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Assessment of Western US Basin Response and Implementation in the 2023 Update of the US National Seismic Hazard Model

The U.S. Geological Survey will update the 50-State National Seismic Hazard Model (NSHM) in 2023, which will include an expanded assessment of sedimentary basin response in the western U.S. Basin effects broadly encompass amplification of long-period (T > 1 s) ground motions due to deep sediments, azimuthal dependence for elongated sediment-filled structures (waveguides) and basin-edge effects including focusing. We target Reno and Las Vegas (Nevada), Portland and Tualatin (Oregon) and the California Central Valley as new focus areas for incorporating basin effects into the NSHM. We compare recently processed ground-motion datasets across Nevada to predicted motions from NGA-West2 ground-motion models (GMMs) to assess GMM performance using new maps of depths to shear-wave (V_S) isosurfaces (Z_1.0 and Z_2.5: depth to V_S = 1.0 and 2.5 km/s, respectively). We perform a similar analysis for the Central Valley using new Z_1.0 and Z_2.5 surfaces created from existing seismic velocity models. We also revisit basins included in the previous (2018) NSHM update using new data or models for improved assessment. We evaluate the treatment of basin effects in Seattle using the newly published NGA-Subduction GMMs, testing an alternative Z_2.5-based model of basin amplification that is independent of the time-averaged V_S to 30 m depth (V_S30). For southern California basins we consider new studies that differentiate between basin and basin-edge sites. We are evaluating whether special treatment of basin-edge sites within the NSHM is warranted, and if so, which basins amplify or deamplify ground motions relative to the default values predicted by the GMMs. In the previous NSHM update, such deamplification was not included in the shallow portion of basins because of an inconsistency with observations at basin edges. We also leverage 3D simulations to develop basin-amplification models both in southern California and in the Seattle basin. This broad effort will provide a framework to assess basin response in areas with varied ground-motion and geophysical data in seismically active regions around the U.S. and worldwide.

Session: 50-State Update of the USGS National Seismic Hazard Models III

Type: Oral

Room: Regency E-G

Date: 4/20/2022

Presentation Time: 02:15 PM Pacific

Presenting Author: Sean K. Ahdi

Student Presenter: No

Additional Authors