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Scrambling to identify the source of Sunday’s magnitude-6.0 earthquake, government scientists said Monday they had located cracks in the ground and in pavement running about six miles on a northwest slant west of Highway 29 on the outskirts of Napa.

The U.S. Geological Survey also reported a one-in-four chance of a “strong and possibly damaging aftershock” in the next seven days.

The cracks, which geologists refer to as surface ruptures, were mapped Sunday by federal and state geologists, and experts spent Monday attempting to find a link between two segments, searching for additional ruptures and refining their interpretation of the field data.

The evidence continues to indicate the West Napa Fault, an obscure 27-mile mile stretch that is part of the San Andreas Fault system, as the cause of the quake that ripped up downtown Napa at 3:20 a.m. Sunday, causing an estimated $1 billion in damage.





“It certainly gives us a much better picture,” said Brad Aagaard, a geophysicist with the U.S. Geological Survey in Menlo Park. The cracks “seem to be aligned in the same direction as the West Napa Fault.”

Geologists also measured a lateral shift of about two inches along the ruptures, a displacement that pales in comparison with the 8- to 10-foot offset along the San Andreas Fault during the 1906 earthquake that devastated San Francisco and downtown Santa Rosa.

Global positioning system receivers operated by the Geological Survey and other agencies also measured a shift of the earth of a few inches because of Sunday’s temblor.

Damage from the quake is “localized in the region surrounding Napa” due to the rupture’s northwest orientation, a U.S. Geological Survey news release said. River valley sediments in the Napa Valley “likely contributed to the amplification of shaking around Napa,” it said.

Dozens of aftershocks recorded since the quake have followed the same orientation, the agency said, estimating the probability of a strong aftershock at one in four over the next week.

Sunday’s quake was the largest in the Bay Area since the deadly Loma Prieta earthquake, magnitude 6.9, in 1989. There had been 49 minor quakes — magnitude 1 to 2 — in the past 20 years in the area around the West Napa Fault, officials said.

A magnitude 5.0 quake at Yountville occurred on the West Napa Fault in 2000. A magnitude 6.3 Mare Island temblor in 1898 happened in the same area as Sunday’s event, the Geological Survey said.

The southern rupture section runs from roughly Bayview Avenue to Old Sonoma Road, with the northern section from about Congress Valley Road to Browns Valley Road, a combined length of about 6 miles, Aagaard said.

The rupture evidently started at its south end and moved north, he said. It may have jumped eastward about half a mile and extended on a north-northwest heading another few miles along an unmapped strand of the West Napa Fault, the agency’s release said.

Experts with the U.S. Geological Survey, the California Geological Survey and other agencies are continuing the search for additional surface ruptures and to refine their interpretations, the release said.

“USGS scientists are working around the clock to understand the earthquake and relay information to emergency managers and the public,” said Tom Brocher, director of the Geologic Survey’s Earthquake Science Center.