According to the USGS, the earthquake occurred at 10:09 a.m., was centered six miles east-northeast of Milpitas and eight miles north-northeast of Alum Rock, and had a depth of 5.6 miles. It was followed by a small 1.6-magnitude quake in the same location at 10:22 a.m.
Both quakes followed another, 2.8-magnitude quake that occurred Wednesday night in the same area, near the intersection of the Calaveras and Hayward faultlines, David Oppenheimer of the Geological Survey told KCBS radio. Another cluster of small quakes occurred the following day.
Oppenheimer said residents have no reason to worry about the quakes, as they are not typically foreshocks of a larger temblor.
"Not every magnitude-4 earthquake is followed by a magnitude-8 earthquake," he said.
The Geological Survey maintains a map of recent quakes on its Web site at earthquake.usgs.gov.