As of 1 p.m., the U.S. Geological Survey had reported 16 quakes in the area, with five of them above magnitude 2.5. The 3.5-magnitude temblor happened at 12:19 p.m. Friday near Diablo, a wealthy community northeast of Danville.

The San Andreas and Hayward Faults

An overview of the San Andreas and Hayward faults.

When BART receives reports of earthquakes, it stops trains until it can get information from the Geological Survey. If the magnitude is lower than 3.0, trains can start running under automatic controls. If it's higher than 3.0, operators run trains at low speeds, under manual control, for about five minutes while conducting visual track inspections.

A 2.8 earthquake at 12:21 p.m. caused BART to halt trains briefly, transit officials said. A 5:53 a.m. quake also led to a 10-minute delay that affected the morning commute.