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Subduction Ground Motion Models for Cascadia in the 2023 USGS National Seismic Hazard Model

Description: 

The U.S. Geological Survey National Seismic Hazard Models (NSHMs) are used to calculate earthquake ground-shaking intensities for design and retrofit of structures in the United States. The most recent 2014 and 2018 versions of the NSHM for the conterminous U.S. included major updates to ground motion models (GMMs) for active and stable crustal tectonic settings; however, the subduction zone GMMs were largely unchanged. With recent development of the Next Generation Attenuation Subduction (NGA-Sub) GMMs, as well as recent progress in utilization of “M9” Cascadia earthquake simulations, we now have access to improved models of ground shaking in U.S. subduction zones and the Seattle Basin. The new NGA-Sub GMMs support “multi-period response spectra” calculations by providing median and variability models that can be used for periods up to 10 s and for eight site classes. They provide global models as well as regional terms specific to Cascadia and terms that account for deep sedimentary basin effects. This study focuses on updates to subduction GMMs for the Cascadia portion of the 2023 NSHM and compares them to the GMMs of previous NSHMs. Individual subduction GMMs, their weighted combinations, and their impact on hazard relative to the 2018 NSHM are discussed. Logic trees are described that include three of the new NGA-Sub GMMs and retain two older models to represent epistemic uncertainty in both the median and standard deviation of ground-shaking intensities at all periods of interest. Epistemic uncertainty is further represented by a three-point logic tree for a given NGA-Sub median model. Finally, in the Seattle region, basin amplification factors are adjusted at long periods based on state-of-the-art M9 Cascadia earthquake simulations. Overall, the GMM changes increase the mean hazard values at shorter periods and short source-to-site distances, but decrease them otherwise, relative to the 2018 NSHM. On deep sedimentary basins, the new models cause hazard decreases for longer periods in the Puget Lowland but increases for shorter periods within the Seattle Basin relative to 2018 NSHM.

Session: The 2023 USGS National Seismic Hazard Model and Beyond - III

Type: Oral

Date: 5/1/2024

Presentation Time: 02:00 PM (local time)

Presenting Author: Sanaz

Student Presenter: No

Invited Presentation: 

Authors