Two sizable aftershocks were reported early Tuesday in southern Napa County, not far from where a magnitude 6.0 temblor struck Sunday, causing significant damage.
A shallow magnitude 3.9 earthquake was reported at 5:33 a.m. one mile from American Canyon at a depth of 6.8 miles, according to the U.S. Geological Survey.
The epicenter was five miles from Vallejo and seven miles south of Napa, the USGS reported. Reports of possible damage were not immediately available.
At 6:45 a.m., a 3.2 earthquake was reported four miles from Napa and at a depth of 6.2 miles, the USGS said. The epicenter was six miles from American Canyon, the agency said.
In the last 10 days, there have now been five earthquakes magnitude 3.0 or greater centered nearby.
More than 65 aftershocks have rattled Napa County since the 6.0 quake on Sunday. Officials have warned the public to be careful around damaged buildings because even a small aftershock could knock debris loose.
In general, seismologists say, the likelihood of big aftershocks goes down with time, as does the chance of a larger quake.
Sunday's quake occurred on the West Napa fault. It's about 20 miles, much shorter than the better-known Hayward and Rodgers Creek faults, which are thought capable of a magnitude 7.2 quake. Still, it produced the largest earthquake in the Bay Area since the deadly 6.9 Loma Prieta quake in 1989.