Scientists uncover link between sea, earthquakes

Tremors in offshore faults more common at high tide

The Nation

October 22, 2004|By KNIGHT RIDDER/TRIBUNE

WALNUT CREEK, Calif. - For more than 30 years, scientists have searched in vain for a connection between tidal forces and earthquakes, hoping for clues to help predict when a fault is ready to slip.

Now, researchers from UCLA and Japan have discovered that certain types of faults near coasts are more likely to let loose during high tides.

The finding supports a theory about how nearby faults interact with each other that could be very important for understanding earthquakes in the San Francisco Bay area and other regions with several active faults.

"There's this idea that earthquakes are interacting by the transmission of small stresses," said geophysicist Ross Stein of the U.S. Geological Survey in Menlo Park, Calif. "This study makes that argument stronger."

The stress exerted by water piling up on an offshore fault during high ocean tides is about the same as the stress exerted by a large earthquake on the other faults surrounding it.

So if the tides can provoke a fault to slip, maybe nearby faults can affect each other as well, said Stein.

Previous studies of all types of faults failed to find a tidal influence on earthquakes, so seismologist Elizabeth Cochran and her colleagues at UCLA and Tohoku University in Japan focused on thrust faults, where the earth on one side of the fault is pushed over the other side.

They zeroed in on shallow faults just offshore where the tidal effect would be the biggest. Their theory was that where tides push in the same direction as the fault moves, the extra shove could cause an earthquake.

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