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Subject: VLF Quakes?
From: "David Saum" DSaum@............
Date: Mon, 1 Apr 2002 14:03:00 -0500

From 3/28 Seattle Times:

Thursday, March 28, 2002 - 11:15 a.m. Pacific

Feel that? If not, it may be a silent quake 

By Eric Sorensen
Seattle Times science reporter
ELLENSBURG - Western Washington is in the midst of an earthquake and a
rather sizeable one, but have no fear: it's silent.
Researchers at Central Washington University, reporting in tomorrow's
issue of the journal Science, say the region has in fact gone through
eight "silent earthquakes" since 1992. One in August 1999 released the
energy of a magnitude 6.7 temblor, nearly as big as last year's 6.8
Nisqually quake. But because the quakes stretch out over six to eight
weeks, their energy is dissipated and barely noticeable.

" 'Slow earthquake' is the other term that's been widely used for
these," said Meghan Miller, a CWU geologist and lead author of the
paper. "The earthquake thing is probably a little bit of a misnomer
because there's no real shaking associated with the event. But that's
what they're called and I kind of like the 'silent earthquake.' It's

Miller and her colleagues were inspired by work published in Science
last year by Herb Dragert, a research scientist with the Geological
Survey of Canada in Sidney, B.C., on the 1999 quake. Both papers rely on
measurements taken at global-positioning sensors arrayed from Seattle to
Neah Bay and on the southern half of Vancouver Island.

Ordinarily, the sensors show Western Washington creeping eastward about
one-half of an inch a year. This is because the oceanic Juan de Fuca
plate is moving east and under the North American Continental plate,
forcing the continental plate to compress and move east. But every 14
months or so, the compression in the continental plate eases and
rebounds westward.

This would be a problem if it rebounded all at once, letting loose a
fast, powerful tremor in the upper plate and beneath the major
population centers of Vancouver, B.C., Seattle and Portland.

But researchers theorize that the movement of silent quakes takes place
in a deep, warm and well-lubricated section of the Cascadia fault -
where the two plates meet - avoiding the jerky motion that happens when
more shallow, cold and stickier portions of the fault give way.

The most recent silent quake began on Feb. 7 near Friday Harbor and has
been spreading across the region for weeks.

The quake is at least magnitude 6, Miller said. 

Researchers say understanding these quakes gives them a new tool for
monitoring the Cascadia fault and how it can cause large earthquakes.
While it probably won't help them predict when earthquakes will happen,
it should help them prepare for the next large earthquake with better
building codes and other safety measures in areas most likely to be

Eric Sorensen can be reached at 206-464-8253 or

....David Saum


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