Simulation‐Based Nonlinear Site Amplification Models for Western United States

This article presents one of the first nonlinear (NL) site amplification models relative to V S30 of 800 m/s for the Western United States (WUS) for both the smoothed effective amplitude spectrum (EAS), described as the orientation‐independent horizontal‐component Fourier amplitude spectrum (FAS) of ground acceleration, and the response spectra ( R S ). The amplification models were derived from a total of 286,254 1D site response analyses using randomizations of base‐case V S profiles, generated through a new slowness model developed using V S measurements at California, and 600 recorded ground‐motion pairs, scaled to match target spectra for 12 earthquake scenarios. The use of slowness model for generating V S randomizations yields a far broader suite of site conditions than from the preceding study employing only eight generic profiles. The simulation‐based EAS‐ and RS‐based NL amplification data exhibits more continuous behavior and alleviates the data sparsity in the low (<0.02 g) to mid (0.5–0.7 g) PGA r (peak ground acceleration at V S30 = 800 m/s) range as compared with prior work. Four NL mean amplification models with corresponding standard deviation relationships are proposed: two EAS‐based named as N1 EAS ( V S30 , EAS r at 5 Hz), and N2 EAS ( V S30 , frequency‐specific EAS r ), and two RS‐based models as N1 RS ( V S30 , PGA r ) and N2 RS ( V S30 , pseudo‐spectral acceleration at V S30 = 800 m/s (PSA r )). The N2 relationships exhibit lower standard deviation values of the residuals compared with their N1 counterparts. The compatibility between NL models’ predictions and observed NL amplification was assessed by incorporating NL relationships into an empirical ground motion model (GMM), resulting in within‐event residuals of GMM closer to zero relative to those of GMM without NL terms, particularly for sites with 200 m/s < V S30 < 500 m/s. The amplification models are applicable for sites with V S30 ≥ 200 m/s and motions with PGA r ≤ 1.0 g or EAS r ≤ 0.3 g/s.