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August 20th, 2010, 12:55 pm
Big earthquakes rock the famous San Andreas fault far more often than previously believed,
UC Irvine researchers have discovered — a finding that will likely increase experts’ estimates of the risk of a big quake happening soon.
The researchers found that large quakes happen on the fault as often as every 45 to 144 years. Previously, the widely accepted estimate was about every 250 to 400 years.
The new time frame is much shorter than the most recent big quake on the San Andreas, an estimated magnitude 7.8 quake in 1857.
“The data we are publishing shows a pattern of much more frequent earthquakes,” said UC Irvine seismologist Lisa Grant Ludwig, the principal investigator on a study to be published the Sept 1 issue of the science journal,
Geology. “We’re talking about ones large enough to reach the surface to form an actual rupture.”
But Ludwig’s research also showed that there are more moderate quakes on the fault than previously thought.
“It’s not all bad news, because the data also implies that they’re not all huge earthquakes,” she said. “The next one may be closer than we like, but it might not be as big as we feared.”
Still, she is urging Southern California residents to be prepared for earthquakes by storing extra water and supplies and developing emergency plans.
“People always ask me if we are overdue,” Ludwig said. “I can’t answer that. But I can tell you that all my colleagues are working hard to try to communicate this to the public, and preparing their own families. The data is pointing in a direction that makes us all want to be prepared.”
Ludwig and Sinan Akciz, lead author on the newest study, spent years digging trenches at the Bidart Fan site on the Carrizo Plain, north of Bakersfield, to pin down the timing and sizes of the last six major earthquakes on the south-central San Andreas fault that occurred over about the past 700 years.
Taking measurements in charcoal deposits left behind by wildfires, they were able to gauge the size of prehistoric ruptures by the breakages in the deposits. Those findings were published in a previous paper.
In the most recent study, the scientists were able to estimate roughly when those quakes occurred using radiocarbon dating of the charcoal.
Ludwig, who was on her way back to the site this weekend to try to take fault measurements with ground-penetrating radar, said she does not wish to be “an alarmist,” but that Southern California residents should take the findings seriously.
“That doesn’t mean panic,” she said. “That is not a wise use of energy. We can’t prevent earthquakes. Use that energy to prepare, because that will minimize impacts.”
http://sciencedude.ocregister.com/20...ndreas/108999/
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