HR: 0800h
AN: S31B-0449    [Abstracts]
TI: Long-period Amplification by Two-Dimensional Resonance in a Deep Alpine Valley
AU: * Roten, D
EM: daniel.roten@sed.ethz.ch
AF: ETH Zürich, Swiss Seismological Service, Zürich, 8093, Switzerland
AU: * Roten, D
EM: daniel.roten@sed.ethz.ch
AF: San Diego State University, 5500 Campanile Dr, San Diego, CA 92182, United States
AU: Fäh, D
EM: donat.faeh@sed.ethz.ch
AF: ETH Zürich, Swiss Seismological Service, Zürich, 8093, Switzerland
AU: Olsen, K B
EM: kbolsen@sciences.sdsu.edu
AF: San Diego State University, 5500 Campanile Dr, San Diego, CA 92182, United States
AU: Bonilla, L F
EM: fabian.BONILLA@irsn.fr
AF: Institut de Radioprotection et de Sûreté Nucléaire, Fontenay-aux-Roses, Cedex, BP17, France
AU: Giardini, D
EM: domenico.giardini@sed.ethz.ch
AF: ETH Zürich, Swiss Seismological Service, Zürich, 8093, Switzerland
AB: Many numerical and empirical studies have shown that deep sedimentary structures may significantly amplify strong ground motion at long periods, and the impact of this low-frequency amplification on tall buildings was demonstrated during many devastating earthquakes. In the framework of the SHAKE-VAL project we are analysing site effects in the Rhône valley, a deep sedimentary basin in Southern Switzerland. We computed site-to-reference spectral ratios from weak motion recorded on a temporary network of 12 seismometers. At most sites we observe substantial amplification between 0.50 and 0.60 Hz. The frequency of amplification is insensitive to the local sedimentary thickness, and the amplification level reaches a maximum of about 12 in the valley center. These observations are consistent with two-dimensional resonance of the deep basin, and the SH₀₀ and SV₀ fundamental modes of resonance can be isolated by rotating the ground motion to the directions perpendicular and parallel to the valley axis. To support our interpretation we performed numerical simulations of the recorded events with a 3-D finite difference method. The synthetic spectral ratios are generally in agreement with the observations at frequencies below 1 Hz. In a last step we estimated how non-linear soil behaviour will affect the long-period part of the signals during strong ground motion. We convolved synthetic time series with the transfer function of the basin and propagated the resulting signals through a shallow sand layer using a fully nonlinear 1-D method. Our results suggest that cyclic mobility will reduce the spectral acceleration at 0.50 Hz by 50% for rock acceleration exceeding 1 ms^-1.
DE: 7212 Earthquake ground motions and engineering seismology
DE: 7290 Computational seismology
SC: Seismology [S]
MN: 2007 Fall Meeting