Posted on 12/15/2009 1:08:32 PM PST by decimon
SAN FRANCISCO – For most of a decade, scientists have documented unfelt and slow-moving seismic events, called episodic tremor and slip, showing up in regular cycles under the Olympic Peninsula of Washington state and Vancouver Island in British Columbia. They last three weeks on average and release as much energy as a magnitude 6.5 earthquake.
Now scientists have discovered more small events, lasting one to 70 hours, which occur in somewhat regular patterns during the 15-month intervals between episodic tremor and slip events.
"There appear to be tremor swarms that repeat, both in terms of their duration and in where they are. We haven't seen enough yet to say whether they repeat in regular time intervals," said Kenneth Creager, a University of Washington professor of Earth and space sciences.
"This continues to paint the picture of the possibility that a megathrust earthquake can occur closer to the Puget Sound region than was thought just a few years ago," he said.
The phenomenon, which Creager will discuss today (Dec. 15) during a presentation at the annual meeting of the American Geophysical Union, is the latest piece of evidence as scientists puzzle out exactly what is happening deep below the surface near Washington state's populous Interstate 5 corridor. He noted that the work shows that tremor swarms follow a size distribution similar to earthquakes, with larger events occurring much less frequently than small events.
The Cascadia subduction zone, where the Juan de Fuca tectonic plate dips beneath the North American plate, runs just off the Pacific coast from northern California to the northern edge of Vancouver Island in British Columbia. It can be the source of massive megathrust earthquakes on the order of magnitude 9 about every 500 years. The last one occurred in 1700.
The fault along the central Washington coast, where the Pacific and Juan de Fuca plates are locked together most of the time but break apart from each other during a powerful megathrust earthquake, was believed to lie 80 miles or more from the Seattle area. But research has shown that the locked zone extends deeper and farther east than previously thought, bringing the edge of the rupture zone beneath the Olympic Mountains, perhaps 40 miles closer to the Seattle area. It is this locked area that can rupture to produce a megathrust earthquake that causes widespread heavy damage, comparable to the 2004 Indian Ocean earthquake or the great Alaska quake of 1964.
Episodic tremor and slip events appear to occur at the interface of the plates as they gradually descend beneath the surface, at depths of about 19 to 28 miles. The smaller tremors between slip episodes, what Creager refers to as inter-episodic tremor and slip events, appear to occur at the interface of the plates a little farther east and a few miles deeper.
"There's a whole range of events that take place on or near the plate interface. Each improvement in data collection and processing reveals new discoveries," Creager said.
Episodic tremor and slip events often begin in the area of Olympia, Wash., and move northward to southern Vancouver Island over a three-week period, but scientists have yet to pin down such patterns among the smaller tremors that occur between the slip events.
Because the two tectonic plates are locked together, stress builds at their interface as they collide with each other at a rate of about 4 centimeters (1.6 inches) a year. The slip events and smaller tremors ease some of that stress locally, Creager said, but they don't appear to account for all of it.
"Each one of these slip events puts more stress on the area of the plate boundary where megathrust earthquakes occur, which is shallower and farther to the west, bringing you closer to the next big event," he said. "There's nothing to tell you which one will be the trigger."
Since the slip events and intervening small tremors don't accommodate all of the stress built up on the fault, scientists are getting a better idea of just what the hazard from a megathrust earthquake is in the Seattle area. One benefit from that is the ability to revise building codes so structures will be better able to withstand the immense shaking from a great quake, particularly if the source is substantially closer to the city than it was previously expected to be.
"We'd like to go back and see how much slip has occurred in these slip events, compared to how much should have occurred," Creager said. "Then we'll know how much of that slip will have to be accommodated in a megathrust earthquake, or through other processes."
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For more information, contact Creager at 206-685-2803 or creager@ess.washington.edu.
Ahhhh! So THAT’S why our gate fell down sometime last night.
When I was researching the Cascadia Subduction Zone about 15 years ago, I talked with Dr. Frank Gonzales of U. of Washington. He told me that geologists and seismologists had found beach sand as far inland as 11 miles up river channels on the Oregon and Washington coasts. The Indian tribes in the area have oral histories that describe how inland Indians went to the coast and found that the coastal villages and people were simply not there.
Dr. Gonzales said that (at that time) there were two working theories about the "big one" that was due; one was that the "big one" would be a magnitude 9 quake that would shake the region for up to 45 minutes, the other was what he described as "the decade from hell" in which the region would be wracked with mag. 8 quakes periodically for about a decade. Either way, it's potentially quite bad. It will make hurricane Katrina look like a tailgate party by comparison.
Great! And I work on the 22nd floor.
All I have to do is wait until my ranch becomes beachfront property.
Look on the bright side - you won't need an elevator to reach street level.
Nope. There exists geologic evidence of tsunamis along the Pacific Northwest coastline, and they were very, very big ones, too.
I live in southwest Washington, 800 ft. above the Columibia River and fifty miles from the Pacific.
My only fear about a large subduction earthquake is that it might happen during the wettest part of the year. Landslides due to water-ladden hillsides collapsing would kill a great many people and block highways preventing disaster relief.
If it improves the traffic problem then whats the issue?
More panicky gotta get attention news that I will make one comment for and then move on..NEXT, gotta save the poor flightless penguins at the North Pole.
Your neighborhood?
After the big one in ‘46, my folks moved away from the Georgia Straight earthquake zone.
At that time, the chimney fell off our house, and my father told me the fields were undulating like the sea.
I live in a mountain valley now, and our only road out is subject to avalanches yearly - every couple of weeks in late winter, actually, due to heavy snowfall.
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Thanks for the correction.
I read some more and learned being on the upper side of the subduction is no protection.
Merry Christmas.
Thanks, and Merry Christmas to you, too.
Best post of the year.
And Queen Christine is in Copenhagen!!!!!!!!!!! What on earth is she thinking? This could all go at any second! What will we do?
That’s ok. We just bought a house on the coast, remember?
It is obviously just a matter of time before the “big one” - and, if just based on the periods between previous events, we are due. I was talking to a environmental geologist and some others because they were laughing about some woman that has a car-topper on her van filled with emergency supplies. She lived in Redmond and works in Seattle. While she did seem a bit hysterical, I did comment that with those supplies, she might do a lot better than others that are stuck in Seattle for a week or more after the big one hits. Access into and out of Seattle is primarily over large bridges, which will only serve as artificial reefs for any rescue efforts.
Ya know, they could save money by combining activities with H1N1 suppositories.
I heard these were the same people who thought inuendo was an Italian suppository.
Nam Vet
“I heard these were the same people who thought inuendo was an Italian suppository.”
Unless of course you’re referring to *explosive* suppositories, most typically wielded by assassins. The two not being mutually exclusive, you understand.
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