The quake occurred at 1:10 p.m. local time and was centered about 6 miles east-southeast of Willits, the USGS said, News.Az reports, citing foreign media.
It struck at a depth of just under 5 miles, a shallow level that can result in noticeable shaking near the surface.
Preliminary data from the USGS indicated light to moderate shaking in the area. Two smaller earthquakes, each measuring less than magnitude 2.0, followed the initial tremor.
A stronger aftershock followed about 90 minutes later, when a magnitude 3.8 earthquake struck roughly 7 miles east-southeast of Willits, the USGS said.
There were no immediate reports of damage or injuries.
The epicenter lies in a rugged, lightly populated region northeast of Ukiah, where earthquakes of this size are not uncommon.
The fault that caused the quake was not immediately known. However, the Willits area is in the vicinity of the Maacama Fault, a lengthy fault running through the interior of California east of and parallel to the San Andreas.
