Three-dimensional ground motion modelling in the Kachchh rift zone, Gujarat, India

Abstract

The strong motion dataset of the 6 April 2006 Bhuj aftershock of Mw5.5 from 16 three-component accelerograph stations are used here to perform a detailed ground motion modelling in the Kachchh rift basin. First, we estimate the 1-D velocity structure model down to the bedrock at 16 accelerograph sites, through the Genetic Algorithm inversion of strong motion waveforms. Then, a 3-D velocity structure model of the basin is constructed using the modelled 1-D velocity structure and constraints from earlier seismological, Vs30 estimation (by PS logging and MASW), and seismic exploration studies in the region. We modelled low-frequency (0.1–1 Hz) ground motions excited by the 6 April 2006 event, using the finite difference method (FDM) assuming a point source, to access the robustness of the constructed velocity structure model. At most of the stations, the observed and simulated velocity waveforms are found to be in good agreement in terms of both amplitude and ground motion duration. We also computed synthetic ground velocities at numerous locations within the basin, for both the 2001 Bhuj mainshock (finite-fault source) and the 2006 aftershock (point source) cases, using the 3-D velocity structure model. For both the cases, we notice significant amplifications of amplitudes and elongation of shaking at stations lying within the basin while smaller amplifications are noticed at stations away from the basin. Our FDM modelling for the 2001 Bhuj mainshock at lower frequency (<1 Hz) predicted a peak ground motion velocity (PGV) of 50–110 cm/s at locations just over the causative fault of the 2001 mainshock. Our study reveals that the presence of low velocity sediments within the Kachchh rift basin plays a key role in modifying the ground motions in 0.1–1.0 Hz, thus, these effects should be quantitatively considered for future seismic hazard analysis in this region.