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3D 0-10 Hz Physics-based Simulations of the 2020 Magna, Utah Earthquake Sequence

The Salt Lake City Segment of the Wasatch fault (WFSLC), where the probability of a M7 earthquake in the next 100 years is estimated to be 16% (McCalpin and Nelson, 2000), is a primary 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, and was a stern reminder of the seismic hazards for the area. Here, we present 3D wave propagation simulations of the Magna earthquake sequence in the Wasatch Front Community Velocity Model (WFCVM, Magistrale et al., 2008) up to 10 Hz to better constrain both linear and nonlinear parameters in the soils of the Salt Lake Valley. We first validated the WFCVM via linear simulations of a M4.5 aftershock in the Magna sequence. By adding a geotechnical layer outside the basin, a statistical distribution of small-scale velocity heterogeneities with 10% standard deviation, and using frequency-dependent anelastic attenuation as Q_s=0.05*V_s (f<1 Hz) and Q_s(f)=0.05*V_s*f^0.4 (f>1 Hz, V_s in m/s), we obtained an overall satisfactory fit to the seismic recordings for the aftershock. Our simulations of the 2020 Magna main shock use finite-fault source models from the Graves-Pitarka rupture generator, and a preferred source model is chosen with minimal bias to data at f<1 Hz, where nonlinear effects are expected to be negligible. Although different plausible source realizations result in factor-of-ten variations in high-frequency synthetic ground accelerations, they generally overestimate those of the recordings, in particular at stations with the largest PGAs, suggesting the presence of nonlinear soil effects. Then, using a fully hysteretic multi-yield-surface 3D method (Roten et al., 2023) we find that Darendeli’s (2001) proposed reference strain-depth relations provide excessive damping of the high-frequency synthetics in the Magna area, with an optimal fit to observations for reference strains 3 standard deviations larger. Our findings provide a basis for improved seismic hazard analyses of the greater Salt Lake City region.

Session: Physics-Based Ground Motion Modeling - I

Type: Oral

Room: Kahtnu 1

Date: 5/3/2024

Presentation Time: 08:45 AM (local time)

Presenting Author: Ke Xu

Student Presenter: Yes

Additional Authors