Monday, August 26, 2019

[CaliforniaDisasters] The 2014 quake does not exempt Napa from another one

The 2014 quake does not exempt Napa from another one

On the fifth year anniversary of the South Napa earthquake, Napa County has ceded the California quake spotlight to Ridgecrest in eastern Kern County.

The series of mid-July Ridgecrest earthquakes were California's strongest since the magnitude 6.0 Napa quake. A U.S. Geological Survey earthquake calculator shows the strongest at magnitude 7.1 released 44 times the energy as the Napa shaker.

Unfortunately, Napa County has the potential to grab the spotlight back. The Association of Bay Area Governments reports a 63 percent chance of a major earthquake along one of the area's major faults rocking the county in the next 30 years.

If earthquake scenario maps on the ABAG website are correct, no city in Napa County is free from the threat of powerful, building-damaging shaking.

There's the West Napa fault that was the source of the South Napa earthquake. A stronger, magnitude 6.7 quake on this fault would jolt the cities of Napa, American Canyon and Yountville, possibly causing considerable damage to even well-built structures, bursting pipes, partially collapsing some homes and overturning heavy furniture, an ABAG shaking map predicts.

Imagine the Aug. 24, 2014 South Napa earthquake with, according to the U.S. Geological Survey earthquake calculator, 11 times the energy released.

Calistoga is no safe haven in the north Napa Valley. A magnitude 7.4 earthquake on the Maacama fault centered in Sonoma County could cause just as much damage there.

For that matter, a magnitude 7.1 Rodgers Creek quake centered in Sonoma County could give all five local cities a strong shaking.

Nor are Lake Berryessa and the small east county communities in an earthquake-free zone. A magnitude 7.1 earthquake on the Berryessa fault could wreak havoc in this sparsely populated area.

Then there's the specter of the long-dreaded Big One on the northern segment of the San Andreas fault. The last Big One was the 1906 magnitude 7.9 earthquake that devastated San Francisco, Santa Rosa and San Jose.

About 20 homes in the then-tiny city of Napa were damaged or destroyed. The south wall of the opera house fell into the Napa Hotel annex, bricks crashing amid guests. The west wall of the Hayes Theater collapsed. A 30,000-gallon metal water tank at the Southern Pacific station was hurled to the ground.

For all of that, no one was killed, the Napa Daily Journal reported after taking a day off from publishing because of damage to its building.

Meanwhile, in the Upvalley, the quake seems to have been mainly a chimney-tumbler. The Weekly Calistogan reported no large damage.

"In Napa County, the shake was not so hard as in other counties, and the loss of property is comparatively light," the newspaper reported after contemplating such scenarios as the destruction of downtown Santa Rosa.

A state earthquake shaking map shows the part of Napa County least likely to be rocked by a quake is the Lake Hennessey area in the mountains east of Rutherford. But it's a matter of picking your disaster threat—a state fire hazard map shows this area to be at moderate-to-high risk for wildfires.

Ridgecrest was hit by a magnitude 6.4 earthquake on July 4 and a magnitude 7.1 earthquake on July 5. The Los Angeles Times reported that scientists were surprised by how relatively little damage the quakes did – mobile homes tossed from foundations, chimneys fallen and a few fires, but most of the homes and business buildings intact.

One difference is the South Napa earthquake caused surface rupturing beneath Browns Valley homes, said Kenneth Hudnut of the U.S. Geological Survey.

Also, Ridgecrest has relatively newer construction than Napa, he said. For example, Napa has a lot of older, brick chimneys. Ridgecrest has more of the stucco-over-metal flue chimneys.

However, the Ridgecrest quake did cause $5 billion in damage to the sprawling Naval Air Weapons Station China Lake, according to more recent government estimates.

One could always flee Napa County and the Bay Area for parts of California less prone to earthquakes. A state earthquake potential map shows supposed safer havens – parts of the central California coast, the Sierra Nevadas and the east side of the Central Valley from Sacramento through Modesto, Fresno and Tulare.

Still, wherever you might move in the state, take care, expert warn. Seismic hazard maps deal in probabilities, not guarantees.

"That map isn't telling places where it's safe," Hudnut said. "California is earthquake country. Even in places with a relatively low hazard, you can have an earthquake."

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