Empirical Corrections for Basin Effects in Stochastic Ground-Motion Prediction

Amplification corrections are presented for the finite-fault stochastic ground-motion simulation model; these corrections represent the total effect of the Los Angeles basin on the ground-motion spectra. Spectral amplification ratios were calculated by dividing the observed spectra for the 1994 Northridge and 1987 Whittier Narrows earthquakes, including shear- and basin-generated waves, by the simulated spectra created assuming an average rock-site condition. Smoothed amplification data were plotted above 3D images of the basin revealing a general correlation between the estimated basin depth and total basin amplification for both earthquakes over three frequency ranges: low (0.2-2 Hz), intermediate (2-8 Hz), and high (8-12.5 Hz). The depth-dependent corrections are derived from the regression of the combined data from both earthquakes in order to reduce an uncertainty caused by the azimuth of incoming waves. Ground-motion duration is defined as the time for 95% of the acceleration spectral energy to pass after the S -wave arrival. Due to ambiguity in defining a basin parameter controlling duration, it was impossible to develop a generic equation that would relate the duration ratio (observed/synthetic) to some characteristic of the basin. Users are cautioned, though, that the durations within the basin may be as much as four times longer than the simulated ones. The procedure is outlined for potential users who wish to use the results of this study in synthesizing more accurate earthquake ground motions, taking into account complicated basin-geometry and near-surface effects. The results are directly applicable to engineering simulation of strong ground motions in a sedimentary-basin environment. Manuscript received 21 May 2002.