Taming the infamous San Andreas fault with buried nuclear bombs or deep grease injections is the stuff of science fiction, but Mother Nature herself may be defusing a major fault in Taiwan, China. Researchers are now showing that typhoons passing over the island have been triggering quakes that harmlessly release fault strain over hours and days rather than destructively over seconds or minutes. And these slow earthquakes, they speculate, may be staving off a big one.
Taiwan is under considerable strain. The Philippine Sea's tectonic plate shoves itself from the east under the island's eastern Coastal Range, pushing it up along the fault. But the fault sticks, refusing to slip, which builds strain until the fault suddenly does let loose in an earthquake that releases the accumulated strain.
In 2005, geophysicists Chi-Ching Liu of the Institute for Earth Sciences in Taipei and Alan Linde and Selwyn Sacks of the Carnegie Institution of Washington's Department of Terrestrial Magnetism in Washington, D.C., first noticed a connection between this changing strain and typhoon activity. As they reported that year at a meeting (Science, 23 December 2005, p. 1899), they soon began to notice that slippage on the fault over hours to days that was recorded as changing strain--that is, slow earthquakes--often coincided with a drop in atmospheric pressure as a typhoon passed over.
Now, with data from 2002 to 2007 that the researchers report in tomorrow's issue of Nature, they find that there were 30 passing typhoons and 20 slow earthquakes, 11 of which started just after atmospheric pressure dropped as the center of a storm passed. "The chances of that being random are 1 in 108," says Linde. That makes the case for typhoons triggering slow Taiwan quakes "unequivocal," he says. And triggering makes physical sense, he notes; the extreme low atmospheric pressure of a typhoon would reduce the squeeze on the fault and let it slip.
Liu and his colleagues take the next step, an admittedly speculative one, and suggest that typhoon-triggered slow quakes may be preventing great earthquakes from rupturing the fault. If typhoons are triggering the harmless easing of strain at other spots on the fault, they reason, the storms may be dividing one long fault that could break in a single great quake into a number of shorter fault segments, each only capable of a far smaller quake. Smaller quakes are all that's been recorded there in the past 100 years.
Geophysicist Herb Dragert of the Geological Survey of Canada in Sidney, British Columbia, is willing to go along with the typhoon-slow quake connection. The one case detailed in the paper is "convincing," he says. But "it's far too early to say" whether triggering is easing enough strain at enough places along the fault to head off a great quake.