A 4.1 earthquake stuck the Bay area at 10:09 AM Thursday morning. The U.S. Geological Service pegged it at 4.1 on the Richter scale. It was 5.6 miles deep and the epicenter was just southeast of Fremont.
Here in Glen Park it came through as a definite jolt, especially for people on second and third stories.
USGS has more info here.
And a nice map here.