2018 Update of the National Seismic Hazard Models for the United States

The USGS has developed new seismic ground shaking hazard models for the conterminous U.S. and Hawaii that update previous models and will be considered for use in building code design procedures and other public policy documents. For the conterminous U.S., we revised the 2014 hazard models for the Central and Eastern U.S. (CEUS) using an updated earthquake catalog and the new NGA-East for USGS ground-motion models, along with additional site factors and ergodic sigmas that were developed for this 2018 update. These models include additional site conditions and oscillator periods that were not available for previous models and allow for further applications by the building-code community. For the CEUS, the new ground motions for both short and long periods (2% probability of exceedance in 50 years) result in increased ground motions near the New Madrid and Charleston seismic zones. For the western U.S., the new hazard models include effects of deep sedimentary basin effects on seismic shaking based on NGA West 2 factors and increased hazard for long-period shaking in portions of the Seattle, San Francisco, Salt Lake City, and Los Angeles regions. The 2018 hazard accounts for additional amplifications over the deepest portions of the basins where ground shaking typically increases 30% at 5 s SA compared to default models that do not specify basin depth. This result is compatible with recent 3-D simulations for the Pacific NW and Southern California regions that predict significantly enhanced ground shaking for urban regions overlying deep basins. The preliminary Hawaii hazard model is updated from the 1999 version with additional seismic information from the 2006 M 6.7 Kiholo Bay earthquake and the 2018 earthquakes associated with volcanic activity in the East Rift Zone. Besides the new earthquake catalog, new ground motion models have been tested to see which models best forecast Hawaii ground shaking. The new models for Hawaii are being generated using the new ground motion models, a new earthquake catalog, and a revised source zonation model. These models are being considered by the engineering code committees for updating the design procedures for the U.S.