Powerful earthquake faults threaten Napa on 2 sides

October 13, 2014 • BARRY EBERLING

When it comes to earthquakes, southern Napa County has more to worry about than the West Napa Fault.

The city of Napa lies between the Northern Hayward/Rodgers Creek Fault that runs through Sonoma County about 15 miles to the west and the Green Valley Fault that runs through Napa and Solano counties several miles to the southeast. Each fault has built up enough tension for a magnitude 7.1 earthquake, according to a new report.

"They've got you on both sides," said Jim Lienkaemper of the U.S. Geological Survey, one of the report's authors.

The result could be a powerful shaking for the city of Napa area if either fault slips, according to researchers from the U.S. Geological Survey and San Francisco State University who wrote the report. An earthquake of magnitude 7.1 would release more than 30 times the energy as the magnitude 6.0 South Napa earthquake on Aug. 24 that has caused an estimated $600 million in damages.

The Green Valley Fault runs from the hills between Napa and Solano counties southward and crosses under Interstate 80 near the entrance to Jameson Canyon/Highway 12. It continues south along the edge of Fairfield's Cordelia Villages area, under Suisun Bay and under Concord.

Historically, the fault moves about every 250 years, Lienkaemper said. It hasn't moved in the last 400 years.

"There's plenty of strain on it," Lienkaemper said.

The Association of Bay Area Governments has released maps showing how major earthquakes along the region's major faults could affect Napa County. While the West Napa Fault poses the biggest shaking threat to southern Napa County, the Green Valley Fault could also cause severe damage.

A magnitude 6.8 earthquake along the Green Valley Fault — it would be 6.3 times bigger than Napa's August earthquake — could cause "strong" shaking in the city of Napa, according to ABAG reports. A few thousand Napa County residents might have to move out of their homes because of earthquake damage, at least temporarily.

Most Napa Valley cities, including Napa and American Canyon, receive water shipped from the Sacramento-San Joaquin Delta through the North Bay Aqueduct, with Napa also receiving water from its own reservoir, Lake Hennessey. The North Bay Aqueduct crosses the Green Valley Fault near Fairfield.

"The North Bay Aqueduct is relatively recent," Solano County Water Agency General Manager David Okita said. "It was constructed in the 1980s. It's fairly up-to-date construction. We've never heard (from the state) that there's a need to retrofit."

But, he said, there are no guarantees that the system would survive an earthquake along the Green Valley Fault undamaged.

The Rodgers Creek Fault runs from Santa Rosa south under San Pablo Bay. ABAG reports show an earthquake of magnitude 7.2 along it could cause "strong" shaking in the city of Napa. The number of people having to leave their homes because of earthquake damage could be in the low hundreds or less.

Napa County Office of Emergency Services Coordinator Kevin Twohey said the Rodgers Creek Fault is a bigger concern to his department than the Green Valley Fault. That's because the U.S. Geological Survey estimates the fault has a 15.2 percent probability over the next 30 years of having an earthquake magnitude 7 or greater, he said.

By comparison, that earlier study said the Green Valley Fault has a 6 percent chance over the next 30 years of having an earthquake of magnitude 6.7.

Still, the West Napa Fault has the potential to cause the most damage in south Napa County, according to ABAG reports. The proximity of the fault matters.

A magnitude 6.7 earthquake on the West Napa Fault could cause "very strong" shaking to the city of Napa, the ABAG report said. More than 10,000 people in the county could be displaced from their homes in such an earthquake, which would be many times more powerful than the Aug. 24 south Napa quake.

http://napavalleyregister.com/news/local/powerful-earthquake-faults-threaten-napa-on-sides/article_d2b1792e-9f5c-5d96-992f-5b42b1e14d55.html