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Home » Earthquakes » San Andreas Fault Earthquake Frequency

Large San Andreas Earthquakes are More Frequent Than Previously Thought

They occur every 45 to 144 years, but the most recent was over 150 years ago


Republished from an August, 2010 press release by UC Irvine.


Studying a 700-Year Earthquake Record



Earthquakes have rocked the powerful San Andreas fault that splits California far more often than previously thought, according to UC Irvine and Arizona State University researchers who have charted temblors there stretching back 700 years.

The findings are published in the September 1, 2010 issue of Geology, conclude that large ruptures have occurred on the Carrizo Plain portion of the fault - about 100 miles northwest of Los Angeles - as often as every 45 to 144 years. But the last big quake was in 1857, more than 150 years ago.


Are We Due for a Major Earthquake?



UCI researchers said that while it's possible the fault is experiencing a natural lull, they think it's more likely a major quake could happen soon.

"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, principal investigator on the study.


A Wake-up Call for Getting Prepared



An associate professor of public health, she hopes the findings will serve as a wake-up call to Californians who've grown complacent about the risk of major earthquakes. She said the new data "puts the exclamation point" on the need for state residents and policymakers to be prepared.

For individuals, that 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 risk.


Dating Charcoal in Fragmented Sediment Layers



Sinan Akciz, UCI assistant project scientist and the study's lead author, was part of a team that collected charcoal samples from carefully dug trenches in the Carrizo Plain, 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 via recently developed radiocarbon techniques to determine time frames for six major earthquakes, the earliest occurring about 1300 A.D.


Previous Recurrence Intervals Too High



The field data confirmed what Ludwig had long suspected: The widely accepted belief that a major earthquake happened on the fault every 250 to 400 years was inaccurate. Not all quakes were as strong as originally thought, either; but they all packed a wallop, ranging between magnitude 6.5 and 7.9.

"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."


Higher Frequency but Lower Strength



"We've learned that earthquake recurrence along the San Andreas fault is complex," agreed 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."

Ken Hudnut, a geophysicist with the U.S. Geological Survey, said the research is significant 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."

Upending previous San Andreas fault modeling is part of a broader shift in seismic research. Experts are increasingly tracking webs of trigger points, smaller faults and more frequent quakes rather than focusing on large, single faults where they assumed there would be well-spaced shakers.


A Time to Prepare



As for the 153-year hiatus 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."


Support for the Study



Funding for the study was provided by the National Science Foundation, U.S. Geological Survey and Southern California Earthquake Center.


About UC Irvine



About the University of California, Irvine: Founded in 1965, UCI is a top-ranked university dedicated to research, scholarship and community service. Led by Chancellor Michael Drake since 2005, UCI is among the most dynamic campuses in the University of California system, with nearly 28,000 undergraduate and graduate students, 1,100 faculty and 9,000 staff. Orange County's largest employer, UCI contributes an annual economic impact of $3.9 billion. For more UCI news, visit www.today.uci.edu.


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the San Andreas Fault at Carrizo Plain
A linear scar marks trace of the San Andreas fault in the Carrizo Plain area. The large stream entering fault zone from left has been displaced by movement on fault to debouch on right (southwest) side into the Carrizo Plain, thus displaying right-lateral slip. View is looking southeastward along the fault. USGS image.




the San Andreas Fault at Carrizo Plain
A sigmoidal pattern of stream channel results from right-lateral offset, followed by stream capture producing left deflection, followed by further right-lateral offset. Fault extends from right to left; stream flows from bottom to top. View southwestward toward the Carrizo Plain. USGS image.



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