Unbiased Moment-Rate Spectra and Absolute Site Effects in the Kachchh Basin, India, from the Analysis of the Aftershocks of the 2001 Mw 7.6 Bhuj Earthquake

What can be learned about absolute site effects on ground motions, withno geotechnical information available, in a very poorly instrumented region? In addition,can reliable source spectra be computed at a temporary deployment? Thesechallenges motivated our current study of aftershocks of the 2001 Mw 7.6 Bhujearthquake, in western India, where we decouple the ambiguity between absolutesource radiation and site effects by first computing robust estimates of coda-derivedmoment-rate spectra of about 200 aftershocks in each of two depth ranges. Crustalattenuation and spreading relationships, based on the same data used here, weredetermined in an an earlier study.Using our new estimates of source spectra, and our understanding of regional wavepropagation, for direct S waves we isolate the absolute site terms for the stations ofthe temporary deployment. Absolute site terms for each station were determined inan average sense for the three components of the ground motion via an L1-normminimization. Results for each site were averaged over wide ranges of azimuths andincidence angles.The Bhuj deployment is characterized by a variable shallow geology, mostly ofsoft sedimentary units. Vertical site terms in the region were observed to be almostfeatureless (i.e., flat), with amplifications slightly 1.0 within wide frequency ranges.As a result, the horizontal-to-vertical (H/V) spectral ratios observed at the deploymentmimic the behavior of the corresponding absolute horizontal site terms, andthey generally overpredict them. This differs significantly from results for sedimentaryrock sites (limestone, dolomite) obtained by Malagnini et al. (2004) in northeasternItaly, where the H/V spectral ratios had little in common with the absolutehorizontal site terms.Spectral ratios between the vector sum of the computed horizontal site terms forthe temporary deployment with respect to the same quantity computed at the hardestrock station available, BAC1, are seriously biased by its nonflat, nonunitary siteresponse. This indicates that, occasionally, the actual behavior of a rock outcrop maybe far from that of an ideal, reference site (Steidl et al., 1996)