Rodgers Creek fault seen increasingly as major threat
If the pair of earthquakes that jolted Santa Rosa and neighboring towns Tuesday also shook residents out of whatever complacency they might possess facing a fourth dry winter in a wildfire-prone region, that’s not a bad thing.
While the story of the moment is drought and the fear of another destructive inferno, the threat of a catastrophic earthquake in the region is no less real.
Not only does an active fault line — the Rodgers Creek fault responsible for Tuesday evening’s tremors — run up the middle of Sonoma County, right under Santa Rosa and ending just north of Healdsburg, but recent studies confirm it is connected with the dangerous Hayward fault. That raises the potential for a major quake if the two faults rupture simultaneously.
Moreover, the particulars of the geology around central Santa Rosa are believed to amplify seismic motion, explaining, for example, the disproportionate effect of the Great 1906 San Francisco Earthquake, which killed 85 of Santa Rosa’s 7,000 people and destroyed its downtown, the U.S. Geological Survey says.
California’s San Andreas fault, running north from the Mexican border to Cape Mendocino is the most famous fault. It runs about 20 miles west of the Rodgers Creek fault in Santa Rosa and was responsible for the magnitude 7.9 quake in 1906 and the 1989 Loma Prieta temblor centered in the Santa Cruz mountains, a magnitude 6.9 killer, and many others.
While the San Andreas poses a threat to the Bay Area, scientists say it is the Hayward-Rodgers Creek fault line that is most likely to produce a major earthquake in the region in the coming years.
They forecast about a 33% chance of at least one magnitude 6.7 quake or greater occurring somewhere on one of the faults, or simultaneously, by 2043.
But Lucy Jones, former science adviser for seismic safety to the city of Los Angeles and founder of the Dr. Lucy Jones Center for Science and Society, said she believes a better predictor is looking at the slip rate and recurrence of fault lines, which would suggest there should be big one on the Rodgers Creek fault every 200 years or so — on average, she stressed.
Some folks around the area may remember two quakes more than an hour apart on Oct. 1, 1969 that measured magnitude 5.6 and 5.7 on the Richter scale. (The Richter scale is a logarithmic scale on which each whole-number jump accounts for a tenfold increase in magnitude and represents the release of more than 31 times more energy.)
The twin quakes damaged many homes and businesses in downtown and central Santa Rosa, and caused more than $7 million in damage, (more than $50 million in today’s dollars) the USGS said.
But the last major, ground-rupturing earthquake on the Rodgers Creek fault appears to have been some time in the 18th century, based on scientific studies. That’s around the same time of a major shaker on the Hayward fault, suggesting the possibility the two fault lines shifted together as part of a single event, though it’s unclear.
Either way, it’s due to release substantial stress built up over the years, and the small burps of energy let go with smaller quakes like Tuesday’s are nowhere near sufficient to relieve enough pressure to forestall something much larger, as some lay people believe, said Suzanne Hecker, a USGS geologist who has studied the Rodgers Creek fault closely for some years.
“They can redistribute the stress a little bit, but in terms of making up for larger ones down the road, they don’t do much,” she said.
Hecker was headed to Windsor on Wednesday, to conduct some trenching and study the pace of the fault’s movement on private property this week. The visit is part of what’s been an increasingly intensive examination of the fault line over the past three-plus decades, in recognition of new revelations of its length, location and connection to the Hayward fault.
Though a fault is, in basic terms, a crack in the earth’s crust, it’s actually much more complex, “like a bunch of cracks along a zone,” she said. And even though the Pacific Plate west of the San Andreas fault is moving northwest, it’s moving faster than the land between San Andreas and Rodgers Creek. That creates friction and stress along the faults.
The Rodgers Creek fault has an average “slip rate” of up to 9 millimeters a year over time, a rate calculated from visible comparisons to landscape features from historic quakes.
However, that 9 millimeters is not steady — it may slowly creep for years, then experience a big lurch, Jones said.
When the plates don’t move, it builds pressure that stores energy that is ultimately released in an earthquake. An earthquake with a magnitude of 4 can indicated a slip of slightly less than 10 centimeters, roughly 10 times the annual slip rate, she said.
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