Cellphones across the Bay Area rang out with another earthquake alarm Sunday afternoon, but this time it was not a false alarm.

An earthquake with a magnitude of 4.0 shook under Sonoma County at 3:30 pm Sunday, with an epicenter about 3 miles west of Glen Ellen.

That was followed by a 3.1M aftershock at 3:38 pm.

The Shake Alert message that came through cellphones in San Francisco said that the initial earthquake was a 4.9M, and said to take cover and expect shaking, though no significant shaking was felt in the city. And a preliminary magnitude assigned to the quake was 4.2.

A friend of SFist in Glen Ellen said that significant shaking was felt there.

This follows a false alarm from the Shake Alert system that was sent out to phones in the Bay Area about an earthquake in Nevada that did not actually occur on December 4.

This is a developing story.