Study Challenges Old Notions About San Andreas

"If you're waiting for somebody to tell you when we're close to the next San Andreas earthquake, just look at the data"

View Comments ()



    This is an image of the San Andreas fault

    More earthquakes have occurred along the San  Andreas fault over the past 700 years that previously thought, according to a study  conducted by researchers at UC Irvine and Arizona State  University.

    The researchers found that large ruptures have occurred as often as every 45 to 144  years along the Carrizo Plain portion of the fault, about 100 miles northwest of  Los Angeles. That's a sharp departure from previous estimates that major quakes  occurred along the fault every 250 to 400 years.

    USGS: Earthquake Preparedness

    But there has been an extended lull. The last large quake along the fault was in 1857, more than 150 years ago.

    "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.

    While the UCI-ASU study found that temblors occurred much more  frequently, it also determined that not all of the quakes were as strong as  originally thought, but they still ranged between magnitude-6.5 and 7.9.

    "We've learned that earthquake recurrence along the San Andreas fault  is complex," said Ramon Arrowsmith, geology professor at ASU. "While  earthquakes may be more frequent, they may also be smaller. That's a bit of  good news to offset the bad."

    Ken Hednut, a geophysicist with the U.S. Geological Survey, said the  researchers work was rigorously field-checked by scientists.

    "I believe they've done a really careful job," he said. "When people  come up with new results challenging old notions, others need to see the  evidence for themselves."

    The study found that the last major quake along the fault was the magnitude-7.8 Fort Tejon quake in 1857.

    "People should not stick their heads in the ground,'' Ludwig said. "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.''