Modeling Nonlinear Site Response Uncertainty in the Los Angeles Basin

We investigate the variability in ground motion predictions that results from the methodology selected for the realization of strong motion site response analyses of typical soil profiles in the Los Angeles (LA) Basin. For this purpose, site-specific regional velocity and attenuation structures are compiled using geotechnical data at three downhole arrays, low-strain velocity and attenuation profiles obtained via waveform inversion of weak motion records, and crustal velocity structures as described by the Southern California Earthquake Centre Community Velocity Model (SCEC CVM IV). Successively, broadband ground motions are simulated by means of a finite source model for rupture scenaria of weak, medium and large magnitude events (M=3.5–7.5) at stations located on a regular 2D grid within distances 5–75km from the fault. Observed estimates of site response at the stations of interest are first compared to simulated small magnitude rupture scenaria. Parametric studies are next conducted for each fixed magnitude (fault geometry) scenario by varying the source-to-site distance and source parameters for the ensemble of site conditions. Elastic, equivalent linear and nonlinear simulations are implemented for the base-model velocity and attenuation structures and nonlinear soil properties, to examine the variability in ground motion predictions as a function of ground motion amplitude and frequency content, and nonlinear site response methodology. Results of this study are currently being used to develop supplementary criteria for the NEHRP site classification system to describe the nonlinearity susceptibility of soft sediments, and thus establish a cost-effective and computationally-efficient framework for site parameterization and response simulation defined as a function of source-path-site-effects and design sophistication level. The reflection of nonlinear site response modeling uncertainty is finally depicted in the estimation of synthetic ground motion amplification factors, which are compared to the currently employed NEHRP factors.