© USGS
The earthquake struck at 5:24 p.m., about 17 kilometers deep, according to the Pacific Northwest Seismic Network. The epicenter was directly east of Scotts Mills, a town of about 350 people in Marion County.
There have been no reports of damage yet. The Marion County Sheriff's Office said deputies are responding to the Scotts Mills area to check on residents as a precaution.
A 4.0 magnitude earthquake is relatively minor, although many people who were indoors felt the shaking.
"If you're really close to the epicenter area it's likely to give you a pretty good jolt, but this is not the type of earthquake that we would expect to see any damage associated with it," said Rafael Abreu, a geophysicist with the U.S. Geological Survey. "Very typical quake for the Pacific Northwest."
Abreu said he does not expect to see any aftershocks.
An earthquake struck in nearly the same spot in 1993. That earthquake was called the "Spring Break quake" and had a magnitude of 5.6.
Comment: The really big one: The next full Cascadia rupture will spell the worst natural disaster in North American history