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San Andreas fault: Could earthquake happen sooner than expected?

The frequency of a major earthquake along a key stretch of California’s San Andreas fault could be greater than thought, according to studies published Thursday in the journal Science.

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The interval between major earthquakes along a key stretch of California's San Andreas fault appears to be shorter than current assessments indicate, according to two related studies published Thursday.

If these results – in the journal Science – hold up under additional scrutiny, they suggest that this section in southern California, which was responsible for the 1857 Fort Tejon quake, may be relatively close to another rupture.

Yet buried within that estimate may be some good news.

The amount of slip produced by the Fort Tejon quake, which was magnitude 7.9, may be roughly half the size previously estimated. If that's the case, estimates of what a San Andreas "big one" is like on that part of the fault "may have gotten a little smaller," says Kenneth Hudnut of the US Geological Survey office in Pasadena, Calif.

A downgrade in magnitude – from 7.9 to perhaps 7.7 – may not sound like a big change, Dr. Hudnut adds. But halving the slip on that section of the fault is a game-changer, reducing to some extent the amount of shaking that the Los Angeles area could experience.

Previously, estimates suggested that major quakes along this part of the San Andreas happened on average every 240 to 450 years. The new work points to an average recurrence rate of 140 years, give or take 46 years.

The results suggest that seismologists may not know the San Andreas as well as they thought they did, according to Lisa Grant Ludwig, a geophysicist at the University of California at Irvine who participated in both studies – in one as the lead researcher.

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