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Next Valley earthquake – not if, but when

Along with the upside of world-class wineries, a robust cuisine culture and Sonoma’s location at the birthplace of the Bear Republic, Valley residents have learned to live with the downside: They’re riding along a strike slip fault zone of two of the planet’s major plates that are moving in opposite directions.

And sometimes, like early last Sunday morning, those plates rasp and heave in a seismic shift that leaves property damaged, power lines down and nerves rattled – or worse.

But don’t think that Sunday’s 6.0 (or 6.1) earthquake in Napa in some way inoculated us against another one. That’s the message that Tom Brocher, the director of the USGS Earthquake Science Center, has for Sonoma residents. The opposite, in fact, is the more likely scenario.

There have been three other significant earthquakes in the area, dating back to the so-called San Francisco Earthquake of April 18, 1906 – estimated at about 7.7 on the “moment magnitude scale” that geologists now use to replace the similar Richter scale. Other notable events that have shaken the county include a pair of Oct. 2, 1969, earthquakes just north of Santa Rosa, that measured about 5.7, and the Oct. 17, 1989, Loma Prieta quake (often remembered as the “World Series Earthquake”) of 6.9 MMS.

While the 1989 quake did very little damage locally, the 1906 quake had an even larger destructive impact on Santa Rosa than it did on San Francisco. Many buildings were damaged in the City by the Bay during that quake, but it was the resultant fire that turned the event into a disaster.

In Sonoma County, structural damage was proportionately even greater. A 2005 United States Geological Survey study determined that, in 1906 “the highest ground shaking intensity occurred in and around Santa Rosa, which was more than 20 miles from the rupture zone.” The quake essentially destroyed the downtown area, electricity was cut off for six days, and gas for even longer, and at least 50 people were killed.

For Sonoma Valley residents, the experience was “an awful ordeal,” according to a Sonoma Index Tribute article headlined, “Fearful Visitation of Earthquake,” on the front page of the April 21, 1906, edition.

“The sleeping inhabitants of this valley were ruthlessly awakened to find the earth in the throes of a mighty earthquake which lasted fully one minute. Buildings cracked and partially collapsed, chimneys toppled and roofs fell in. There were the sound of falling glass and the cries of the panic stricken as they rushed pell-mell from their homes to the streets.”

Local destruction in the Valley was primarily to buildings and, according to reporting, no one was killed or notably injured. Several stone buildings suffered collapsed walls, the high school was rendered unfit for use, all the drugs were thrown off the shelves of L.S. Simmons pharmacy and “not a chimney in town is standing,” read the coverage.

It was over 60 years before the next earthquake of significance struck the region, and once again Santa Rosa’s downtown bore the brunt of local damages. The twin Oct. 2 1969, temblors – spaced less than 90 minutes apart – again brought down chimneys and crumbled many older buildings, buckled sidewalks and ruptured underground pipes. In fact, 49 structures, almost exactly half of the 99 damaged buildings in Santa Rosa, were in the downtown area, many of them in the vicinity of Courthouse Square.

Today’s on-going controversy over the future of Courthouse Square is in many ways fallout from those 1969 quakes, which resulted in creation of a downtown redevelopment district. Though the old Courthouse had been demolished in 1966, the quakes contributed to the drive for urban renewal that culminated in the indoor shopping mall at Santa Rosa Plaza, completed in 1983, which took several other historic buildings with it.

“Santa Rosa is built on top of a sedimentary basin,” said Brocher by phone from his USGS office in Menlo Park. “It’s basically an area that’s subsiding and gets filled in with young deposits. So it shakes harder.”

The 1969 quakes took place along the Healdsburg Fault, one of a series of faults that radiate like cracks in an ice sheet along the Rodgers Creek Fault Zone north of San Francisco Bay. The West Napa Fault, implicated in Sunday’s earthquake, is part of a separate fault system that runs up the East Bay and includes the Concord, Calaveras and Green Valley faults. Sonoma Valley sits pretty much dead center between these two fault systems.

“But you’re not entirely off the hook,” warned Brocher. “The map only shows the faults for which there’s really good evidence – but other faults are suspected.” He names the Franklin Fault, diving under Carquinez Strait, as likely extending into Sonoma.

“When I was a geology student eons ago,” said Brocher, “we were taught that the San Andreas Fault was the boundary between the Pacific plate and the North American plate – you could step across it.” Brocher, 60, has his graduate degrees from Princeton University.

“Now we realize that it’s actually a very wide plate boundary, at least 40 or 50 miles, if not wider. There’s evidence that almost the entire state of California is moving northward, and you could almost argue that the width of California is the plate boundary.” Instead of separate fault systems, Brocher likens it to a “shear zone.”

Which brings up the question: What is a fault? The answer is surprisingly simple. “Geologically, a fault means it’s a defect. You have layers of rock that are juxtaposed with rocks that shouldn’t be there naturally. They didn’t form there, they’ve been brought there by movement.”

But both the Rodgers Creek and Hayward fault zones might be thought of as subsets of the much larger and more notorious San Andreas Fault zone, stretching from Cape Mendocino down to the Salton Sea –roughly 800 miles long. And that in itself is part of the problem.

“That’s significant because, in California, it’s really the length of the fault that controls the maximum magnitude earthquake it can produce,” noted Brocher. If the West Napa Fault, at only about six miles long, could create a 6.0 earthquake, the potential of the Rodgers Creek fault zone – which includes the Healdsburg Fault, and possibly the Hayward Fault to the south – could be 150 miles long.

Another bit of folk wisdom that Brocher challenged was that an earthquake can relieve the pressure built up on a fault, and reduce the likelihood of another. “Actually it more transfers the stress,” he said.

“Maybe along the segment of the fault that ruptured it’s reduced, but it’s shoved into something else – and that something else has higher stress.”

All this makes any sense of relief from risk purely temporary, especially for those of us in Sonoma County. “Rodgers Creek Fault has probably the highest probability (of a quake) in the Bay Area,” said Brocher. “The Rodgers Creek and the Hayward Fault, but especially the Rodgers Creek. Because it’s been so long since it’s had an earthquake and it’s been loaded by these movements of the earth’s plates, the thinking is it’s probably the most likely of the earthquake zones to go next.”

And according to David Schwartz, another USGS scientist, “The Rodgers Creek historically has produced a large earthquake, over a 7.0 magnitude, every 220 years, and it’s been 300 years since the last one.”

Earthquake scientists today say there’s a 62 percent chance of an earthquake with a magnitude 6.7 or greater to strike the Bay Area in the next 30 years – and the worst place to be is on the Rodgers Creek Fault – stretching from Petaluma, Cotati and Rohnert Park in the south to Healdsburg up north.

“Now’s a good time to make sure your emergency supplies are ready,” said Brocher. “We have a preparedness page on earthquake.usgs.gov, where we have a checklist of things you should know.”