Working Group on California Earthquake Probabilities (WGCEP)

The ongoing WGCEP is developing a time-dependent, statewide earthquake-rupture forecast that uses “best available science”. This model, called the Uniform California Earthquake Rupture Forecast (UCERF), will have the endorsement of SCEC, USGS, and CGS. The California Earthquake Authority, which holds about two-thirds of all homeowners earthquake insurance policies throughout the state, will use the model to set insurance rates. Development of this model is tightly coordinated with the USGS National Seismic Hazard Mapping Program. Specifically, our time-independent component will be used in the 2007 USGS/CGS California hazard map. We are deploying the model in an adaptable, extensible framework where modifications can be made as warranted by scientific developments, the collection of new data, or following the occurrence of significant earthquakes (subject to the review process). Our implementation strategy is to add more advanced capabilities only after achieving more modest goals.

The following are examples of SCEC activities that could make direct contributions to WGCEP goals:

• Extend our UCERF, which gives the magnitude, average rake, and rupture surface of all possible earthquakes throughout the state, to include different viable slip time histories for each of these ruptures (A).
• Develop models that give fault-to-fault rupture probabilities as a function of fault separation, difference in strike, and styles of faulting (A9).
• Refine estimates of observed earthquake rates and their uncertainties, both statewide and as a function of space. This could include associating historic events with known faults (A4 and C).
• Further refinement of fault models including geometries, seismogenic depths, and aseismicity parameters (C).
• Development of deformation models that give improved slip- and stressing-rates on known faults, as well as off-fault deformation rates elsewhere (A3).
• Further constrain viable magnitude-area relationships, especially with respect to how they are being used in this project (A4).
• Develop moment-balanced rupture models that predict a long-term rate of earthquakes that is consistent with the historical record (e.g., no discrepancy near magnitude 6.5) (A6).
• Develop methodologies for computing time-dependent earthquake probabilities in our model. These methodologies could include approaches that invoke elastic-rebound-theory motivated renewal models, earthquake triggering effects that include aftershock statistics, or physics-based earthquake simulations (A6).
• Develop easily computable hazard or loss metrics that can be used to evaluate and perhaps trim logic-tree branch weights (B6, C).
• Develop a community-standard hazard-to-loss interface (i.e., that can be used by anyone from academics, government officials, and consulting companies) (B6, C). Further suggestions and details can be found at http://www.WGCEP.org, or by speaking with the project leader (Ned Field).