The area where the quake struck was centered was about a mile from San Ramon. The temblor hit at around 11:43 a.m. and was felt all across the East Bay, including the cities of Danville, Concord and Oakland.
The earthquake popped up along the Calaveras fault. It had initially measured as a 4.0 quake before being downgraded.
Aftershocks that registered 2.6 and 3.0 happened about 15 minutes after the first shaking.
Alameda County Supervisor Board President Richard Valle stopped his meeting briefly at 11:44 a.m. to note that the building in Oakland shook for a moment before resuming a presentation.
Shaking was felt in KTVU's newsroom in Jack London Square as well.
There were no immediate reports of damage or injury.