– Knight Ridder Newspapers

WALNUT CREEK, Calif. – Tides may trigger earthquakes in coastal areas, according to a new study.

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

But now, researchers from UCLA and Japan have discovered 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 that looked at all types of faults failed to find a tidal influence on earthquakes, so UCLA seismologist Elizabeth Cochran and her colleagues at UCLA and Tohoku University in Japan focused on a specific type of fault.

The team looked at thrust faults, where the earth on one side of the fault is pushed over the other side, and they zeroed in on shallow faults just offshore where the tidal effect would be the biggest. Their theory was that where the tides are pushing in the same direction as the fault moves, the extra shove could cause an earthquake.

The team studied earthquakes that were magnitude 5.5 or greater on these faults over the entire globe from the last 25 years. They found that during times of large tidal fluctuations, these faults were three times more likely to have an earthquake during high tides than during low tides.

“When the tides are big, some times of the day are more dangerous than other times of the day,” said geophysicist John Vidale of UCLA.

These unusually high tides typically pile five or more feet of water onto coastal faults in places like Alaska, South America and Tonga.

California doesn’t experience tides this big, and most of the dangerous faults along the California coast are strike-slip faults where the earth on one side of the fault slides past the other side, rather than thrust faults.

But Vidale believes tides could just as easily affect strike-slip faults. His previous study of California’s offshore faults found that even the relatively small tides in this area may be causing a one or two percent increase in earthquakes.

The research probably won’t help geologists predict earthquakes, but it could be a big step in uncovering what causes them.

“It helps us understand what forces it takes to trigger an earthquake,” Cochran said.

Geologists have been working on the idea that a large earthquake on one fault loads stress on to some surrounding faults and reduces the stress on others. But questions remain about whether the stress is large enough to actually affect earthquakes on those surrounding faults.

For a while after the Loma Prieta earthquake hit the San Andreas fault in 1989, the nearby Hayward fault stopped experiencing the very small, imperceptible shocks it normally has. And the San Gregorio fault, which is usually much quieter started having more of these tiny quakes.

The tidal study supports this evidence that faults can affect each other, said Stein. “That’s a very positive step.”

(c) 2004, Contra Costa Times (Walnut Creek, Calif.).

Visit the Contra Costa Times on the Web at http://www.contracostatimes.com.

Distributed by Knight Ridder/Tribune Information Services.

AP-NY-10-21-04 2153EDT

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