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Are earthquake probabilities or forecasts the same as prediction?

No. Forecasts are rather like weather forecasts, while earthquake probabilities are more like climate. Here are some distinctions:

Probabilities

Most earthquake probabilities are determined from the average rate of historical events. Assuming the annual rate is constant, one can make a probability statement about the likelihood of such an event in the next so-many years. These probabilities might range from one in thirty to one in three hundred.

For some faults, historical occurrences are not available, but rate of slip along the fault can be estimated. Assuming a particular magnitude, one can estimate the number of years it would take to accumulate the required amount of slip. This estimate can be used to give an annual rate and used in the same manner as historical rates. These probabilities might range from one in three hundred to one in three thousand.

Finally, given a large event, there is a declining rate of aftershocks. From this rate, the probability of an aftershock can be determined. These probabilities might be larger than one in thirty.

Predictions

Predictions usually occur as a result of some event supposed to be indicative of an earthquake occurring in the near future. Such an event may be a swarm of small earthquakes, increasing amounts of radon in local water, unusual behavior of animals, increasing size of magnitudes in moderate size events, or a moderate-magnitude event rare enough to suggest that it may be a foreshock.

Unfortunately, most such precursors frequently occur without being followed by an earthquake. This means that the forecast must be made in probabilistic terms. Estimates of such the probabilities seem to be no greater than one in three, to one in ten and hence the forecasts have low reliability. A succession of unreliable forecasts is likely to do more harm than good.

One might hope that before a major earthquake several such precursor would occur. This might increase the reliability of a forecast. An earthquake forecast was made in China several decades ago, based on small earthquakes and unusual animal activity. People were able to sleep outside of their homes and thus were spared when the main quake struck and caused widespread destruction.

However, unfortunately, many earthquakes are preceded by no precursory events whatsoever. The next large Chinese event was entirely unheralded and scores of thousands of Chinese died.

(contributed by Dave Perkins)

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