Site- and Motion-Dependent Parametric Uncertainty of Site-Response Analyses in Earthquake Simulations

Abstract

We investigate the propagation of uncertainty in site-response analyses from the soil model parameters to the ground surface motion at three downhole array sites in the Los Angeles (LA) Basin. For this purpose, we develop realistic stochastic models of elastic and nonlinear dynamic soil properties using extensive site-specific and generic geotechnical data on the variability of soil properties. We also generate synthetic ground motions using a finite source dynamic rupture model over a wide range of magnitudes and distances and use this statistically significant number of ground motions in the analysis. For each of the three sites, we evaluate the effects of soil parameter uncertainty as a function of the seismic input intensity and frequency content. We show that the frequency range, where the ground-motion variability due to soil parameter uncertainty is maximized, is a function of both the site and the seismogram characteristics. We compare our results with previously published studies and show that different soil models, statistical descriptions of soil parameters, or groundmotion scenarios may yield substantial differences in the estimated site-response scatter. We conclude that the effects of nonlinear soil property uncertainties on the ground-motion variability strongly depend on the seismic motion intensity, and this dependency is more pronounced for soft soil profiles. By contrast, the effects of velocity profile uncertainties are less intensity dependent and more sensitive to the velocity impedance in the near surface that governs the maximum site amplification.

Critical Parameters Affecting Bias and Variability in Site-Response Analyses Using KiK-net Downhole Array Data

“…Many studies have demonstrated the limitations of the underlying assumptions of analytical soil models, even for relatively small-strain records (e.g., Baise et al, 2003;Boore, 2004;Kwok et al, 2008;Stewart and Kwok, 2008;Li and Assimaki, 2010;Rathje et al, 2010;Thompson et al, 2012). Site-response analyses are burdened with significant uncertainties in the input (rock) motion, soil parameters, constitutive model parameters, constitutive model assumptions, and other general assumptions (such as the 1D representation of the soil column that cannot account for horizontally propagating surface waves).…”

Empirical Site Response and Comparison with Measured Site Conditions at ANSS Sites in the Vicinity of Reno, Nevada

“…Amplification effects due to the entire velocity-depth profile are likely to prove more useful to provide frequency-based amplification spectra. For future studies, it may be useful that processing of surface geophysical data used to measure V S profiles (e.g., ReMi, as used for this study, Louie, 2001;Scott et al, 2004Scott et al, , 2006Pancha et al, 2007) be revised to determine V S and its uncertainties as a function of depth (Li and Assimaki, 2010;Molnar et al, 2013) rather than averaged velocity values to selected depths (e.g. 30 m and 100 m as presented in this study).…”

Mapping the uncertainty in modulus reduction and damping curves onto the uncertainty of site amplification functions

“…Kwok et al [7] performed similar analyses on the Turkey Flat site by including three sets of MRD curves with full correlation between layers and found values ranging from 0.1 to 0.2 at short periods and very small values at long periods. Li and Assimaki [8] analyzed two sites in La Cienega using more than 50 MRD curve realizations and using a correlation coefficient of 0.15 between layers for nonlinear soil properties. For the first site, they computed standard deviation values of AFs due to varying MRD curves ranging from 0.3 and 0.4 at short periods and 0.2 at long periods.…”