Which fault is at fault for South Napa Earthquake?

FRESNO, Calif. (KFSN) -- Sunday's South Napa Earthquake was the strongest to hit the region in 25 years.

Fresno State geologist Dr. John Wakabayashi says it could be days before we know exactly which fault is at fault, but he says we do have a prime suspect, this after instruments at US Berkeley and the U.S. Geological Survey in Menlo Park were able to find the epicenter and zero in to fraction of a mile.

But beyond that, Wakabayashi says it's difficult to pinpoint. That's because most larger faults -- such as the San Andreas -- have multiple, interweaving strands referred to as fault zones.

This earthquake happened about 4 miles northwest of American Canyon and 9 miles south-southwest of Napa, meaning it's within the San Andreas Fault system but right next to two others: the Hayward-Rodgers Creek system to the west and the Concord-Green Valley system to the east.

There's also a couple of faults there: the West Napa and Carneros-Franklin faults. Geologists initially suspected the quake was centered along the bigger West Napa Fault, but now believe it was caused by a 2-1/2-inch lateral shift along the lesser known Carneros Fault, which has shown no signs of activity for the last 100,000 years.

"It has not been studied as much as a number of the other faulted regions within the San Francisco Bay Area so there's still work to be done, and I think this event will focus more attention on this and the need to kind of learn more," said Wakabayashi.

Wakabayashi submitted proposals to study this region back in the 1990s, but was not successful. Since then, he's kept tabs on research in the area and says it should be of interest and concern for seismic hazards because it shows these obscure faults can come alive at any time.

He says while another quake may not happen for another 100 years or so, he says people should prepare now as if one could occur tomorrow.