Earthquake Theory Shaken

San Francisco 1906. Scientists may have to rethink their ideas about when the next "big one" will hit California.

A widely accepted theory about earthquakes has received a major shakeup. A team of geologists studying the San Andreas fault near Los Angeles found that bigger earthquakes aren't necessarily preceded by longer periods of inactivity on the fault.

The going wisdom about earthquakes is that the longer a fault goes without a major earthquake, the bigger the quake will be when it finally strikes. The theory seems particularly apt for the San Andreas fault, which marks the boundary between two tectonic plates. As the Pacific plate moves northward relative to North America at about 4.5 cm per year, friction stops the fault from slipping. The longer that strain jacks up, the farther the plates will jolt when they finally let go and the larger the resulting earthquake will be. Or so the thinking went. But a group led by Ray Weldon of the University of Oregon, Eugene, found that the theory doesn't hold up for the segment of the fault near the southern California town of Wrightwood.

Weldon and his team dug 45 trenches across the fault and uncovered evidence of at least 30 earthquakes over the last 6000 years. They used carbon-14 dating to determine when each event occurred and measured how far the fault slipped each time. The result is the most complete long-term record of activity for any fault in the world. And it contradicts the conventional wisdom: Shorter quiet periods of less than a century were generally followed by larger earthquakes, and longer periods of several hundred years preceded smaller quakes. Although this appears counterintuitive, the larger pattern is more logical. It appears that strain is not released entirely with each earthquake but continues to accumulate through four or five or more earthquake cycles. Finally, the strain is released by one big quake or a cluster of smaller shocks. The research appears in the September issue of GSA Today.

The authors caution that they don't know yet if the activity at the Wrightwood fault segment is typical of the entire San Andreas fault or of faults in general. But if it is, the research could change the way scientists estimate the probability of earthquakes, says geophysicist Paul Segall of Stanford University: "My guess is this will force a lot of people to think hard about the assumptions they make about earthquake recurrence."

Related Sites
USGS Earthquake Hazards Program
Ray Weldon's research
The GSA Today article

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