Deepest quakes hit in surprise seasonal cluster

摘要

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 the Scripps Institution of Oceanography in San Diego, 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 [Geophysical Research Letters, doi.org/7w7). The seasonality could be related to the tidal forces felt by Earth due to the sun's gravity, Zhan says, or to a wobble in Earth's rotational axis due to seasonal sloshing of water and air around the planet.