Abstract
We analyse statistically the long-term properties of several instrumental earthquake catalogues. Complete catalogues exhibit both short- and long-term clustering for earthquakes of all depth ranges. After accounting for the effect of short-term clustering, we find that in residual (declustered) catalogues, long-term clustering, not periodicity, characterizes the occurrence of all earthquakes—shallow, intermediate, and deep. The degree of clustering in residual catalogues is the same for earthquakes in different depth ranges. Circumstantial evidence indicates that the long-term variation of seismicity is governed by a power-law temporal distribution; as in short-term clustering, it is scale invariant. The fractal dimension of an earthquake set on the time axis is of the order of 0.8–0.9. Therefore, mainshock occurrence is closer to a stationary Poisson process than standard aftershock sequences of shallow earthquakes.
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