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.