A stochastic forecast of California earthquakes based on fault slip and smoothed seismicity

We present a testable stochastic earthquake source model for intermediate- to long-term forecasts. The model is based on fundamental observations: the frequency-magnitude distribution, slip rates on major faults, long-term strain rates, and source parameter values of instrumentally recorded and historic earthquakes. The basic building blocks of the model are two pairs of probability density maps. The first pair consists of smoothed seismicity and weighted focal mechanisms based on observed earthquakes. The second pair corresponds to mapped faults and their slip rates and consists of smoothed moment-rate and weighted focal mechanisms based on fault geometry. We construct from the model a "stochastic event set," that is to say, a large set of simulated earthquakes that are relevant for seismic hazard calculations and earthquake forecast development. Their complete descriptions are determined in the following order: magnitude, epicenter, moment tensor, length, displacement, and down-dip width. Our approach assures by construction that the simulated magnitudes are consistent with the observed frequency-magnitude distribution. We employ a magnitude-dependent weighting procedure that tends to place the largest simulated earthquakes near major faults with consistent focal mechanisms. Nevertheless, our stochastic model allows for surprises such as large off-fault earthquakes, events that comply with the observation that several recent destructive earthquakes occurred on previously unknown fault structures. We apply our model to California to illustrate its features.