The US Geological Survey says the 3.9-magnitude quake struck at 5:33 a.m. Tuesday about 7 miles (11 kilometers) south of the city ofNapa.
A sheriff's dispatcher says the station shook a little, but there have been no calls reporting damage or injuries.
California's wine capital is still recovering from Sunday's 6.0-magnitude earthquake, which is estimated to have caused at least $1 billion in property damage.
There have been a series of small aftershocks, mostly in the 2.0-magnitude range.
The earthquake that jolted California's wine capital may have caused at least $1 billion in property damage, but it also added impetus to the state's effort to develop an early warning system that might offer a few precious seconds for residents to duck under desks, trains to slow down and utility lines to be powered down before the seismic waves reach them.
California's senior senator, Democrat Dianne Feinstein, joined a chorus of renewed calls on Monday for the quick deployment of a quake activity alert system such as the ones already in operation in Mexico and Japan.
"Officials in Washington and along the West Coast should partner with the private sector to make an interoperable earthquake early-warning system a reality, and we should do so as soon as possible before a much larger earthquake strikes," Feinstein said.
Such a system may be closer to reality than most Californians realize, though it's still years away. A bill signed by Gov. Jerry Brown last year ordered his Office of Emergency Services to develop a comprehensive statewide system and by 2016, identify sources of funding for it. An early-warning system would cost an estimated $80 million.
The office's director, Mark Ghiladucci said Monday that he pictures a network, financed with both private and public money, made up of a government "backbone" and supplemented with data from private sources in more remote areas.
"This is sort of the last mile out of the 20-year effort for scientists that have been working on this for us to pull it all together," Ghiladucci said. "California is unique. It is a long, complicated, highly populated state, and we have to have a system that is 100 percent reliable so that people can count on it."
Richard Allen, director of the University of California, Berkeley, Seismological Lab, said his lab received a 10-second advance warning of light shaking when the seismic waves from Sunday's quake arrived there. Allen is among the researchers testing the earthquake warning system that is not yet available for public use, but is envisioned as the basis for the state's system.
The magnitude-6.0 temblor was centered near the city of Napa and caused several injuries, left four mobile homes destroyed by gas-fed fires and damaged wineries, historic buildings and hotels.
Allen, of the UC Berkeley Seismological Lab, said though Berkeley is about 40 miles from the quake's epicenter and did not experience any damage during Sunday's quake, in a more violent temblor, 10 seconds could have made a big difference.
"A few seconds means that you can move to your safe zone, that you can get under that sturdy table; that way you are not going to be injured by falling fireplaces and ceiling lights. We see a large number of injuries resulting from these kinds of incidents."
The systems can't predict quakes, and are not effective at the epicenter, where the tremors go out almost simultaneously. The warning people receive — a few seconds to tens of seconds — depends on the distance from the epicenter.
Napa would have received at most a second of warning if California already had a system in place, said Thomas Heaton, a professor of engineering seismology at California Institute of Technology in Pasadena.
"It's important for people to keep this in perspective," he said. "It's a new kind of tool, but it's not a panacea."
Business owners in Napa spent Monday mopping up high-end vintages that spilled from barrels and bottles and sweeping away broken glass in the rush to get the tourist hotspot back in shape for the summer's final holiday weekend. Government and tourism officials assessing its economic and structural impact encouraged visitors to keep flocking to the charming towns, tasting rooms, restaurants and spas that drive the Napa Valley economy.
While cleanup will take time and broken water mains remained a problem, they said, the worst damage and disruption was confined to the city's downtown, where a post office, library and a 141-room hotel were among more than 160 homes and buildings either deemed unsafe to occupy or enter. Two hotels and 12 wineries were still closed Monday, as well as gift shops, restaurants and other downtown businesses, Clay Gregory, president of tourism organization Visit Napa Valley, said.
"Clearly, we are concerned that people are going to see that it was a catastrophe, and it certainly wasn't good, but it wasn't a catastrophe by any means," Gregory said as workers at a shuttered downtown visitor's center updated lists of open wineries and surveyed hotels about cancellations. "The real story is that it has impacted a very small part of the valley."
Local officials have an early working estimate that Napa Valley suffered $1 billion in property damage, but they hope the long-term economic impact of the quake to businesses will be modest, Napa County Supervisor Bill Dodd said. August, September and October grape harvest represents the busiest time of year for both the valley's 500 or so vintners and the visitors who come from all over the world to see them work.
If people "think Napa is devastated, it's anything but devastated. We're only 24 hours out from an earthquake, and we're on our way back," Dodd said.
The Napa Valley Wine Train, which offers tourists a three-hour journey through 18 miles of wine country, canceled its service Monday but planned to resume trips Tuesday. Other tour operators said they were taking it one day at a time, tweaking their itineraries as wineries and their workers dealt with the damage.
At the famed Robert Mondavi Winery outside Napa, where visitors sat in a sunny outdoor tasting area sipping glasses of wine on Monday, gift shop supervisor Kevin Seeman said there had been only a small number of canceled reservations and that the people who worked there were more on edge than the patrons.
"A lot of the staffers are worried," Seeman said. "Some of them their homes are full of rubble; they are worried because they can't find their cats. The visitors seem not so worried about it all."