May 18, 2004 Right on schedule, a slow earthquake apparently has started deep beneath western Washington. And while it could end up releasing as much energy as the magnitude 6.8 Nisqually earthquake did in 2001, no one will feel a thing.
University of Washington scientists believe measurements of the current seismic tremor will help their understanding of processes at work deep in the Earth. Their objective is to gain insight into the accumulating stresses that will eventually lead to the region's next major earthquake.
Seismologists at the UW, the Geological Survey of Canada and Central Washington University in Ellensburg have documented at least nine previous so-called slow earthquakes going back to 1992. They seem to occur every 14 months or so, and last about a month. The last one happened in February and March of last year.
The first sign that something was up this year came in late April when seismograph stations south of Puget Sound began to record weak isolated bursts of energy from deep in the Earth. Those bursts, or deep tremors, were only slightly larger than typical background noise such as weather, ocean waves and truck traffic, and they became more frequent as April went along, said Wendy McCausland, a University of Washington pre-doctoral research associate in Earth and space sciences.
"I actually look for them every day," she said. "It was on April 26th that we began to see them regularly. I saw them throughout that day."
The tremors were initially detected in the Centralia-Chehalis area, the farthest south they have been observed by the Pacific Northwest Seismograph Network. Typically the bursts move toward the northwest, past the Strait of Juan de Fuca along Vancouver Island. This latest round of tremors did not begin moving northward until last weekend, McCausland said.
Otherwise, the activity is similar to what was observed last year, she said.
The regional seismograph network has more than 25 permanent seismograph stations in the region to provide general information on where the deep tremors are taking place.
But this year, three special seismic arrays have been installed, one on the Olympic Peninsula near Sequim, one on Lopez Island in the San Juan Islands and one on Vancouver Island west of Victoria, British Columbia. In the international effort, scientists from Canada and the Vesuvius Observatory in Italy provided equipment and assistance.
The three temporary arrays can be focused on specific areas to determine with greater precision the depths of the deep tremors. Current estimates place them 12 to 25 miles deep. McCausland hopes information from the arrays and the seismograph network also will help establish the relationship between deep tremors and slow-slip events.
Previously, slow-slip events, or slow, deep earthquakes, have been documented by Global Positioning System data. Normally the North American tectonic plate that lies under western Washington is pushed to the northeast about one-half inch a year along the coast and about one-quarter inch a year farther inland. In a slow-slip event, the plate's direction is actually reversed for several weeks, and the Pacific Northwest Geodetic Array in Ellensburg is able to detect the gradual movement of GPS stations to the southwest.
A big question, McCausland said, is whether the deep tremors and the slow slip are the same event, or if they are separate events that are somehow interrelated.
Making that determination will help in understanding whether they are adding to or relieving stress in the Cascadia subduction zone off the Washington and B.C. coast, where the Juan de Fuca plate dives beneath the North American plate. The subduction zone is capable of generating great earthquakes, and scientists recently determined that a Cascadia earthquake in 1700 measured about 9.0 in magnitude.
Judging by past deep tremors, McCausland expects the current event to continue for another week or two.
"It started farther south than previous episodes, so it might last longer if it migrates its full path, all the way up to Vancouver Island," she said.
For more information, see:
The "Deep Tremor Page" at http://www.pnsn.org/WEBICORDER/DEEPTREM/arrays.html
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