San Andreas fault: Could earthquake happen sooner than expected?

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.

Indeed, the latest work represents an important advance in understanding the south-central San Andreas, says Hudnut, who was not part of the research team. But it's also controversial, he adds.

The research team relied heavily on LIDAR data – a radarlike instrument that uses light instead of radio waves. The small wavelength of light, compared with radio, means that the images show far more detail. The data was initially gathered as part of the B4 Project, led by Ohio State University's Michael Bevis.

The concept behind the B4 project: Use LIDAR and other remote-sensing techniques to gather detailed "before" images along faults. That information will allow seismologists to track fault movement with time, as well as provide a means of comparing postquake landscape features with prequake positions.

Based on the B4 LIDAR data, the team reporting these latest results identified several surface features that earlier surveys had overlooked. They suggest that a five-meter slip is typical of big quakes along the south-central San Andreas – rather than the previously estimated nine-meter slip.

But more work will be needed, Hudnut cautions.

For instance, some argue that the nine-meter slip is the correct figure, based on years of study of the Fort Tejon quake. Over the past decade or more, seismologists have come to appreciate the misdirection that surface features can present.

For instance, in some cases after further study, large slip distances seen in surface features have been triggered by more than one quake. Thus, the nine-meter number may represent two temblors, rather than one.

Despite this and other uncertainties, Hudnut says, the work adds to a growing body of results pointing to major earthquakes happening more frequently along the San Andreas than previous estimates suggest.

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