In a study of historic seismic activity along the fault, researchers with the University of California-Irvine and Arizona State University found that large ruptures have occurred on the Carrizo Plain partion of the fault, about 100 miles northwest of Los Angeles, as often as every 45 to 144 years.
The accepted scientific wisdom has predicted major earthquakes along this part of the San Andreas fault every 250 to 450 years. The new study found big quakes occurred much more often - every 88 years, on average.
The last major earthquake was the magnitude 7.8 Fort Tejon quake in 1857, more than 150 years ago.
Seismologist Lisa Grant Ludwig (Photo courtesy UCI)
"If you're waiting for somebody to tell you when we're close to the next San Andreas earthquake, just look at the data," said UCI seismologist Lisa Grant Ludwig, an associate professor of public health who is principal investigator on the study.
Not all the six earthquakes studied were as strong as originally thought, but they all were big quakes, ranging in magnitude from 6.5 to 7.9.
"We've learned that earthquake recurrence along the San Andreas fault is complex," said co-author Ramon Arrowsmith, a geology professor at Arizona State. "While earthquakes may be more frequent, they may also be smaller. That's a bit of good news to offset the bad."
Sinan Akciz, UCI assistant project scientist and the study's lead author, was part of a team that collected charcoal samples from trenches they dug in the Carrizo Plain. The study is based on analysis of these samples along with earlier samples that Ludwig had stored for decades in her garage.
The charcoal forms naturally after wildfires, then is washed into the plain by rains, building up over the centuries in layers that are fragmented during earthquakes.
Akciz dated the samples using recently developed radiocarbon techniques to determine time frames for six major earthquakes, the earliest occurring about 1300 A.D.
"What we know is for the last 700 years, earthquakes on the southern San Andreas fault have been much more frequent than everyone thought," said Akciz. "Data presented here contradict previously published reports."
The scientists will publish their study in the September 1 issue of the journal "Geology." Funding for the study was provided by the National Science Foundation, U.S. Geological Survey and Southern California Earthquake Center.
Ken Hudnut, a geophysicist with the U.S. Geological Survey, said the research is important because it revises long-standing concepts about well-spaced, extremely strong quakes on the 810-mile fault.
"I believe they've done a really careful job," he said, adding that the work was rigorously field-checked by many scientists. "When people come up with new results challenging old notions, others need to see the evidence for themselves."
As for the 153-year lull since the magnitude 7.8 Fort Tejon quake, Ludwig said, "People should not stick their heads in the ground. There are storm clouds gathered on the horizon. Does that mean it's definitely going to rain? No, but when you have that many clouds, you think, ‘I'm going to take my umbrella with me today.' That's what this research does. It gives us a chance to prepare."
For individuals, preparation means having ample water and other supplies on hand, safeguarding possessions in advance, and establishing family emergency plans.
For regulators, Ludwig advocates new policies requiring earthquake risk signs on unsafe buildings and forcing inspectors in home-sale transactions to disclose degrees of earthquake risk.
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