Site responses based on ambient vibrations and earthquake data: a case study from the meizoseismal area of the 2001 Bhuj earthquake

The 2001 Mw 7.6 earthquake sourced in the Kachchh rift of northwest India led to extensive damage in the city of Bhuj, located similar to 70 km southwest of its epicenter. The building stock of this densely populated city was a mix of modern, single, and multistoried structures as well as traditional and non-engineered abodes, most of which were not designed to withstand severe shaking effects. Although there was extensive liquefaction and ground failure in the meizoseismal area, they were not observed in Bhuj, but the damage was severe here. In this study, we apply horizontal to vertical spectral ratio method to ambient vibrations (HVSR-AV) to obtain fundamental resonance frequency (f0) and H/V peak amplitude (A0) to examine if site response had any significant role in the observed damage. The patterns of H/V curves as well as spatial distributions of f0 (0.6-1.4 Hz) and A0 (1.5-4.4) suggest absence of any strong impedance contrast within the subsurface. Similar results obtained for ambient vibrations and earthquake signals suggest the efficacy of the HVSR-AV method as most useful for regions of low-level seismicity. The weathered sandstone that is generally exposed in the city represents the resonating layer whose thickness is approximately estimated as similar to 66-155 m, based on 1D assumption. The current set of available data precludes any quantitative modeling, but our preliminary inference is that site effects were not significant during the 2001 earthquake damage observed in Bhuj