HR: 08:20h
AN: S41C-02    [Abstracts]
TI: Absolute site effects in Kachchh, India, determined from aftershocks of the 2002 Bhuj earthquake.
AU: * Malagnini, L
EM: malagna@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, Rome, 00143 Italy
AU: Mayeda, K
EM: mayeda@s136.llnl.gov
AF: Lawrence Livermore National Labs, P. O. Box 808, L-205, Livermore, CA 94550 United States
AU: Bodin, P
EM: pbodin@memphis.edu
AF: CERI, U. of Memphis, Memphis, TN 38112 United States
AU: Akinci, A
EM: akinci@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, Rome, 00143 Italy
AB: What can be learned about absolute site effects on ground motions from recordings of aftershocks at ten temporary seismic stations, none of which could be considered a "reference" (hard rock) site, and for which no geotechnical information is available? This challenge motivated our current study of Bhuj aftershocks; and our answer, briefly put, is: quite a bit. We started by constraining the regional attenuation and geometric spreading: this was the result of an earlier study [Bodin et al., BSSA 2004], the goal of which was to be able to reproduce the general character of the observations with a constrained set of stochastic synthetic ground motions. Our present work is based on the same aftershock data we used in the prior study. We first produced stable and reliable, unbiased source moment-rate spectra using the technique described by Mayeda et al., [BSSA, 2003]. With these known "absolute" source spectra, and the propagation terms we quantified in the previous study we inverted for the site response using only the largest ~200 earthquakes (M$>$2.8) in each of two depth ranges (0-25 km, and 20-40 km), to yield the "absolute" site terms for horizontal and vertical ground motions. We were able to obtain stable results in the 1-14 hz frequency band. The results reveal that the site terms generally share a common character: small amplifications (near unity) at the longer-period end of the pass-band, and decreases (perhaps due to attenuation or near-site scattering) at the higher frequency end. This character is evident in a similar study of earthquake ground motions in the Alps at sites on hard rock [Malagnini et al., BSSA 2004]. In contrast to Alpine hard rock sites, however, the vertical site terms at our sediment and soft-rock sites are generally rather flat and featureless. We observe differences in site response between stations which appeared to be on similar geologic conditions, and vice versa. For sites that appear to be on deep unconsolidated soils, amplification of horizontal motion is evident at the longer periods.
DE: 7200 SEISMOLOGY
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting