The biggest Bay Area earthquake in a quarter-century rattled the region early Sunday morning, with a 6.0 rattler waking up nervous locals, knocking out power to tens of thousands of buildings and sending items off shelves in homes and stores.
The slow rumble was reported at 3:20 a.m. and was centered close to Buchli Station Road, near American Canyon in Napa County, and was about 6.7 miles deep, according to the U.S. Geological Survey.
An estimated 2.3 million people spanning 100 miles from the Santa Cruz area to Wine Country were affected by the quake -- with several thousand people quickly reporting that they had felt the rumble, the USGS reported.
More than 20 aftershocks were reported, topping out at about magnitude 2.5.
The last time an earthquake of this size hit the Bay Area was in 1989, when the infamous Loma Prieta quake at magnitude 6.9 caused severe damage. The largest on record was the historic 7.8 earthquake that hit San Francisco in 1906.
On Sunday, the reports of damage began to pour in. Some street lights in Napa were knocked out, and there were scattered reports of fires, the California Highway Patrol said. People posted pictures on social media of a Walmart and a grocery store with bottles knocked over and shattered. Some residents posted pictures of their kitchens in disarray. One man posted a picture of his chimney knocked over. In Vallejo, the CHP closed some roadways because of damage. There were fears of gas leaks.
PG&E's online outage map showed more than 30,000 customers without power in Napa shortly before 5 a.m. Another 15,000 customers lost service in Sonoma, with more than 10,000 still in the dark in Santa Rosa. Outages of between 1,000 and 5,000 were reported in St. Helena, Vallejo and Pinole.
Seconds after the shaking, which lasted upward of 30 seconds in some parts, social media was flooded with witness accounts of the earthquake, including from law-enforcement and emergency personnel in the area. People reported swaying chandeliers, pictures falling off walls and bottles shattering on the floor.
There were no immediate reports of damage to Bay Area bridges.
According to initial USGS data, it was categorized as "severe" in how the shaking was perceived, though user-reported data deemed it on a lower degree, as "strong." The earthquake was initially reported as having a 6.1 magnitude before being quickly downgraded to 6.0.
An earthquake of Sunday's size, while large, is not uncommon to the area historically, said David Schwartz, an earthquake geologist with the USGS office in Menlo Park. He noted that since 1969, earthquakes in the range of a 6 pop up every few years: a pair of 5.7 and 5.8 in Santa Rosa in 1969, a 5.8 and 5.9 duo in Livermore in 1980, and a 6.2 in Morgan Hill in 1984.
"It's a fairly common-sized earthquake that we've seen historically. There are there sitting in the background," Schwartz said.
It is not, he added, in the range of the next "big one" anticipated by Bay Area residents since Loma Prieta.
"That's not the magnitude we're expecting. We're looking at a 6.7 or larger" for the next major event, Schwartz said.
Whenever an earthquake is rated at 5 or higher, the USGS automatically sends out an alert that there's a 5 to 10 percent chance that an equal quake or larger could occur in the next 72 hours, based on historic activity in California.
"It doesn't mean a large earthquake will occur," Schwartz said. "We don't know if this is a foreshock or a main event followed by aftershocks, which is what seems to be happening here."
If an aftershock occurs, the USGS recommends that people who are indoors stay there, taking shelter under a piece of furniture, in a hallway or against an inside wall, away from windows, fireplaces and heavy objects.
If you are outdoors, get into the open away from buildings, power lines and other things that could fall. If driving, stop carefully and move out of traffic. Avoid bridges, trees and other falling objects. Stay in your car until the shaking stops.
Schwartz said based on the north-northwest trend of aftershocks, Sunday morning's quake was parallel to the West Napa fault, about two miles east, but it's not clear which fault it rumbled out of.
"Nothing shows up on our maps as an active fault," he said. "This will be one of the things we'll be looking into, what the source was."
The USGS "ShakeMap" feature compares and contrasts both agency and user-reported data to rate how strong an earthquake was felt. The USGS system reported it as a "violent" quake with potential for "heavy" damage, while user reports deemed it "very strong" and "moderate" in those respective categories.
Schwartz said that damage cannot be directly predicted by a magnitude rating, noting an array of variables.
"There's a high water table along the Napa River, so you have poor soils. When you have saturated soils, it tends to amplify the shaking," he said. "You put the same earthquake in different locations, with different soils, sediments and rocks, you're going to get different kinds of damage."
The last major earthquake in the Bay Area was the 1989 Loma Prieta earthquake -- a magnitude 6.9 quake that struck on Oct. 17, 1989, in the Santa Cruz Mountains. It killed 62 people and caused $6 billion in damages. Before Sunday morning's quake, the largest Bay Area earthquake since Loma Prieta was located near Alum Rock Park in 2007, with a magnitude 5.4 temblor.
California straddles the boundary between two of the Earth's tectonic plates -- as a result, it is broken by numerous earthquake faults. Literally thousands of small earthquakes occur in California each year, providing scientists with clear indications of places where faults cut the Earth's crust.
In 2007, a panel of experts estimated there is a 63 percent chance that in the next 30 years the San Francisco Bay Area will experience a catastrophic earthquake at least as powerful as the magnitude 6.7 Northridge earthquake that rocked Southern California in 1994. There is a far greater chance -- 99 percent -- that an earthquake that size will strike somewhere in the state during that time.
Check back later for updates to this story.