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Earthquakes on the Salt Lake City Segment of the Wasatch fault (WFSLC) represent the most significant seismic hazard to the Salt Lake Valley, populated by 1 million+ people. The 2020 Magna, UT, earthquake, which likely occurred on the WFSLC, generated peak ground accelerations (PGAs) as large as 0.55 gin the Salt Lake Valley. Here, we present three-dimensional (3D) physics-based wave propagation simulations of the Magna earthquake sequence in the Wasatch Front Community Velocity Model (WFCVM) up to 10 Hz to better constrain both linear and nonlinear parameters in the soils of the Salt Lake Valley. We first calibrate the WFCVM via linear simulations of a $M_w$4.59 Magna aftershock, obtaining the best fit between the recordings and synthetics, including a statistical distribution of small-scale heterogeneities with 10% standard deviation and $Q_S=0.05V_S$for frequencies $<1$ Hz and $Q_S=0.05V_S{f}^{0.4}$for frequencies $>1$ Hz ( $V_s$in m/s). Spectral ratios from our simulations of the 2020 Magna mainshock using a finite-fault source model generally overestimate those for the recordings in the linear regime at higher frequencies, in particular at stations with the largest PGAs, suggesting the presence of nonlinear soil effects. Using a fully hysteretic multi-yield-surface 3D nonlinear modeling approach, we find that damping from the reference strain–depth relations proposed by Darendeli significantly reduces the bias in terms of spectral amplification ratios at stations with the shortest epicentral distances. We find an optimal fit between the recordings and nonlinear synthetics for reference strains at about 2 standard deviations below Darendeli’s relations, with reduction of the spectral amplification bias by more than a factor of two. Our findings suggest significant nonlinear soil effects in the Salt Lake Valley and provide a basis for improved seismic hazard analysis of the greater Salt Lake City region. 

Abstract The ShakeOut scenario of an M 7.8 northwestward rupture on the southern San Andreas fault (SSAF) (Jones et al., 2008) predicted significant long-period ground-motion amplification in the greater Los Angeles, California, area, caused by a waveguide from interconnected sedimentary basins. However, the early ShakeOut ground-motion simulations omitted important model features with immature versions of the velocity structure and fault geometry. Here, we present 0–1 Hz 3D numerical wave propagation simulations for the ShakeOut scenario including surface topography, as well as updated high-resolution velocity structures and SSAF geometry. Spectral accelerations at 3 s are increased by the local high-resolution basin models (25%–45%) but decreased from complexity in velocity and density updates outside the basins (65%–100%) and inclusion of surface topography (∼30%). The updated model reduces the simulated long-period ground motions in the waveguide by 60%–70%, bringing the predictions significantly closer to the values by a leading Next Generation Attenuation-West2 ground-motion model. 

ABSTRACT We have implemented and verified a parallel-series Iwan-type nonlinear model in a 3D fourth-order staggered-grid velocity–stress finite-difference method. The Masing unloading and reloading behavior is simulated by tracking an overlay of concentric von Mises yield surfaces. Lamé parameters and failure stresses pertaining to each surface are calibrated to reproduce the stress–strain backbone curve, which is controlled by the reference strain assigned to a given depth level. The implementation is successfully verified against established codes for 1D and 2D SH-wave benchmarks. The capabilities of the method for large-scale nonlinear earthquake modeling are demonstrated for an Mw 7.8 dynamic rupture ShakeOut scenario on the southern San Andreas fault. Although ShakeOut simulations with a single yield surface reduces long-period ground-motion amplitudes by about 25% inside a waveguide in greater Los Angeles, Iwan nonlinearity further reduces the values by a factor of 2. For example, inside the Whittier Narrows corridor spectral accelerations at a period of 3 s are reduced from 1g in the linear case to about 0.8 in the bilinear case and to 0.3–0.4g in the multisurface Iwan nonlinear case, depending on the choice of reference strain. Normalized shear modulus reductions reach values of up to 50% in the waveguide and up to 75% in the San Bernardino basin at the San Andreas fault. We expect the implementation to be a valuable tool for future nonlinear 3D dynamic rupture and ground-motion simulations in models with coupled source, path, and site effects.