"Large tides have a significant effect in triggering earthquakes," said Elizabeth Cochran, a UCLA graduate student in Earth and space sciences and lead author of the Science paper. "The earthquakes would have happened anyway, but they can be pushed sooner or later by the stress fluctuations of the tides."

"Scientists have long suspected the tides played a role, but no one has been able to prove that for earthquakes worldwide until now," said John Vidale, UCLA professor of Earth and space sciences, interim director of UCLA's Institute of Geophysics and Planetary Physics, and co-author of the paper. "Earthquakes have shown such clear correlations in only a few special settings, such as just below the sea-floor or near volcanoes."

"There are many mysteries about how earthquakes occur, and this clears up one of them," Vidale said. "We find that it takes about the force arising from changing the sea level by a couple of meters of water to noticeably affect the rate of earthquakes. This is a concrete step in understanding what it takes to set off an earthquake."

Cochran, Vidale and co-author Sachiko Tanaka are the first researchers to factor in both the phase of the tides and the size of the tides, and are using calculations of the effects of the tides more accurate than were available just three years ago. Tanaka is a seismologist with Japan's National Research Institute for Earth Science and Disaster Prevention.

Cochran and Vidale analyzed more than 2,000 earthquakes worldwide, magnitude 5.5 and higher, which struck from 1977 to 2000. They studied earthquakes in "subduction zones" where one tectonic plate dives under another, such as near the coasts of Alaska, Japan, New Zealand and western South America. "These earthquakes show a correlation with tides because along continent edges ocean tides are strong," Vidale said, "and the orientation of the fault plane is better known than for faults elsewhere."

Cochran conducted a statistical analysis of the earthquakes and tidal stress data, using state-of-the-science tide calculations from Tanaka and the best global earthquake data, which came from Harvard seismologists. This research follows up on a 2002 study by Tanaka. The current research was funded by the National Science Foundation and the Laurence Livermore National Laboratory.

Cochran and Vidale found a strong correlation between when earthquakes strike and when tidal stress on fault planes is high, and the likelihood of these results occurring by chance is less than one in 10,000, Cochran said. They found that strong tides impose enough stress on shallow faults to trigger earthquakes.

If the tides are very large, more than two meters, three-quarters of the earthquakes occur when tidal stress acts to encourage triggering, she found. Fewer earthquakes are triggered when the tides are smaller.

In California, and in fact in most places in the world, the correlation between earthquakes and tides is considerably smaller, Vidale said. In California, tides may vary the rate of earthquakes at most one or two percent; the overall effect of the tides is smaller, he said, because the faults studied are many miles inland from the coast and the tides are not particularly large.