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Puget Sound hit with swarm of small earthquakes

SEATTLE -- You can't feel them, but the Puget Sound is suffering from a reoccurring case of the tremors.

Seismologists from the Pacific Northwest Seismic Network began noticing the tremors east of Vancouver Island on December 22nd.

Nearly two weeks after it started, more than 8,000 quakes have been registered between North Vancouver Island and nearly all the way to Seattle -- and the tremors are still moving south.

"It's interesting, we don't understand it but we don't think it's dangerous," said John Vidale, Director of the PNSN, who has observed the event several times since the pattern was first discovered nearly 15 years ago.

"It seems regular, but in fact it's not," said Vidale. "The last couple of years it's been a more complicated pattern, but two years before that it was pretty regular with 14 month intervals."

Scientists know what's going on but they aren't sure what's triggered this batch of rippling tremors. Vidale says it's caused by two tectonic plates colliding under our feet. The North American plate and the Juan de Fuca plate are moving about an inch over the course of a couple of weeks.

"They kind of rattle as they are moving and so we see tremors," said Vidale.

The noted authority on earthquakes says the tremors are a form of stress relief very deep on the subduction zone, which is very different than the shallow areas that load up stress for 500 years and then release it all in five minutes.

That was the case with the Nisqually Earthquake 15 years ago that caused wide spread damage in western Washington.

In August of 2010, scientists from the University of Washington installed seismic equipment ahead of tremors heading toward the Olympic mountain range. The research that was gathered during that period has helped scientists anticipate and explain what has become a nearly yearly event.

"It's just some slow slip that takes a couple of days to happen at any point and takes weeks for it to migrate through until it runs out of energy," said Vidale.

That energy could run out at any time or last another week when the tremors where the rolling tremors are expected to end underneath Olympia.

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