A Fault Runs Through It

Amid a flurry of smaller quakes, geophysicists drill deep in anticipation of the next Big One

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Aerial view of the San Andreas fault in the Carrizo Plain, South-Central California.

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SAFOD's subterranean spyglass is aimed at a geophysical sweet spot on the San Andreas that is a miniature earthquake machine. The size of a football field, it rattles with microearthquakes--in this case, earthquakes of magnitude 2--with surprising regularity. Right next door, within a 2-mile radius, are more microquake clusters. In the coming years, Ellsworth anticipates, SAFOD will record fine-grained portraits of thousands of tiny temblors, many not much bigger than magnitude 0. By closely examining those portraits, scientists should be able to tell how closely one event resembles another and whether earthquakes, at least in principle, are predictable.

On this point, Parkfield's seismic history seems suggestive. For more than a century, the area just south of the drill site produced magnitude-6 earthquakes on a roughly 22-year cycle--or so it seemed in the mid-1980s, when a USGS team threw a net of instruments over the area, hoping to catch the next iteration. The last quake occurred in 1966, so scientists figured the next would come around 1988. Instead, the 1966 quake was followed by a 38-year pause. Some speculate that another earthquake, which occurred on a nearby thrust fault in 1983, reset the seismic clock by altering the local stress field.

Now scientists are trying to gauge how last year's Parkfield quake affected the broader San Andreas system. Stress has been off-loaded to the section of the fault directly south of the rupture, and that has at least the potential to set the stage for a larger upheaval. In 1857, for example, a moderate temblor at Parkfield was followed within hours by a major earthquake that started in the vicinity of Cholame, 15 miles away, and ripped south for 225 miles. In some places the ground moved more than 25 ft.

Most geophysicists don't believe sufficient stress has accumulated along this section of the fault to power another 1857-style spasm. That one approached a magnitude of 7.9, making it even stronger than the 1906 quake that devastated San Francisco. Still, experts acknowledge, it's not inconceivable that the next moderately strong shake-up at Parkfield could lead to the unzipping of a longer section of the fault, spawning a quake of, say, magnitude 7. If that happens, SAFOD would provide scientists with more than they bargained for--a near ringside seat at the start of, if not the Big One, something pretty close.

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