USGS says the quake hit at 4:27 p.m. It was centered 26 miles west of Eureka in the Pacific Ocean.
"It was huge -- one of the biggest earthquakes we've had up here in 20 years," Judd Starks told the Associated Press. Starks is the kitchen manager at a bar and restaurant known as The Alibi. "The whole town is kind of freaked out right now. All the power is out, people are out walking around," Starks said.
The Eureka fire department says they responded to some gas leaks and downed power lines.
A spokesman at the Ferndale Fire Department said damage was confined to broken windows and damage inside homes - from objects like dishes falling off shelves. One grocery store in Ferndale had quite a clean up job on all aisles.
There were no immediate reports of injuries.
This is the third earthquake in Northern California since Thursday. The other two were under 5.0 and both hit in the South Bay near Milpitas.
A sign posted in Ocean Beach is part of San Diego's effort to become what the National Weather Service's website calls Tsunami Ready. A title 11 areas in California can claim, only one of which, Imperial Beach, is here in San Diego.
According to geologist Pat Abott, it could happen once every couple of thousand years.
"It's something that has happened in the past. It will happen again," he said.
Still, he says the reminder can't hurt.
"What's the worst that could happen? You're understanding more about the environment and the world you live in, and to me, that's always a good thing," Abott said.
Back in September, a tsunami warning was issued for San Diego after an earthquake in the South Pacific, but no damage was recorded.
There have been six damaging Tsunamis recorded along California's coast since 1812.