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Use of the 2018 USGS National Seismic Hazard Model in Forthcoming U.S. Building Codes

Session: Recent Engineering Uses of National Seismic Hazard Models II

Type: Oral

Date: 4/22/2021

Presentation Time: 05:15 PM Pacific

Description: 

For building codes in the United States, the 2020 NEHRP Recommended Seismic Provisions for New Buildings and Other Structures (Provisions for short) have updated the ground motions used in designing earthquake-resistant structures. The updates stem from the 2018 U.S. Geological Survey (USGS) National Seismic Hazard Model (NSHM) and recommendations from the Building Seismic Safety Council “Project ’17,” a collaboration with the USGS predominantly funded by the Federal Emergency Management Agency. Project ’17 recommended updates to how the design ground motions of the Provisions should be derived from USGS NSHMs. Like previous editions, the 2020 Provisions continue to take the lesser of probabilistic and deterministic ground motions, although Project ’17 did consider discontinuing this deterministic capping of probabilistic values. The probabilistic ground motions are so-called risk-targeted spectral accelerations (RTSAs), as introduced in the 2009 Provisions. Unlike the uniform-hazard (e.g., 2,475-year) ground motions of previous editions, RTSAs are derived from the entirety of the hazard curves (of return period vs. spectral acceleration) output from the USGS NSHM. Prior to the 2020 Provisions, the deterministic caps on the RTSAs corresponded to “characteristic earthquakes on all known active faults.” Since the Uniform California Earthquake Forecast used in the latest two updates of the USGS NSHM (i.e., UCERF3) no longer defines characteristic earthquakes, the deterministic ground motions of the 2020 Provisions now correspond to scenario earthquakes from hazard disaggregation at the probabilistic ground motion values. In contrast to the single magnitude that was previously chosen for each characteristic earthquake, the mean magnitude for each scenario earthquake averages over all magnitudes that could occur on a fault. Further, unlike characteristic earthquakes, the disaggregation-based scenario earthquakes are fully consistent with the UCERF3 rupture inventory.

Presenting Author: Nicolas Luco

Student Presenter: No

Authors