Fault suspected in Napa County quake fairly obscure

As Bay Area earthquake fault lines go, the West Napa Fault - tentatively identified as the source of Sunday’s magnitude-6.0 temblor - is a relative stranger.

A statewide assessment six years ago of major earthquake risks, for example, did not mention the 27-mile-long fault, which runs through the hills west of Napa, hit hard by the quake at 3:20 a.m.

But the West Napa Fault runs just 4 miles east of the formidable Rodgers Creek Fault, which hammered Santa Rosa in October 1969, leading to major downtown redevelopment, and decades later prompted Sutter Medical Center to build a new hospital to open soon next to the Wells Fargo Center for the Arts.

The old Community Hospital, part of the current Sutter facility on Chanate Road, lies near the Rodgers Creek Fault.

In Northern California, the Rodgers Creek Fault was rated as the most likely to trigger a magnitude 6.7 or larger quake, with a 31 percent probability over the next 30 years, according to the Uniform California Earthquake Rupture Forecast issued in 2008. The Rodgers Creek, West Napa and other faults lie within the 44-mile-wide San Andreas Fault system, which forms the boundary between Pacific and North American tectonic plates.

The 2008 report listed the state’s seven most threatening faults but noted that many others have similar potential and that “the next big quake in California is just as likely to occur” on one of them.

California sits on the boundary between the two massive tectonic plates, which grind past each other at a rate of about two inches a year, building up pressure that can only be released by earthquakes. Hundreds, if not thousands, of faults splay out from the boundary, spreading the threat of large quake ruptures through most of the state.

Brad Aagaard, a geophysicist with the U.S. Geological Survey in Menlo Park, said the source of Sunday’s temblor had not been pinpointed, describing the West Napa Fault as the “fault of interest’’ in the ongoing investigation.

USGS geologists were out Sunday looking for “surface rupture,” visible evidence of the temblor’s tie to the West Napa Fault, Aagaard said. Without such evidence, he said it would take several days to establish a “more definitive picture” of the quake.

The duration of Sunday’s quake is also uncertain, he said, noting that it increases with distance from the epicenter because of the different speeds at which seismic waves travel.

Other faults, including the Hayward and Calaveras faults running through the East Bay, are “considerably more active” than the West Napa Fault, Aagaard said.

There have been 49 minor earthquakes - magnitude 1 to 2, recorded by seismographs but barely felt by people - within 6 miles of Sunday’s epicenter in the past 20 years, he said. Many were near the West Napa Fault, but its below-ground geometry is not well enough known to determine which ones were on the fault.

The deadly 1989 Loma Prieta quake at magnitude 6.9 is the largest event on the San Andreas system in the Bay Area since the San Francisco earthquake of 1906. Sunday’s temblor was the first magnitude 6 quake in the Bay Area since then. A magnitude 6 temblor has a seismic energy yield of 15 kilotons, the equivalent of the atomic bomb dropped on Nagasaki.

“I wouldn’t call it a surprise,” Aagaard said, noting that several other faults in the area are capable of such a jolt.

He said it was more surprising that the area had gone 25 years without a major quake.

“Things have been relatively quiet in the Bay Area for a while,” Aagaard said.

You can reach Staff Writer Guy Kovner at 521-5457 or guy.kovner@pressdemocrat.com.