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‘Silent slips’ complicate earthquake patterns

/ Source: The Associated Press

Liquids under high pressure deep beneath Earth’s surface allow some faults to move in a slow, gradual way, potentially delaying earthquakes in some areas but increasing the threat elsewhere, according to a study.

A “silent slip” is caused by a pocket of high-pressure fluid in the deepest part of a fault where two rock masses grind together, Shuichi Kodaira and colleagues report in a paper in Friday’s issue of the journal Science.

The silent movements have been noted only in the last few years. Kodaira, of the Japan Agency for Marine-Earth Science and Technology, was able to propose the mechanism after studying movement in the Nankai Trough off the coast of Japan.

Similar movement has been recorded in the Cascadia Trough, running from Puget Sound to Vancouver Island off the west coast of the United States and Canada, said William L. Ellsworth, chief scientist of the U.S. Geological Survey’s Western region earthquake hazards team.

“Not a lot is known about silent slip,” Ellsworth said; the Nankai study describes one active hypothesis.

New processes identified
He said two important geological discoveries in recent years have been silent slip, which occurs without serious quakes, and nonvolcanic tremors that take originate deep in subduction zones where one of the earth’s giant plates is moving beneath another.

Scientists are working to determine what those processes can reveal, said Ellsworth, who was not part of the Japanese study group.

The Japanese researchers said the movement was discovered using sensitive Global Positioning System tracking.

While silent slip movement is gradual enough not to be felt, Kodaira said it may not ease the threat of a a major temblor. That’s because, while the movement may ease stress where it occurs, it could increase pressure on adjacent rocks that are locked together and don’t have fluids to ease their movement.

“A silent slip may function as precursor or trigger of the earthquake slip in the locked zone,” said Kodaira.

Complicates predictions
The finding complicates predictions of how often quakes will recur, the researchers added.

In the Nankai Trough earthquakes occur about every 100 to 150 years, but the Tokai section where the silent slip was found is an exception, Kodaira said.

“We do keep in our mind that the silent slip may trigger a large earthquake, after several recurrences of the silent slips,” Kodaira concluded.

The study was funded by the Japan Agency for Marine Earth Science and Technology and the Earthquake Research Institute, University of Tokyo.