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A statistically significant series of earthquakes in Southern California, 1934-1957, variously filtered to avoid aftershocks and events outside a localized area, has been tested for a Poisson distribution. It has been found that, for any of several methods of filtering, the data do not fit a Poisson distribution and hence a causal connection between weak earthquake events is inferred. The conclusion is in accord with that of Aki for large events.
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L. Knopoff (1964) studied this question.
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