Most seismic hazard analyses assume that earthquakes have a Poisson distribution and that data from recorded earthquakes can be used to predict future seismic activity in a region. Historic catalogs, however, may show different earthquake occurrence rates for regions that are geologically similar, suggesting, on a local scale at least, temporal variations in seismicity. The simple two-state model presented here allows a region to have both seismically “active” and “inactive” states; in this model, earthquakes have a Poisson distribution during both states, but occur at a higher rate when the region is “active.” The probability of changing from one state to the other at any time is constant. The two-state model reduces to the usual Poisson model if the region is always “active” or if the rate is the same for both states.

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