HR: 16:45h
AN: S24A-04    [Abstracts]
TI: Calibration of the Ground Motions in the Bay Area: Regional Propagation and Absolute Site Responses
AU: * Malagnini, L
EM: malagnini@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Rome, Ita 00143 Italy
AU: Mayeda, K
EM: mayeda2@llnl.gov
AF: Lawrence-Livermore National Laboratory, P.P. Box 808, Livermore, CA 94551 United States
AU: Uhrhammer, R
EM: bob@seismo.berkeley.edu
AF: University of California, Berkeley, 215 McCone Hall # 4760, Berkeley, CA 94720-4760 United States
AU: Akinci, A
EM: akinci@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Rome, Ita 00143 Italy
AB: We studied the ground motion scaling in the Bay Area, in the 5-170 km hypocentral distance range, and 0.25 - 20 Hz band, using 5769 waveforms from 281 events recorded by the Berkeley Digital Seismic Network (BDSN). Using the absolute moment-rate spectra provided by a previous study by Mayeda et al. (2005), and the extension to their technique given by Malagnini et al (2004), we computed the absolute site responses at the BDSN stations. In a log log space, the regional attenuation within and around the Bay Area can be modeled by using a bi-linear geometrical spreading function with a 30 km cross-over distance, coupled with the anelastic parameter Q(f)=180 f^0.45. g(r)=1/r is used to model the close-distance attenuation of the Fourier spectral amplitudes, whereas g(r)=r^-0.6 is needed to model the attenuation of the spectral amplitudes at distances beyond 30 km. Once the crustal attenuation is quantified, the absolute 3-D site responses can also be calculated at each station, as well as the H/V spectral ratios (HVSR). The latter quantities are widely considered as estimates of the true horizontal site amplification, although some recent papers seem to question the robustness of this method. For some stations of the BDSN, we compared the HVSR's with the corresponding estimates of the absolute horizontal site responses. Our results show that the estimates of the site transfer functions obtained through the H/V spectral analysis cannot be considered estimates of the horizontal site transfer functions without further investigations. We observed that rock sites may have their own site responses, with possible exceptions like the very hard rock sites of previously glaciated regions, which will be the focus of our future work.
DE: 7212 Earthquake ground motions and engineering seismology
SC: Seismology [S]
MN: Fall Meeting 2005