Little-known fault draws renewed attention

If you hadn’t heard of the West Napa Fault before Sunday morning’s 6.0 shaker, you’re not alone.

Even though a magnitude 5.0 earthquake occurred along that same fault in Yountville in 2000, few locals – and few geologists, for that matter – have given it much thought.

And yet, on Sunday at 3:20 a.m., it appears to have hosted the Bay Area’s most powerful earthquake in 25 years.

“Those faults are not very well known,” said Diane Moore, a research geologist with the U.S. Geological Survey in Menlo Park, referring to a complex web of North Bay fault lines that includes the West Napa Fault. As a result, she said, there are no strong estimates on the likelihood of quakes occurring near the region.

“If you look at the probability elements for the area, that is not even included on it,” she said.

The Uniform California Earthquake Rupture Forecast, a 2008 report prepared in part by the U.S. Geological Survey, did not list West Napa among the faults considered most likely to trigger a major earthquake within the next 30 years. Another recent report, the San Francisco Region Earthquake Probability Map of 2012, also does not list the West Napa Fault as a top concern. (Both reports were gauging the likelihood of a magnitude 6.7, and Sunday morning’s quake was only a magnitude 6.0).

That all could change, said Moore, who was not ready to make any prediction regarding North Bay earthquakes except for one: “This region is going to get a lot more study in the future.”

Such study will include setting up more seismometers in the area as well as surface mapping and other techniques, she said.

In any case, a Monday news release by the USGS put the West Napa Fault back on the map, describing it as “the most seismically active of the faults mapped between the longer Rogers Creek Fault on the west and the Concord-Green Valley Fault to the east.” The Rogers Creek Fault is located along the west side of Sonoma Valley and skirts the foot of Sonoma Mountain.

USGS also gave a name to the recent earthquake that damaged parts of downtown Napa, tore open Highway 121 and shattered untold bottles of fine wine: the “South Napa Earthquake.”

In terms of physical damage, the quake could have been much worse. For example, the U.S. Army Corps of Engineers inspected Coyote Valley Dam at Lake Mendocino and Warm Springs Dam at Lake Sonoma, and found “no damage or issues resulting from (the) quake.”

Though most geologists shy away from predicting earthquakes, there is one in Sonoma Valley who is known for it: Glen Ellen resident Jim Berkland.

“That was the second worst of my life,” Berkland said Monday in reference to the weekend’s quake. Given its strength, he said, “We were amazed that it wasn’t more than just a couple pictures off the wall and a couple teacups knocked out.”

Berkland said he did predict the weekend’s quake — or, more precisely, he had given probability windows for a larger one happening, and this fell into one of those windows.

“Today’s the day of the new moon,” he said. “And so it’s the proper time. Plus we’re two weeks out to the moon perigee, the ‘supermoon’ that we had.”

More generally, he said, “I was certainly anticipating this summer to be quite shaky.”

That worst earthquake of Berkland’s life was, of course, the Loma Prieta quake of 1989 — which Berkland famously foresaw days in advance in the pages of a local newspaper.

So will more big quakes be coming soon? “Yeah, but not locally,” Berkland said. “I think we’ve shot the local one pretty well. But the Los Angeles area and Seattle are really under the gun.”