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Swarm of Undersea Quakes Draws Scientists

March 8, 2005

SEATTLE (AP) — A swarm of undersea earthquakes off the Pacific Northwest coast has scientists from the University of Washington scrambling in hopes of glimpsing of two tectonic plates pulling apart.

The 209-foot RV Thomas G. Thompson, a research ship from the university, headed for the Endeavor Hot Vents over the weekend after seismic equipment had detected nearly 3,800 small quakes as of late Thursday.

Among those on the ship are scientists from Oregon State University and the National Oceanic and Atmospheric Administration, said Robert P. Dziak, an oceanographer at the Hatfield Marine Science Center in Newport, Ore.

“These earthquake swarms are associated with sea floor spreading,” Dziak said.

“The speculation is it might be a volcanic eruption or a magma event on the ridge,” Garry Rogers, a seismologist with the Geological Survey of Canada in Vancouver, British Columbia, said Sunday.

“Thousands of earthquakes occurring over a few days, it’s a tremendous amount of energy, but it’s way offshore,” Rogers said. “They are too far to be felt, so as far as we know there is no threat.”

The swarm included a 4.1 magnitude quake at 5:01 a.m. Sunday, “just to remind us it’s a very active earthquake area,” he said.

The vents are in a 6.2-mile segment of the Northern Juan de Fuca Ridge beneath about 7,425 feet of water northwest of the mouth of the Columbia River and about 165 miles off southern Vancouver Island.

The ridge, an area where the Juan de Fuca and Pacific plates of the earth’s crust are moving apart, experiences intense occasional bursts of activity between periods of quiet, Rogers said.

“We saw them in 1999 and 2001 going on for a week or so with a thousand quakes, but there’s never been a ship out there when this is happening to see exactly if there is magma being ejected on the sea floor,” he said.

Experts agree the subduction zone where the Pacific Plate plunges beneath the Juan de Fuca plate will at some point generate a monster earthquake that could devastate much of the Pacific Northwest coast. A subduction quake in 1700 generated a tsunami that caused widespread destruction as far as Japan.

One of the biggest problems in trying to predict when the next big one will hit is not knowing how fast the two plates are moving, and that is a key reason the research vessel was dispatched, Rogers said.

“Is it millimeters over many years? Is it all of a sudden centimeters or even meters in the period of a few minutes or seconds during these earthquake swarms? We really don’t know that,” he said.

What does seem clear is that the latest quake swarm is unlikely to signal a jolt big enough to generate a tsunami, Rogers said.

“We don’t feel there is a tsunami threat, even though there is vertical motion, in that we have never seen a tsunami generated from a mid-ocean ridge,” he said.

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