Evolution of Seismic Design Values over the Years

The 2020 NEHRP Provisions, and ASCE 7-24 that is based on it, adopt a new USGS ground motion model that incorporates stie class and basin effects directly into the calculation of gridded seismic design values. For the first time, these values are available only through an on-line seismic hazard data base and are not printed in conventional maps. A review of the evolution of seismic design values over the years and the basis for adoption of the current approach is presented.

The 2018 Update of the USGS National Seismic Hazard Model

Updates to the design ground motions of the 2020 NEHRP Recommended Seismic Provisions come from two main sources: 1) updates for the 2018 U.S. Geological Survey (USGS) National Seismic Hazard Model (NSHM), which improved the scientific modeling of earthquake sources and ground motions, and 2) recommendations from the Building Seismic Safety Council (BSSC) Project ’17 committee, which updated the design ground motion procedures. Major updates for the 2018 NSHM included: 1) incorporation of new ground motion models and site amplification factors in the central and eastern U.S., including the new “NGA-East” models; 2) incorporation of deep sedimentary basin effects in the four regions of Los Angeles, San Francisco Bay, Salt Lake City, and Seattle; 3) relatively minor modifications to the western U.S. crustal and subduction zone ground motion models; and 4) updates to the seismicity catalogs outside of California. USGS computed the design ground motions of Chapter 22 by combining hazard results from the 2018 NSHM with the new BSSC design ground motion procedures. One of the major updates to the design procedures was the recommendation to use Multi Period Response Spectra, which also affected the 2018 NSHM update (in particular, decisions made in selection of ground motion models). This connection and the implications for design ground motion values will also be briefly discussed.

Learning Objectives:

• The collaborations between the U.S. Geological Survey (USGS) and the Building Seismic Safety Council (BSSC) Project '17 will be explained, including how the recommendation to use Multi Period Response Spectra (ground motions at 22 periods and 8 site classes) affected the updates to the USGS hazard model.
• The science behind the 2018 update of the USGS national seismic hazard model, which was used for the development of MCER and MCEG in the 2020 Provisions, will be outlined.