STUDY OF BASIN-EDGE INDUCED SURFACE WAVES AND ASSOCIATED DIFFERENTIAL GROUND MOTION

Abstract

In this dissertation, the effects of basin-edge, soil layering in the basin, basin-edge slope on the ground motion characteristics is documented. Seismic responses of various models were simulated using software based on parsimonious finite difference staggered grid approximation of 2.5 D elastodynamic wave equation. Study is carried out to understand the effects of basin-edge on the characteristics of the induced surface waves and associated differential ground motion. Simulated results revealed generation of surface waves near the basin-edge, which was confirmed by comparing the response with the flat layer response, computing the group velocity, differential ground motion and the snapshots at different times. The response of basin-edge model using different source location revealed that this phenomenon is not azimuth dependent. Spectral analysis of the attenuated surface waves reveals that the anomalous earthquake intensity or large strain may generate in a zone parallel to basin-edge at an offset of 0.5-1.0 km and persists up to several kilometers. Seismic responses of various models with different number of soil layers but for a fixed thickness of soil deposit, fundamental frequency of soil and impedance contrast revealed extended duration of ground shaking with increase in layers in the basin because of increased dispersion. It is inferred that when soil layers are more in the basin low frequency Rayleigh waves°'appear and differential ground motion is very large near basin-edge. Differential ground motion drops suddenly with increase in distance from edge, in the Rayleigh waves but not in Love waves. Further, it was inferred that with increase of layering in the basin there was shift of dominant frequency in induced surface waves towards the higher value. Decrease of amplitude of Rayleigh waves with increase of edge-slope was obtained after analyzing the edge-induced surface waves in basin having different edge-slopes, even there was minor increase of amplitude of incident wave. An extra phase causing large differential ground displacement was noticed. This may be a higher mode of Rayleigh waves moving with a large velocity as compared to the fundamental mode. The findings of this study reveals that differential ground motion and basin-edge induced surface waves need special attention during the seismic microzonation or seismic hazard prediction since it may play an important role in damage distribution during earthquakes.