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WHAT CONSTITUTES AN EARTHQUAKE PREDICTION? An earthquake prediction must specify the expected magnitude range, the geographical area within which it will occur, and the time interval within which it will happen with sufficient precision so that the ulti- mate success or failure of the prediction can readily be judged. Only by careful recording and analysis of failures as well as successes can the eventual success of the total effort be evaluated and future direc- tions charted. Moreover, scientists should also assign a confidence level to each prediction; it is clear that—particularly in the early stages of the effort—some predictions will merit considerably greater assurance than others, but even low-confidence predictions should be considered and evaluated. A prediction of locality without specifica- tion of time, while valuable in itself, and constituting the principal basis of present-day seismic zoning maps, is not an acceptable predic- tion in the sense that the word is used in this report. It is clear that the time uncertainty for some predictions will necessarily be much greater than for others. Very broad time predic- tions (e.g., "within 25 years") can have significant value in encouraging permanent social responses such as the development of realistic building codes, land-use planning, and long-term disaster preparation. Neverthe- less, we recognize that the word "prediction" is more commonly inter- preted—particularly by the public—to imply a much smaller time uncertainty. The expression "short-term prediction" as used here means an earthquake prediction that has a sufficiently precise time estimate, close enough to the time at which the prediction is made, so that only temporary or transient, but nevertheless very significant, social re- sponses are possible—such as the alerting of emergency forces, possibly emergency strengthening of certain special structures, and the evacua- tion of questionable structures or areas. Even in the absence of evidence adequate to permit a low-confidence prediction, unexplained geophysical anomalies in a given area may still be of sufficient concern to cause it to be designated an "area of inten- sive study." This designation has been used in Japan to avoid undue public alarm in situations where no realistic prediction is possible in spite of recognized anomalies that are possible precursors to earthquakes and that clearly warrant accelerated investigation. Any capability for reliable prediction, no matter how long the time scale, is more useful than none at all. For the highly seismic State of

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8 California, we can now say only that certain densely populated areas will almost certainly experience a great earthquake at some time within the next l00-200 years. Although effort and money are now being expended to design structures so as to avoid collapse during such an earthquake, a great deal is being left undone because of the lack of a sense of urgency that a more precise short-term prediction might induce. For example, many structures in metropolitan areas of high population densi- ty and high seismicity will surely fail in a great earthquake, but because of the lack of precision in our estimates of the time of their occurrence it is unlikely that much will be done to avoid such disasters. A reliable prediction that a great earthquake will occur at a given place and time would alert utility companies to possible difficulties and induce community leaders to take actions that would minimize loss of life and property damage.