Our lack of success in predicting earthquakes has caused earthquake program managers, even in Japan, to cut back on prediction research and focus on earthquake engineering, the effects of earthquakes, and the faults that are the sources of earthquakes. Yet in a more limited way, we can say something about the future; indeed, we must, because land-use planning, building codes, and insurance underwriting depend on it. We do this by adopting the strategy of weather forecasting—20 percent chance of rain tonight, 40 percent tomorrow.
Earthquake forecasting, a more modest approach than earthquake prediction, is more relevant to public policy and our own expectations about what we can tell about future earthquakes. The difference between an earthquake prediction and an earthquake forecast has already been stated: a prediction specifies time, place, and magnitude of a forthcoming earthquake, whereas a forecast is much less specific.
Two types of forecasts are used: deterministic and probabilistic. A deterministic forecast estimates the largest earthquake that is likely on a particular fault or in a given region. A probabilistic forecast deals with the likelihood of an earthquake of a given size striking a particular fault or region within a future time interval of interest to society. An analogy may be made with hurricanes. The National Weather Service can forecast how likely it is that southern Florida may be struck by a hurricane as large as Hurricane Andrew in the next five years; this is probabilistic. It could also forecast how large a hurricane could possibly be: 200-mile-per-hour winds near the eye of the storm, for example. This is deterministic.