Abstract

We predict broadband (BB, 0–10 Hz) ground motions for M 7 earthquakes on the Salt Lake City segment of the Wasatch fault (WFSLC), Utah, which include the effects of nonlinear site response. The predictions are based on low‐frequency (LF, 0–1 Hz) finite‐difference (FD) simulations for six different rupture models generated during a previous study (Roten et al., 2011), which we combine with high‐frequency (HF, 1–10 Hz) shear‐to‐shear (S‐to‐S) back‐scattering operators to generate BB synthetics. Average horizontal spectral accelerations at 5 and 10 Hz (0.2‐s SAs and 0.1‐s SAs, respectively) calculated from the linear BB synthetics exceed estimates from four recent ground‐motion prediction equations (GMPEs) at near‐fault (<5  km) locations on the sediment by more than one standard deviation, but agree with the GMPEs at larger rupture distances. The overprediction of the near‐fault GMPE values is largely eliminated after corrections of the BB synthetics for nonlinear soil effects are applied, reducing the SAs from the simulations by up to 70%. These corrections are based on amplitude‐, frequency‐, and site‐dependent correction functions from 1D nonlinear simulations at ∼450 locations in the Salt Lake basin, using a simple soil model based in part on published laboratory experiments on Bonneville clay samples. We obtain geometric mean 1‐s SAs from from the six scenarios of more than 0.75g on the hanging‐wall side of the fault. Geometric mean 0.2‐s SAs exceed 1g on the hanging‐wall and on the footwall sediments in the central Salt Lake basin, and peak horizontal ground accelerations range from 0.45 to >0.60g in the same general locations.

Online Material: Table of coefficients and amplitude‐dependent correction functions for nonlinear soil effects, and figures showing maps of SAs at various frequencies, PGA and PGV, with and without correction for nonlinear soil effects, results of 1D nonlinear simulations, and comparison to ground motion prediction equations.

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