Three-dimensional 0–10 Hz physics-based simulations of the 2020 Magna,Utah,earthquake sequence

Abstract

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 g in 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.