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Some temblors probably were triggered by tides

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11:54am, November 29, 2004
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Detailed analyses of past large earthquakes suggest that some of the temblors may have been triggered by strong tides in Earth's crust and its oceans.

Such tides, induced by the gravitational attraction of the sun and moon, cause variations in the stress inside rocks and can significantly affect the pressure that holds together the two sides of a fault zone. An Earth-crust tide can increase or decrease this pressure by 500 kilograms per square meter, and an ocean tide can have 10 times that effect by sloshing into or away from a shoreline adjacent to an inland fault, says John E. Vidale of the University of California, Los Angeles.

To see whether tide-induced stress variations might be triggering earthquakes, Vidale and his colleagues looked for effects of tides, if any, during more than 2,000 large earthquakes that occurred in the last quarter-century on fault zones at rock depths of less than 40 kilometers, where such forces can have significant effects.

If tide-induced stress variations had no influence on earthquakes, half of the temblors would occur when the pressure holding together the two sides of a fault is below average and half would occur when the pressure is above average, says Vidale. However, the researchers' analysis showed a different pattern.

Of the 19 earthquakes that happened when the stress along a fault at a temblor's epicenter either increased or decreased by more than 2,000 kg/m2, 14—or 74 percent—occurred when the pressure holding together the fault was lower than average.

Small variations in stress didn't produce such lopsided statistics, Vidale and his colleagues report in the Nov. 12 Science. These patterns suggest that tidal stress can trigger some earthquakes, says Vidale.

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