The past few years have seen an unusual number of epically large earthquakes, with several—in Sumatra, Chile, and Japan—reaching magnitudes of roughly 9.0. This has led a number of people to wonder whether large earthquakes cluster and, if they do, whether we should be getting nervous about when the next one will hit. A new analysis in PNAS, however, suggests the elevated activity is nothing unusual, although the long gap between recent activity and past monster quakes was statistically unlikely.

The authors went through the US Geological Survey's historic records, identifying every earthquake above magnitude 7.0 that occurred between 1900 and 2011. To eliminate aftershocks and local strain caused by initial earthquakes, the authors set a cutoff: any smaller earthquakes within three years and 1,000km of a quake were considered its aftershocks, and not incorporated into the analysis. This is a fairly liberal definition of aftershock, and takes two recent monster quakes out of the analysis, both over 8.5 and near the site of the first Sumatran quake. But it is consistent with what we know about how major quakes can add strain to areas at a considerable distance from where the fault actually ruptured.

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