Vintners in California's prestigious wine country on Monday were shaking off the effects of a major earthquake and thanking the very soil from which their prized vintages stem.

Winemakers who endured the 6.0-magnitude South Napa Quake were quick to conclude that how much damage was suffered depended not only on proximity to the epicenter, but the "terroir" in which grapes are rooted.

Rocky soil seen as ideal for powerful red wines also seemed to hold up well, while spots richer in clay or sand were more inclined to shiver dangerously, like gelatin, beneath foundations.

Early estimates put the cost of the damage into the billions of dollars, with much of that done to businesses like hotels, restaurants and shops that cater to tourists who flock to the area to visit wineries known around the world.

Damaged buildings and broken water lines could have an array of businesses off limits during the coming three-day Labor Day weekend in the United States, prime time for tourism here.

- Barrels smashed -

Many wineries were reporting damage no worse than bottles or barrels smashing to floors in wine cellars or tasting and barrel rooms.

Gates were barred to many aspiring visitors Sunday due to electric power being out of service, not because of quake damage.

"We had no damage whatsoever," said Jim Sullivan of Castello di Amorosa, a winery built as a replica of a 13th century Tuscan castle.

"While it has rock more than 100 years old on the outside, behind it is concrete and (steel reinforcement) rebar."

Darioush winery, whose owners are from Iran, boasts free-standing columns in a design putting modern touches on ancient Persian architecture.

The structures were unscathed by the quake, which did only slight damage to wine there, according to marketing directory Alessandra Boscarino.

"We were really fortunate," Boscarino said of Darioush. "The difference between my home and here was quite astounding. At home, everything that was elevated came down."

The number of lost wine barrels tallied about 20 at Bouchaine Vineyards, where the crew had begun the labor intensive crushing of grapes the same weekend the quake hit.

A wine barrel holds enough to fill 300 standard bottles.

A building at Trefethen Family Vineyards was knocked out of shape by the quake, which also caused two 20,000-liter tanks of Cabernet to spill at a Hess Collection facility, according to local news reports.

"Ways property was affected had to do with the soil," Boscarino said.

"The terroir contributes to the wine with its different compositions and structures, so you wonder."

- Quake trumped 'crush' -

Wine makers here were cleaning up quickly and getting on with the freshly-started grape harvest, known as the crush because juice must be quickly squeezed from fruit.

"I can tell you that 99 percent of all the businesses in this valley are open and ready for our guests," Bourassa Vineyards vintner Vic Bourassa said in a blog post.

"Napa is still one of the prettiest places on earth."

The earthquake that rocked California's scenic Napa Valley wine country a day earlier was the strongest to hit the region in a quarter of a century, seriously injuring three and jolting thousands from their sleep.

Data collected from activity tracking UP bracelets indicated that 93 percent of wearers in the cities of Napa, Sonoma, Vallejo and Fairfield woke when the quake hit at 3:20 am in California.

Slightly more than half of UP wearers further away in San Francisco and Oakland were roused by the shaking, according to bracelet maker Jawbone.

"Once awaken, it took the residents a long time to go back to sleep, especially in the areas that felt the shaking the strongest," Jawbone said, noting that many did not make it back to sleep.

No deaths were reported, but authorities said a child was in critical injured after being crushed by a fireplace and that some 130 people sought minor medical care and that.

Governor Jerry Brown declared a state of emergency in the wake of the quake, which sparked fires, burst water mains, caused gas leaks and even cracked roads.

The US Geological Survey said the temblor was the most powerful to hit the San Francisco Bay area since the 1989 Loma Prieta earthquake, at magnitude 6.9.

Water main breaks added to woes, including those of wineries eager to save precious waters in the face of an enduring drought here.