SEATTLE -- A magnitude 4.6 earthquake shook Seattle and the Puget Sound region just before 3 a.m. local time Friday, according to the United States Geological Survey.
"It's been widely felt throughout the Seattle area," said Paul Caruso, a USGS geophysicist.
The shaking emanated from Three Lakes, Snohomish County, about nine miles east of downtown Everett. The earthquake was relatively shallow, originating about 14 miles beneath the surface, according to a USGS map.
There were no immediate reports of damage in Snohomish County, according to a tweet from the Sheriff's Office. Police in nearby Lake Stevens reported no damage to city infrastructure. The Washington State Department of Transportation said in a tweet that the agency would be inspecting bridges Friday morning, but had no reports of damage.
A second quake, measured at magnitude 3.5, was reported near Monroe a few minutes afterward. A handful of smaller aftershocks followed, according to the Pacific Northwest Seismic Network.
Initial USGS reports had described the larger trembling as a magnitude 4.4 earthquake that began less deep in the earth's surface.
Caruso said the earthquake was the product of a thrust fault, in which one side of a fault pushes upward in relation to its opposite side. Thrust faults are common in the Cascade Range, Caruso said.
He said the earthquake did not have any connection to recent tremors in California, and that it was too shallow to have originated in the Cascadia subduction zone off the Washington coast, where "stress and strain has been building for a long time."
Joan Gomberg, a USGS seismologist and affiliate faculty member at the University of Washington, said "the likelihood of a bigger one is small. It's not impossible, but it's small."
She said scientists were working Friday morning to better quantify those chances.
The biggest fault near the earthquake is the Southern Whidbey Fault Line, but Gomberg said researchers don't suspect this event occurred on that fault.
Scientists think another fault, further beneath the earth's surface where scientists have less understanding, is likely responsible. That fault is probably not mapped and likely lacks surface expression features like cracks in the ground or offsets in topography that can be spotted with powerful lidar surveying technology, Gomberg said.
"If there's earthquakes, there's a fault," Gomberg said. "There has been activity in this same spot previously."
The larger earthquake was felt across the Canadian border, the USGS map reports. People reported feeling the earthquake to the south in Olympia, to the west in Port Angeles and to the east in Wenatchee.
Kieran Smith, 23, a Western Washington University student in Bellingham who lives in a fourth-floor apartment, said he felt his bed shake and the building sway.
On social media, many reported waking up or sensing the earthquake.
In Arlington, Tristan Halsen, 20, an Everett Community College student, was sitting on his couch working on homework when he heard a "really loud rumbling like a stampede," he said in a Twitter message. A wall-mounted TV began shaking. At first, he thought it was a thunderstorm. Then, his house began shaking "for what seemed like forever," he said.
"I think this was my first 'big' earthquake that I can remember and it was interesting to experience one this big," he wrote.
Leah Kennebeck, who lives in Seattle's Queen Anne neighborhood, said in a Twitter message that she woke up from a dream to her bed shaking and her ceiling light fixture rattling.
She "sat straight up and froze" trying to feel if the shaking was getting worse, she said.
"Once I realized things were rattling in my room longer than if it was from something like a truck going by, I knew it was an earthquake," she said.
Kennebeck, who last felt shaking during the 2001 magnitude 6.8 Nisqually earthquake in grade school, said the quake left her with an unsettling feeling, particularly after a string of powerful earthquakes in California last week.
"Scared the crap out of me. And it was only a 4.4. My 4th grade self would be ashamed," she wrote.
She said she'll be putting together an updated emergency kit later Friday.
The Puget Sound region is one of the most hazardous areas in the country for earthquakes. The Cascadia Subduction Zone lurks offshore and could produce a damaging magnitude 9.0 earthquake, an event expected about every 500 years. But some have come only 200 years apart, and it has been 319 years since the region was hit with an earthquake that large. A Seattle Times analysis in 2016 found that about 5.4 million people live in a part of Washington endangered by a magnitude 9.0 earthquake.
Scientists have discovered more than two dozen faults across Washington state that could also produce damaging shaking.
The region is largely unprepared. The state of Washington does not require old, rigid brick and concrete buildings to be retrofitted. Thousands of children attend classes in vulnerable school buildings. The state would need to spend hundreds of millions more dollars to retrofit its bridges. Many county emergency-management departments are staffed with a single employee, The Seattle Times found in 2017.
The Pacific Northwest Seismic Network is developing an early earthquake warning system to provide people notice an earthquake might soon give them a shake.
Researchers have been "working furiously" to install new monitoring sites, Gomberg said. A pilot group of businesses, government agencies and schools has early access to the warning system, as its being tested and refined.
Gomberg said the system worked during this earthquake. It provided a few seconds of warning to Seattle that an earthquake was coming. It also assessed, within a few tenths of a point, the magnitude of the shaking, she said.
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