DEVELOPING: Earthquake Swarm Rattling Residents in Seattle
After more than a dozen earthquakes, including two early Thursday, struck around the Kitsap Peninsula since early May, another small temblor off Whidbey Island shook the Seattle area.
The first quake hit at 12:34 a.m. Thursday, with a magnitude of 3.6 and an epicenter 3.4 miles from Bremerton, according to the University of Washington’s Pacific Northwest Seismic Network (PNSN). More than 700 people reported to the U.S. Geological Survey (USGS) that they felt some intensity of shaking in places from Vashon Island to Bellevue to Lynnwood.
The second, smaller temblor came at 2:36 a.m., with a magnitude of 2.6 and an epicenter 2.1 miles from Bremerton.
Earthquakes registering between 2.0 and 3.9 on the Richter scale are considered minor.
The earthquakes struck near the Seattle fault, a zone of thrust faults that run through Seattle and across the Puget Sound.
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More than 200 people reported to the USGS that they had felt the 3.4 magnitude earthquake off Whidbey Island.
John Vidale, director of the PNSN, said it’s likely the swarm that has been detected will end soon — and it’s likely not connected to the Whidbey quake.
“There’s no reason to believe the Bremerton swarm would trigger the Whidbey earthquake, 50 kilometers away,” he said. “It’s probably a coincidence.”
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Scientists are working to find out whether the quakes struck directly on the Seattle fault, “a structure we are concerned about,” Vidale said. “We’re trying to figure out if it’s on or near the fault.”
Later Thursday, the network’s spokesman, Bill Steele, said there was still some uncertainty about the swarm but that the quakes appeared to be striking below the fault.
He said the region gets similar swarms “every few years.”
“They’ve happened before and will probably happen again,” he added.
The last time the fault, which is a collection of fractures that run under the Seattle region, ruptured in a big way was about 1,100 years ago. The resulting quake measured at least magnitude 7 and thrust up shorelines in West Seattle and Bainbridge Island by 20 feet or more. It also triggered a tsunami that swept through Puget Sound.
Pinpointing the exact location of the fault has been difficult, though, because it only breaks the surface in a few places on Bainbridge Island and near Lake Sammamish.
Based on surveys that measure the magnetic properties of rocks, scientists have assembled a fuzzy picture of a wide swath of fractures that extend from Bremerton, passes south of downtown Seattle and continues into the Puget Sound foothills.
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