A new survey of fault movement along the southern stretch of the San Andreas reveals an unprecedented view of its potential to tear open an area populated by such cities as Palm Springs, San Bernardino and Riverside.

Ruling out other explanations for why this 100-mile section has remained quiet for nearly 300 years, Yuri Fialko of the Scripps Institution of Oceanography at the University of California, San Diego, provides substantial evidence that pressure is building up on the fault, making it long overdue for a big quake. "At present time the fault is strained to what it has never experienced before," says Fialko, who reports the result of his research in today's issue of Nature.

Fialko based his analysis on a combination of high-quality satellite images, global positioning system (GPS) readings and ground-based seismic measurements collected between 1985 and 2005. The data, which has a resolution down to 20 meters, show how much one side of the North American plate has been slipping past the Pacific plate.

When Fialko calculated the numbers, he found that the sides of the plates were lagging six to eight meters behind in how fast they should be sliding past each other. They appear to be locked together and accumulating strain. As the tectonic pressure builds, something will have to give. "That amount of slip is equivalent to a great earthquake between magnitude 7.5 and 8," Fialko explains. It may not happen all at once, he notes, but for that kind of pressure to release in smaller events, it would require a moderate earthquake every day.

When such an event will happen is difficult to predict. But previous geologic research has determined that the southern section of the San Andreas fault has seen a major earthquake every 200 to 300 years. "We seem to be overdue with respect to the earthquake cycle," Fialko observes.

Fialko also found that a branch of the southern San Andreas system, the San Jacinto fault, is moving twice as fast as previously thought--20 millimeters as opposed to 10 to 12 millimeters--building up the pressure and the potential for a magnitude 7 quake. If the San Andreas doesn't release the next big one, the San Jacinto just might.

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