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Earthquake science
A new chance for Parkfield

For more than a decade, seismologists have been waiting, instruments ready, in a small town in the hills of Southern California for another earthquake to shake along the San Andreas fault. The town is Parkfield, a name associated with the U.S. Geological Survey’s (USGS) failed earthquake prediction experiment during the 1980s. The 25-kilometer-long stretch of the San Andreas has experienced at least five magnitude-6.0 quakes since 1857. The latest earthquake was in 1966. Based on the periodic nature of these events, USGS researchers predicted an earthquake recurrence every 22 years with the next earthquake shaking things up by 1993. That earthquake never came.

Caza Drilling rig hands tighten the pipe joints on the drill rig for the SAFOD pilot hole. Image courtesy of Stephen Hickman, USGS.

But Parkfield, the most comprehensively instrumented section of a fault anywhere in the world, is now offering seismologists another chance. It is here that in July, a team of seismologists drilled a 7,100-foot pilot hole for the proposed San Andreas Fault Observatory at Depth (SAFOD), a key component of the National Science Foundation’s (NSF) $180-million EarthScope project to study North America’s tectonics and formation (Geotimes, April 2002).

About a mile from the San Andreas fault, crews drilled the pilot hole down about 1.5 miles, and placed inside a string of 40 seismometers to pinpoint the locations of small, magnitude-2.0 earthquakes. By looking at these clusters of so-called microearthquakes, seismologists such as Ross Stein of the USGS hope to learn more about what initiates and governs an earthquake.

Stein studies how earthquakes and faults interact through the transfer of stress. He says that with completion of the seismically instrumented pilot hole, the observing power at Parkfield has taken a new leap. “Perhaps in the end, Parkfield’s delay will appear providential,” Stein says.

Indeed, for a good number of researchers, SAFOD holds the promise of answering many questions about Earth, and the Parkfield pilot hole is a great place to start. After drilling the hole, for example, researchers took fluid samples. “What we’re very interested in is the pore fluid chemistry both in and near the fault,” says Mark Zoback of Stanford University, the drilling project’s co-principal investigator. Determining the fluids’ origins and properties will allow him and his colleagues to test the theory that fluids rising from the upper mantle somehow trigger earthquakes.

The pilot hole culminates 10 years of joint research between Zoback and USGS seismologists Stephen Hickman and William Ellsworth, who have been studying how faults work. “We’ve learned a lot, gathered a lot of good information, and we’ve really set the stage for SAFOD, which is what the pilot hole is all about,” Zoback says.

SAFOD would consist of a wider hole 2.4 miles deep that would actually cross the fault at that depth. “If we get EarthScope funding, then we’ll come back to this exact same site, drill vertically in more or less the same way we have already and then turn the direction of the hole toward the San Andreas fault, so that we intercept the fault at a depth of about 4 kilometers,” Zoback says.

Until Congress decides whether or not to appropriate the $35 million President Bush requested for “ in next year’s budget, the pilot hole’s success is bittersweet. Support for the pilot-hole study is independent of EarthScope, as the International Continental Scientific Drilling Program, NSF and USGS have cooperatively supplied its $1.5 million in funding. As of July, EarthScope’s prospects looked good as the Senate moved to appropriate $20 million.

In the meantime, Zoback is hopeful that the pilot hole will not only provide researchers a good test site for SAFOD, but also begin revealing the secrets of faulting that many seismologists have spent their lives exploring.

“What the structures are, what the fault is made out of, what the fluid pressure is and so on — all of these things have been questions that have plagued us for several decades and now we’ll finally be able to constrain literally dozens of published hypotheses.”

Lisa M. Pinsker


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