Using Gridded Seismicity to Forecast the Long-Term Spatial Distribution of Earthquakes for the 2025 Puerto Rico and U.S. Virgin Islands National Seismic Hazard Model

Abstract Gridded (or background) seismicity models are a critical component of probabilistic seismic hazard assessments, accounting for off-fault and smaller-magnitude earthquakes. They are typically developed by declustering and spatially smoothing an earthquake catalog to estimate a long-term seismicity rate that can be used to forecast future earthquakes. Here, we present new gridded seismicity models for use in the 2025 National Seismic Hazard Model (NSHM) for Puerto Rico and the U.S. Virgin Islands (PRVI). The previous PRVI NSHM was released in 2003, and our new models incorporate updates to both data and methodology. We utilize an updated earthquake catalog based on improved Puerto Rico Seismic Network data with newly characterized completeness epochs. The catalog is divided into crustal, subduction interface, and intraslab seismicity using new methods and Slab2 subduction zone geometries. To forecast the long-term spatial distribution of earthquakes, we use an updated methodology developed for the 2023 U.S. 50-state NSHM, considering three declustering methods and two spatial smoothing methods based on 2D Gaussian kernels. To adapt it for the complex seismotectonics of the region, we also adopt probabilistic methods to account for events with unknown depths and uncertainties in tectonic classification, and develop a new method for spatial scaling to counteract the effects of spatial variability in network coverage while maintaining the use of smaller events. Finally, we test the performance of these spatial models in forecasting the location of Mw≥5 earthquakes in the region. Our updated methodology improves the representation of epistemic uncertainty relative to the 2003 model, and our results demonstrate the effectiveness of the new measures we have introduced to address heterogeneities in network detection and systematically evaluate forecast performance.