Semi-Empirical Nonlinear Site Amplification from NGA-West2 Data and Simulations

We analyze NGA-West2 data and simulations to develop a site amplification model that captures ground motion scaling with V-S30 and soil nonlinear effects. We parameterize nonlinearity as the gradient of site amplification with respect to peak acceleration for reference (firm) sites. Both data analyses and simulations indicate nonlinearity for sites with V-S30 < 500 m/s and spectral periods T < similar to 3 s. Following approximate removal of nonlinear effects from the data, we evaluate V-S30-scaling of ground motions, which is most pronounced for T >= similar to 0.2 s and saturates for hard rock sites. Regional trends in V-S30-scaling and nonlinearity were not found to be sufficiently robust to justify inclusion in our model. We apply the site amplification model to derive site factors now approved for building code applications. Principal causes of changes relative to previous values are reduction of the reference velocity (at which amplification is unity) to 760 m/s and reduced nonlinearity.