Forecasting the Long-Term Spatial Distribution of Earthquakes for the 2023 U.S. National Seismic Hazard Model Using Gridded Seismicity

Probabilistic seismic hazard analyses such as the U.S. National Seismic Hazard Model (NSHM) typically rely on declustering and spatially smoothing an earthquake catalog to estimate a long‐term time‐independent (background) seismicity rate to forecast future seismicity. In support of the U.S. Geological Survey’s (USGS) 2023 update to the NSHM, we update the methods used to develop this background or gridded seismicity model component of the NSHM. As in 2018, we use a combination of fixed and adaptive kernel Gaussian smoothing. However, we implement two additional declustering methods to account for the fact that declustering is a nonunique process. These new declustering methods result in different forecasts for the locations of future seismicity, as represented by spatial probability density functions that are later combined with a rate model to produce a full gridded earthquake rate forecast. The method updates, particularly in the separation of the spatial and rate models as well as revised regional boundaries, in some places cause substantive changes to the seismic hazard forecast compared to the previous 2018 NSHM. Additional updates to catalog processing and induced seismicity zones also contribute to changes in the gridded seismicity hazard since 2018. However, these changes are well understood and reflect improvements in our modeling of gridded seismicity hazard.

The authors thank Charles Mueller, John Anderson, and Iason Grigoratos for discussions about the various declustering and smoothing methods and providing codes. The authors also gratefully acknowledge the invaluable assistance, codes, and discussions from the late Ilya Zaliapin. Scott Ausbrooks, Noel Bartlow, Mike Brudzinski, Shannon Fasola, Steve Horton, Paul Ogwari, Shelby Peterie, Alexandros Savvaidis, Robert Skoumal, Jake Walter, and Will Yeck all provided information that was critical to defining the areas of induced seismicity described in the  Appendix. Robert Skoumal and Elizabeth Cochran added valuable feedback on an earlier version of the  Appendix, and the authors thank Julie Herrick for help making the map figure in the  Appendix. The authors also thank David Shelly, Associate Editor Sanjay Bora, and an anonymous reviewer who provided comments that helped greatly improve the article. The Scientific Colour Maps Hawai?i and vik (Crameri, 2018) are used in this study to prevent visual distortion of the data and exclusion of readers with color‐vision deficiencies (Crameri et al., 2020). Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.