A Synoptic View of the Third Uniform California Earthquake Rupture Forecast (UCERF3)

Probabilistic forecasting of earthquake-producing fault ruptures informs all major decisions aimed at reducing seismic risk and improving earthquake resilience. Earthquake forecasting models rely on two scales of hazard evolution: long-term (decades to centuries) probabilities of fault rupture, constrained by stress renewal statistics, and short-term (hours to years) probabilities of distributed seismicity, constrained by earthquake-clustering statistics. Comprehensive datasets on both hazard scales have been integrated into the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3). UCERF3 is the first model to provide self-consistent rupture probabilities over forecasting intervals from less than an hour to more than a century, and it is the first capable of evaluating the short-term hazards that result from multievent sequences of complex faulting. This article gives an overview of UCERF3, illustrates the short-term probabilities with aftershock scenarios, and draws some valuable scientific conclusions from the modeling results. In particular, seismic, geologic, and geodetic data, when combined in the UCERF3 framework, reject two types of fault-based models: long-term forecasts constrained to have local Gutenberg-Richter scaling, and short-term forecasts that lack stress relaxation by elastic rebound.