HR: 11:50h
AN: S22B-06    [Abstracts]
TI: Source-Averaged Basin Effects from 3D Ground Motion Simulations
AU: * Day, S M
EM: day@moho.sdsu.edu
AF: San Diego State University, Dept of Geological Sciences 5500 Campanile Drive, San Diego, CA 92182
AU: Bielak, J
EM: jbielak@cmu.edu
AF: Carnegie-Mellon University, Dept Environmental and Civil Engineering 5000 Forbes Ave., Pittsburgh, PA 15213
AU: Dreger, D
EM: dreger@seismo.berkeley.edu
AF: University of California, Berkeley, Berkeley Seismological Lab 301 McCone Hall, Berkeley, CA 94720
AU: Graves, R
EM: Robert\_Graves@urscorp.com
AF: URS Corporation, 566 El Dorado Street, Pasadena, CA 91101
AU: Larsen, S
EM: shawn@s109.llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA 94550
AU: Olsen, K B
EM: kbolsen@geology.sdsu.edu
AF: San Diego State University, Dept of Geological Sciences 5500 Campanile Drive, San Diego, CA 92182
AU: Pitarka, A
EM: arben\_pitarka@urscorp.com
AF: URS Corporation, 566 El Dorado Street, Pasadena, CA 91101
AU: Ramirez, L
EM: lramirez@cs.cmu.edu
AF: Carnegie-Mellon University, Dept Environmental and Civil Engineering 5000 Forbes Ave., Pittsburgh, PA 15213
AB: We simulate long-period (0-0.5 Hz) ground motion time histories for a suite of sixty scenario earthquakes (Mw 6.3 to Mw 7.1) within the Los Angeles basin region. Fault geometries are based upon the Southern California (SCEC) Community Fault Model, and 3D seismic velocity structure is based upon the SCEC Community Velocity Model. The ground motion simulations are done using 5 different 3D finite difference and finite element codes, and we perform numerous cross-check calculations to insure consistency among these codes. The nearly 300,000 synthetic time histories from the scenario simulations provide a resource for ground motion estimation and engineering studies of large, long-period structures, or smaller structures undergoing large, nonlinear deformations. By normalizing spectral accelerations to those from simulations performed for reference hard-rock models, we characterize the source-averaged effect of basin depth on spectral acceleration. For this purpose, we use depth (H) to the 1.5 km/s S velocity isosurface as the predictor variable. The resulting mean basin-depth effect is period dependent, and both smoother (as a function of period and depth) and higher in amplitude than predictions from local 1D models. For example, relative to a reference hard-rock site, sites with H equal to 2.5 km (corresponding to some of the deeper L.A. basin locations) have a predicted mean amplification factor of approximately 5.5 at 2 s period, and approximately 7.5 at 10 s period. We compare long-period (5 s) spectral amplitudes from the reference simulations with standard regression relationships for sites nominally classified as "rock" in empirical studies. From that comparison, we infer that the average nominal rock site represented in the empirical regression models has response approximately a factor of 2 higher than our hard-rock reference model. Hence, the basin-depth factors can be scaled down by a factor of approximately 2 to convert them to correction factors to empirical rock-site relationships. For the H=2.5 km example, the resulting correction to rock-site relationships is about 2.75 at 2 s period, and 3.75 at 10 s period. The correction factors are lower for shallower basin sites, and the period dependence has the reverse sense. For example, for H=0.75 km, the corresponding factors are approximately 1.7 (at 2 s) and 1.4 (10 s). The factor of 2 long-period bias between hard-rock simulations and empirical rock-site regression models suggests that the standard rock-site classification actually incorporates sites in which relatively low S velocities (less than 2 km/s) extend, on average, to depths of the order of 0.5 km.
DE: 7260 Theory and modeling
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting