Predicting earth's phenomena is all about finding the right tools for the job. Scientists studying the movement of the San Andreas Fault—the 800-mile-long boundary where the North American Plate grinds against the Pacific Plate—may not yet be able to tell us when to expect a big California earthquake, but they are now able to rake in enough data to estimate where.

Yuri Fialko, an associate professor at the Institute of Geophysics and Planetary Physics at the University of California San Diego, uses a combination of data from satellites (GPS and microwave radar) to pinpoint to the millimeter the movement of tectonic plates. It's called Interferometric Synthetic Aperture Radar (InSAR), a technology whose efficacy in studying seismic movement was proven in the early 1990s.

How it works: A satellite sends a microwave pulse toward the Earth's surface, and then records the radar "echo." From that, computers create a digital terrain map that spans hundreds of square miles, with a resolution of about two meters. For the past decade, a satellite has passed over the San Andreas area roughly once a month, giving Fialko a wealth of data on the shifts of the fault. He has learned, for example, that the southern portion of the fault is moving about 25 millimeters per year and showing a record amount of elastic strain. This strain is released by earthquakes—the greater the elastic stress on the plates, the greater the earthquake required to release it. Since the southern San Andreas, which runs from the Salton Sea to the San Bernardino Mountains, has not had an earthquake in over 300 years, Fialko believes a big one could be on the way. The question, of course, is when.

"Using interferometry, we're able to get an idea of the likely location and strength of an earthquake," says Fialko, "but the repeat frequency of a large quake can vary widely." Still, with four InSAR-equipped satellites currently orbiting the Earth, and NASA looking into launching one of its own, Fialko could someday have enough data to make long-term predictions possible. "We're now closer to understanding the timing of seismic events than we've ever been," he says.

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