杏吧原创

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SOME earthquakes seem to hit in a seasonal pattern 鈥� and no one knows why. 鈥淧hysically, it doesn鈥檛 make any sense,鈥� says .

Zhan, of the Scripps Institution of Oceanography in San Diego, California, and his colleague 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鈥檛 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, ).

The seasonality could be related to the tidal forces felt by Earth due to the sun鈥檚 gravity, Zhan says, or to a wobble in Earth鈥檚 rotational axis due to seasonal sloshing of water and air around the planet.

The solar tides may help explain how earthquakes happen at all at such depths, where the rocks are under high temperatures and pressures that render them soft and pliable 鈥� and unlikely to suddenly rupture, causing a quake. 鈥淭he high pressure makes people think there shouldn鈥檛 be any earthquakes,鈥� says Zhan.

Although Zhan鈥檚 methods are sound the result may only be a statistical aberration, and may lack a physical mechanism, says . 鈥淚f you鈥檙e searching for some possible explanation, the solar tide would be your candidate. But it鈥檚 not a very strong candidate because it鈥檚 not a very strong tide,鈥� he says. As early as the 1930s, geologists claimed to have found a seasonal trend in earthquake activity. Often, as new data comes to light, the correlations weaken, he adds.

Zhang says it will take a few more decades of new data to see if the seasonal trend is more than chance. A confirmed trend may lead to a better understanding of how quakes are triggered and hence better prediction tools, Zhan says.

This article appeared in print under the headline 鈥淒eepest quakes hit in surprise seasonal cluster鈥�