Three back-to-back aftershocks -- one reaching a magnitude-3.9 -- struck Northern California on Tuesday only miles from Napa, which was rocked by a magnitude-6.0 quake two days ago, the U.S. Geological Survey reports. There were no immediate reports of injury or damage. The USGS said the epicenter of the first temblor, a magnitude-3.9, was 7 miles south of Napa, near American Canyon, Calif. It was followed two minutes later by a magnitude-2.7 quake only a half mile away and 37 minutes later by a magnitude-2.8 quake at the same location.