Rupture Direction, Basin, Distance and Hanging-wall Effects on Ground Motions from M7 Earthquakes on the Salt Lake City Segment of the Wasatch Fault, Utah

Nan Wang, Daniel Roten, Kim B. Olsen, & James Pechmann

Published August 14, 2017, SCEC Contribution #7692, 2017 SCEC Annual Meeting Poster #245

We have analyzed numerical simulations of six M7 earthquakes on the Salt Lake City segment of the Wasatch fault, Utah, to better understand the long-period ground motions that these simulations predict in the adjacent Salt Lake Valley (SLV). The peak spectral accelerations at 2s (SA-2s) and 3s (SA-3s) were calculated in the Wasatch Front Community Velocity Model (v3d) as well as in a 1D rock model using the same source descriptions. The SAs from the 1D model are generally smoother and smaller in amplitude due to the lack of underlying 3D basin structure. 3D/1D ratios of the SA values depict the 3D basin effects in the SLV, such as focusing, defocusing, and wave entrapment. We developed regression models for the 6-scenario ensemble ground motions as a function of depth to the isosurfaces of Vs=1.0 and 1.5 km/s in the SLV. These models show amplification factors of up to ~2.7 and ~3.7 above the deepest part of the basin for SA-2s and SA-3s, respectively. The individual scenario SAs show correlations with the underlying rupture parameters, in particular, the slip distribution. The directivity factors for the Bayless and Somerville (2013) model, computed using a 10-section rupture surface approximation, increase the SAs by less than 20% for the scenarios and help little to increase the similarity between the SAs from the NGA-West2 GMPEs and the simulations. As compared to 4 NGA-West2 GMPEs, the long-period scenario ground motions for soil sites increase from Rrup (rupture distance) of 0 to 1-1.5 km and decrease from Rrup ~4 to 10 km. The maximum in simulated ground motions for Rrup ~1-4 km appears at least in part to be caused by basin-edge effects and entrapment of waves in the deeper basin, combined with scenario specific conditions not captured by the GMPEs. The GMPEs do a reasonably good job of predicting the increased ground motions over the hanging wall for the simulated SA-3s; however, for SA-2s, the simulations show minimal increase over the hanging wall and the GMPEs overpredict the simulations.

Citation
Wang, N., Roten, D., Olsen, K. B., & Pechmann, J. (2017, 08). Rupture Direction, Basin, Distance and Hanging-wall Effects on Ground Motions from M7 Earthquakes on the Salt Lake City Segment of the Wasatch Fault, Utah. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions