SOME earthquakes seem to hit in a seasonal pattern – and no one knows why. “Physically, it doesn’t make any sense,” says Zhongwen Zhan. Zhan, of Scripps Institution of Oceanography at UC San Diego in California, and his colleague Peter Shearer examined a global catalogue of earthquakes that have occurred since 1900. Out of 60 large, deep quakes in the catalogue, 42 hit between April and October. The trend doesn’t appear in shallower or smaller quakes, but stands out clearly for those above magnitude 7.0 and at a depth greater than 500 kilometres.