Uncertainty of site amplification derived from ground response analysis

Site-specific geotechnical ground response analyses (GRAs) are typically performed to evaluate stress and strain demands within soil profiles and/or to improve the estimation of site response relative to generic site terms from empirical prediction equations. Implementation of GRA results in probabilistic seismic hazard analysis (PSHA) requires knowledge of the mean and standard deviation of site amplification from GRA. We provide expressions for evaluating within-event standard deviations of ground motion given the parameterization of site amplification for a particular intensity measure into a nonlinear equation. A key component of those expressions is the standard deviation of site amplification, φlnY. We evaluate from the literature estimates of φlnY from ground response simulations using variable inputs and from interpretations of ground motion recordings. We find data-based studies to exhibit relatively consistent values of φlnY ≈ 0.3 over a wide range of oscillator periods, whereas simulation-based studies produce much higher values below the fundamental period of the analyzed soil column and lower values at longer periods. Values derived from the data-based studies are recommended for application