Assessment of Western U.S. Empirical Basin Response for the 2023 Update of the National Seismic Hazard Model

Sean K. Ahdi, Morgan P. Moschetti, Brad T. Aagaard, Oliver S. Boyd, Grace A. Parker, & William J. Stephenson

Published September 11, 2022, SCEC Contribution #12483, 2022 SCEC Annual Meeting Poster #239

The United States Geological Survey (USGS) is undertaking a 50-state update to the National Seismic Hazard Model (NSHM) for 2023. We assess amplification of ground motion for local site conditions (shallow sediments) and regional basins (deeper sediments) for several new regions as part of the 2023 update. These areas include the cites of Reno and Sparks in Nevada, Portland and Tualatin in Oregon, and the Great Valley in California. The 2018 NSHM update included local amplification for only the Los Angeles, San Francisco, Seattle, and Salt Lake City regions. For each of the new regions, we determine whether using local models for Z1.0 and Z2.5 (depth to the shear-wave speed of 1.0km/s and 2.5km/s isosurfaces, respectively) as inputs to ground motion models (GMMs) result in a better match to recorded ground motions—as compared to average Z1.0 and Z2.5 values derived from VS30 (the time-averaged shear-wave velocity in the top 30m).

In the Reno and Sparks area, GMMs using the new Z1.0 maps by Simpson and Louie (2020) improve the fit to recorded ground motions. However, when compared to basins in California for which the GMMs were originally developed, the relatively shallow depth of the Reno/Sparks basins (Z2.5 < 0.5km) renders implementation in the NSHM as a special zone unviable. We plan to continue the practice from the 2018 NSHM update to not deamplify ground motions with respect to the average VS30 site condition for such shallow basins or basin-edge regions. In the Great Valley, inclusion of new Zx maps on average improves GMM predictive power with respect to recorded ground motions; however, the presence of very thick sequences of basin sediments (Z2.5 > 4km) tends to lead to over-amplification of ground motions at such sites for all periods, warranting further investigation about potentially capping such amplifications within the NSHM. Finally, the geologically coupled Portland (relatively shallow Z1.0 < 0.5km) and Tualatin (deeper Z2.5, up to 5km) basins provide different signatures of deep-sediment amplification within geographically proximal basin structures, with less long-period amplification in shallower Portland sub-basin sites than in deeper portions of the Tualatin basin. The inclusion of new Zx maps only moderately improves GMM performance across all periods for the entire region, which warrants investigating separate implementation of the Portland and Tualatin sub-basins within the NSHM to better describe basin-specific amplification.

Key Words
NSHM. Basins, Site response, Basin response, GMM

Citation
Ahdi, S. K., Moschetti, M. P., Aagaard, B. T., Boyd, O. S., Parker, G. A., & Stephenson, W. J. (2022, 09). Assessment of Western U.S. Empirical Basin Response for the 2023 Update of the National Seismic Hazard Model . Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions