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2020 NEHRP Provisions Design Ground Motions Based on Multi-Period Response Spectra (MPRS) and Their Implications for USGS Hazard Models

The U.S. Geological Survey (USGS) computes design ground motions by combining hazard results from the USGS National Seismic Hazard Models (NSHMs) with the site-specific design code procedures of the NEHRP Provisions. Prior to the 2020 NEHRP Provisions, these design code procedures were applied to the USGS ground motions at three spectral periods (0, 0.2 and 1s) at the reference site class (BC). These procedures included risk-targeted calculations, multiplication by maximum-direction factors and deterministic capping. To obtain design ground motions for other site classes, multiplication by NEHRP site amplification factors was required. Design ground motions for other spectral periods were then derived from a Two-Period-based Response Spectrum (TPRS) shape. As a result, the updates of the USGS NSHM have traditionally given much attention to the behavior of ground motion models (GMMs) at the three mentioned periods and one site class.

For the 2020 NEHRP Provisions, the design code procedures have been updated to use a Multi-Period Response Spectrum (MPRS) in order to overcome the recognized shortcomings of the TPRS and site amplification factors. As a result, the site amplification factors were eliminated from the provisions, which now rely on the USGS to provide hazard results for more periods (22) and site classes (8). We discuss how the MPRS affects the selection and modification of GMMs in the 2018 and future updates of NSHMs. The 2018 NSHM update for the conterminous U.S. (CONUS), which is used to compute the design ground motions of the 2020 NEHRP Provisions, selected and modified GMMs such that they are applicable at all periods and site classes of interest and in some regions considered sedimentary basin effects. Outside of the CONUS (e.g., Hawaii and Alaska), where the USGS has not yet developed new hazard models, the design ground motions of the 2020 NEHRP Provisions are calculated using newly developed generic spectral shapes to achieve estimates of MPRS.

Presenting Author: Sanaz Rezaeian

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