UCLA

In August 2024, the National Seismic Hazard Model (NSHM) Steering Committee appointed a 14-member panel (herein referred to as “The Panel” or “Panel”) to review the time-independent earthquake rupture forecast (ERF) for the 2025 update of the Puerto Rico and U.S. Virgin Islands (PRVI) component of the NSHM (herein referred to as “PRVI25-ERF”). This report summarizes the Panel’s findings and recommendations. The primary materials for Panel review were nine papers documenting the PRVI25-ERF draft model. The Panel was also informed about the process to update the ERF in three briefings by the U.S. Geological Survey (USGS) development team (herein referred to as the “USGS Team” or “Team”). The PRVI25-ERF model is a substantial improvement over the current model, which was released in 2003 (PRVI03-ERF; Mueller et al., 2003, 2010). The USGS Team has incorporated substantial new information obtained about the region and its tectonic environment over the past twenty years, and they have combined this information with state-of-the-art probabilistic seismic hazard modeling techniques to produce a complete probabilistic ERF that, for the most part, reflects the best available earthquake science. The publications and reports on the model development are generally excellent and comprehensive. State-of-the-art techniques have been employed in the neotectonic and paleoseismic evaluations of fault activity, in the analysis of earthquake catalogs, and in the incorporation of slip-rate estimates from paleoseismic and geodetically constrained tectonic block models. The model of fault slip rates has been generalized to a probabilistic representation, and the epistemic uncertainties of these rates have been incorporated into the logic-tree formulation using a novel stochastic sampling method. The fault system inversion techniques developed for the CONUS23-ERF for the western U.S. (Field et al., 2023; Milner and Field, 2023) have been successfully applied to the PRVI fault systems. The Panel raised questions about the adequacy of the PRVI seismicity data, which are compromised by magnitude inconsistencies; the structure of the crustal fault model and its accommodation of strain partitioning; the weakness of the geodetic constraints on seismic coupling factors; and the low amplitudes of the epistemic uncertainties in the hazard derived from the ERF logic tree. Several of the Panel’s 16 actionable recommendations are directed towards a more complete assessment of the ERF uncertainties. Of particular concern is the bias and uncertainty related to the seismic coupling factors. The Panel also offers 19 aspirational recommendations for model improvements that could be implemented in future PRVI updates. The Panel recommends that, after suitable responses to the actionable recommendations in this review, the PRVI25-ERF model be adopted as a component of the NSHM.