakes using a 3D underground velocity model of Kathmandu Basin. Nine different locationseight at the edges (one at rock and seven at soil sites) and one at the center were marked in the model to obtain synthetic earthquake data (up to 0.8Hz) of four different aftershocks of Gorkha earthquake (M7.3, M6.7, M5.5 and M). We compared the data obtained in different stations in both time and frequency domain. The comparisons have shown that the basin edges have profound amplification at the frequency band of 0.45-0.8 Hz when compared with the rock site and central basin site. The central station is placed above thicker sediment layers. So, as a matter of fact, its response is expected to be the largest among all. Nevertheless, the stations at the edges showed significant amplifications. The two stations each placed at the eastern and western edges showed higher amplification than the centrally placed station in all four respective earthquakes. Relatively higher amplitudes were found due to larger earthquakes (M7.3 and M6.7). The station located at the western side has high values at the frequency band of 0.4-0.8 Hz and eastern side at 0.3-0.6 Hz for all the magnitudes of the given earthquakes. Similarly, the PGV values obtained from time domain analysis is in sync with the frequency records. Edge stations are found more amplified than the station at the center in most of the cases. The stations at the northern most and southern most edges showed amplifications lower than that of the central station and higher than the station at rock site. It is found that the generation of local surface waves occurs near the margin of basins and that these waves travel back and forth, resulting in a long-duration and highly amplified reaction. The unusual long-period amplification obtained at the edges, demonstrate the basin edge effect.