LONG-TERM EARTHQUAKE CLUSTERING

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.