The earthquake that rocked Napa on Sunday stressed three adjacent faults, but the likelihood that it could trigger a major new earthquake is a diminishing risk, federal geologists said Monday.
The 6.02 magnitude earthquake on the West Napa Fault that trashed part of downtown Napa and shook awake tens of thousands of sleeping Bay Area residents put additional stress on the Green Valley Fault to its east and the Rodgers Creek Fault to its west, said research scientist David Schwartz of the U.S. Geological Survey.
The quake also boosted stress on the northern part of the Hayward Fault under San Pablo Bay, he said. Other major faults in the Bay Area, such as the San Andreas and Calaveras, were not affected.
"That little push could trigger something bigger, but generally it does not happen," said Schwartz.
The increased tension "will last a little while, and then die off," he said. When a fault moves, it changes the stress on the tips of adjacent faults, he explained.
"There is a slight chance that it is a foreshock to a larger event. It is very small -- significantly less than 5 percent. But that chance does exist," added Joern Kaven, research geophysicist at the USGS office in Menlo Park.
USGS geologists predict 12 to 20 smaller tremors along the West Napa Fault within the next week.
There is a 5 to 10 percent chance within the next week of a repeat earthquake of magnitude 6 or larger and a 16 percent chance of a slightly smaller earthquake, measuring between magnitude 5 to 6, said Kaven.
"The longer it takes, the lesser the chances" of such earthquakes, he said.
The West Napa Fault has long been known to be an active fault. It had a 4.9 magnitude earthquake in 2000.
On Aug. 5, a 3.6 magnitude foreshock on the fault shook the area.
Sunday's "strike-slip" quake shifted the fault's two faces by 2.5 inches, making a long crack in the Napa landscape.
The Napa quake was the Bay Area's largest since the 1989 Loma Prieta earthquake, magnitude 6.9, in the Santa Cruz Mountains.
Strike-slip faults are the dominant form of fault in the Bay Area. The most famous example is California's San Andreas Fault, a many-branched system stretching about 600 miles from Southern California to the Point Reyes Peninsula and then out to sea. The "Great Quake" of 1906 that ravaged San Francisco struck along this fault.
The sideways motion of the fault is caused by the Pacific Ocean's crustal plate moving to the northwest under North America's continental crust.
Shock waves from Sunday's early morning temblor were felt as far east as Winnemucca, Nevada, as far south as Bakersfield and up to Yacult, Washington.
On Monday, scientists were studying clues in the six-mile long and six-mile-deep fractured fault.
"The faulting and shock waves -- how far it extended and its orientation -- that informs us in understanding the fault system better and assessing what the risk is in the future," said Kaven.
Contact Lisa M. Krieger at 650-492-4098.