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Group B, Poster #168, Ground Motions

3D nonlinear multi-surface simulations of the 2020 Magna, UT, earthquake sequence

Ke Xu, & Kim B. Olsen
Poster Image: 

Poster Presentation

2023 SCEC Annual Meeting, Poster #168, SCEC Contribution #13239 VIEW PDF
The primary seismic hazard in the Salt Lake Valley, home to more than a million residents, is a large earthquake on the Wasatch fault. McCalpin and Nelson (2000) estimated the probability of an M7 earthquake occurring on the Salt Lake City Segment of the Wasatch fault (WFSLC) during the next 100 years to be 16%. Thus, it is imperative to be able to predict accurate ground motions for a future large WFSLC event. The Mw5.5 2020 Magna, UT, earthquake likely occurred on the Wasatch fault, generated Peak Ground Accelerations (PGAs) as large as 0.55g in the Salt Lake Valley, and was a stern reminder of the seismic hazards for the area. In addition, the seismic dataset recorded from Magna event and... aftershocks provides invaluable constraints for the structure of the Salt Lake basin. Here, we have carried out 3D wave propagation simulations of the Magna earthquake sequence in the Wasatch Front Community Velocity Model (WFCVM, Magistrale et al., 2009) up to 10 Hz to better constrain both linear and nonlinear parameters in the soils of the Salt Lake Valley. The simulations used the discontinuous mesh finite difference code AWP-ODC, with support for modeling nonlinear soil effects via a multi-yield-surface (Iwan-type) 3D approach (Roten et al., 2023). We first validated the WFCVM via linear simulations of a Mw4.6 Magna aftershock assuming a point source. After adding a geotechnical layer from Vs30 outside the basin following Ely et al. (2010), as well as a statistical distribution of small-scale velocity heterogeneities to the WFCVM, linear simulations provided overall satisfactory fit of the synthetics to seismic recordings for the aftershock. Then, we simulated 3D wave propagation for the Mw5.5 Magna main shock, with finite source models obtained from the Graves-Pitarka rupture generator (Pitarka et al., 2021) and moment tensors following Pang et al. (2020). The simulations of the main shock generally overestimate the recorded ground motions, in particular at the stations with the largest PGAs and frequencies >2Hz, suggesting the presence of nonlinear soil effects. Finally, we applied the fully hysteretic multi-yield-surface approach in AWP-ODC to refine the nonlinear parameters including reference strains to obtain an optimal fit between synthetics and strong motion data for the Magna main shock.