A Feasibility Study of Medium-term Earthquake Forecasting Using Numerical Earthquake Simulators: Comparison to the WGCEP

We have carried out simulations for earthquakes on models of California's fault system (Virtual California -- "VC") for simulation runs over time intervals from tens of thousands of years to millions of years. Using these simulations, we have now developed techniques to assimilate observed earthquake variability into the simulations. Our technique is based on mining the simulation data to identify time intervals that look most like the recent past history of earthquakes on the California fault system. We then use these optimal time intervals to "look into the future" and forecast the likely locations of future major earthquakes. We note that the parameters that enter into the model are set using the long term average properties of the fault system -- earthquake and plate rate variability are not used at this stage of the simulation. Here we describe this method and carry out a feasibility study of its application. We develop fault-based relative spatial probabilities that can be compared with recent results from the Working Group on California Earthquake Probabilities (WGCEP 2008). Both VC and WGCEP forecast elevated relative probabilities for the Southern San Andreas fault (40.4% VC; 35.5% WGCEP). However, the relative probabilities are significantly different for the Northern San Andreas fault (22.6% VC; 12.7% WGCEP); the Calaveras fault (13.5% VC; 4.2% WGCEP); the Hayward-Rodgers Creek faults (5.0% VC; 18.7% WGCEP); and the San Jacinto fault (10.5% VC; 18.7% WGCEP). An important qualification is that since our model has not been systematically validated, these first probabilistic results should be treated with caution.