By Paul Gottlieb
Peninsula Daily News
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Crews are doubling the number of seismic recording devices between Port Angeles and Port Townsend to measure tremors that make their presence known every 15 months or so in northwestern Washington, Pacific Northwest Seismic Center Director John Vidale said Wednesday.
Staff from the University of Washington-based center this week are increasing seismic stations from 10 to 20 at each of eight sites on the North Olympic Peninsula.
The "tremor-and-slip" events have occurred about every 15 months since they were first detected in 2002.
The latest were felt Sunday north of Olympia and west of Tacoma
They are expected to travel north under the Peninsula to Vancouver Island.
The tremors are expected to last about three weeks and chances are "very small" that they will be felt on the surface, Vidale said.
Over the course of several weeks, these silent tremors can release as much energy as a magnitude 6 earthquake, UW scientists said.
They emanate from the Cascadia subduction fault zone about 50 miles off the Washington coast.
Timing a mystery
Scientists hope the data that's gathered helps them understand "what controls the timing of these events," Vidale said.
The surface of the fault "is right under your feet there" about 20 miles below surface, he said.
"We don't understand why they are such weak events," he added.
"It lets out few centimeters every year, compared to the big ones, which let out 10 meters of slip every 500 years," or 33 feet, Vidale said.
"Another mystery is why it takes two or three weeks for all this movement to occur rather than a couple of minutes like a giant earthquake," he said.
"We are trying to understand what is happening with this kind of faulting."
The last subduction zone earthquake in the Pacific Northwest was in 1700, so the chance is "very small" that a large earthquake will occur during the present round of tremors, Vidale said.
"Essentially, the big earthquake is likely to come in the next 300 years, so the tremor will happen 250 times before we are pretty sure we had an earthquake," he said.
The sensors are spread out over a mile at each site, the locations of which are being kept secret "so people don't go to visit them and make off with the batteries," Vidale said.
The sensors are about the size of a quart Coke bottle that's attached by cable to a data recorder size of a milk carton, he said.
Senior Staff Writer Paul Gottlieb can be reached at 360-417-3536 or at email@example.com.