The earthquake that rolled through the Bay Area and beyond Sunday is suspected of striking along a little-known section of the West Napa Fault in the seismically active North Bay region, where parallel faults have been rupturing for millions of years, scientists at the U.S. Geological Survey said.

Scientists said early-detection equipment had sent out alerts of imminent ground shaking to distant locations, raising the possibility that a more extensive system could one day give precious seconds of warning that would save lives.

A network of seismographs maintained by the USGS and UC Berkeley indicated that the quake ruptured underground at a depth of 6.7 miles - typical for temblors in the Bay Area. The Geological Survey said late Sunday that the magnitude was 6.0, though at times during the day it said the quake had been measured at 6.1.

The quake appears to have ruptured along the Browns Valley section of the West Napa Fault, about 3 miles northwest of American Canyon, said USGS seismologist David Oppenheimer.

However, the precise location of the 3:20 a.m. temblor remains uncertain. The quake could have struck on any one of the faint traces of parallel fault strands on the west side of the Napa Valley, said geophysicist David Schwartz of the USGS.

Analyzing all the data from the main shock and its aftershocks in coming days "will give us a more definitive understanding of the quake's origin," Schwartz said.

Varying duration

Although many Bay Area residents described the quake as a long, rolling one, seismic instruments recorded ground shaking that lasted for just four to five seconds in the Napa region and 10 to 14 seconds in various parts of San Francisco, according to USGS geophysicist Brad Aagaard. The length of shaking can differ depending on several factors, including type of soil and underlying rock.

The quake instantly triggered the incomplete Earthquake Early Warning System headquartered at UC Berkeley's Seismological Laboratory, said Richard Allen, the system's director.

"The system worked very well," Allen said, and alerted instruments in Berkeley that the campus would undergo strong ground shaking within 10 seconds after the quake struck. Test users in San Francisco would have received the warning 12 seconds ahead of ground shaking, and for San Jose the alert would have been received 25 seconds ahead, Allen said.

Of the 300 seismic stations in the network, four are needed to trigger an alert, and the first of those stations recorded the rupture within one-third of a second, Allen said. The slowest station detected the quake two seconds later.

Looking for funding

The alert system has been largely funded by the Gordon and Betty Moore Foundation. Seismologists hope that eventually the state Office of Emergency Services will find $80 million to expand the system statewide.

Backers say even seconds of early warning can be critical in alerting people to take cover, stop trains and power down industrial equipment. In Japan, an early-warning system touched off alarm bells just before an earthquake and sent out cell phone alerts.

"In the case of this earthquake, the warning that the ground would be shaking could have been twice as fast if planned improvements to the system were in place," Allen said. "We need even more speed if the system is to provide faster warning that ground shaking is about to happen."

The entire Bay Area experienced what seismologists call "dynamic shaking" in response to the rupture at the quake's epicenter, said USGS scientist Oppenheimer - in other words, a "flurry" of very small quakes that affected nearly every fault in the region, and particularly in the unstable ground of the Geysers geothermal field in Sonoma County.

Loose soil worsens shaking

The quake's suspected source, the West Napa Fault, extends about 35 miles along the west side of the Napa Valley, from St. Helena in the north to the Carquinez Strait near Benicia on the south. It is mostly on unconsolidated ground, where shaking is more severe.

In 2000, a magnitude 5.0 quake struck on or near the fault. Known as the Yountville quake, the temblor caused an estimated $50 million in damage.

Oppenheimer said Sunday's quake may have somehow been related to one that historical records show caused severe damage around Sonoma County in 1898.

Its magnitude was 6.3, and it damaged the Mare Island Navy Yard and the railroad community of Schellville near Sonoma, as well as houses along Petaluma Creek. Several buildings collapsed, according to the USGS records.

Oppenheimer says it appears Sunday's quake happened in nearly the same spot as that long-ago earthquake.

A major concern for quakes throughout the North Bay is that they rupture the ground near the well-known Rodgers Creek Fault, which in turn is a northern extension of the infamous Hayward Fault. Quakes on smaller faults can increase the likelihood of a more destructive temblor on major faults such as the Hayward-Rodgers Creek, seismologists say.

The Hayward Fault has long been considered the most dangerous in Northern California. The last major quake on the Hayward was in 1868, and scientists consider it overdue for another one.

Strike-slip faults

Like most faults in Northern California, the West Napa Fault is known as a right lateral strike-slip fault. Most such faults in the Bay Area run north-south, and in an earthquake the parallel sides slide abruptly past each other.

The Hayward-Rodgers Creek is also a strike-slip fault. There is about a 30 percent chance of a deadly temblor along the fault within the next 25 years, according to forecasts by a group of more than 70 scientists at the USGS, the Southern California Earthquake Center and the California State Geological Survey.

That same team of experts has estimated the probability of a magnitude-6.7 earthquake or larger somewhere in the Bay Area before 2030 at 62 percent.

And a "strong and deadly earthquake" is virtually certain to strike somewhere on one of California's many major faults before 2038, the group has warned. That probability is 99 percent.