San Andreas Drillers Find a Strangely Weak Fault

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Science  23 Dec 2005:
Vol. 310, Issue 5756, pp. 1898
DOI: 10.1126/science.310.5756.1898a

SAN FRANCISCO, CALIFORNIA— Almost 12,000 earth and planetary scientists (a new record) of every stripe met here 5 to 9 December to discuss topics as varied as the inner workings of the San Andreas fault and ancient muck on Mars. Drillers have punched kilometers down through the San Andreas fault for the first time. A first glance at this “natural earthquake machine” reveals that the fault is relatively weak but not what weakened it, researchers reported at the meeting. In their quest to understand how quakes get started and why almost all fizzle out, investigators will drill straight through the heart of San Andreas quakes as they build the San Andreas Fault Observatory at Depth (SAFOD).

The SAFOD drill bit broke through the fault zone in central California last summer, just north of last year's moderate Parkfield earthquake. Drillers were extending the hole begun west of the fault by bending the hole toward the east and through the fault at a depth of almost 3 kilometers, close to a 100-meter patch on the fault that was breaking every 2 years in magnitude-2 quakes.

Before they divert drilling toward their ultimate target, the quake patch, SAFOD workers are looking around a bit, reported geophysicist Mark Zoback of Stanford University in California. He is a co-principal investigator of the SAFOD component of the EarthScope project (Science, 26 November 1999, p. 1655) funded by the U.S. National Science Foundation. Zoback and his colleagues are particularly curious about the stress that builds along a fault and eventually drives fault rupture.

Deep reach.

Scientific drilling into the San Andreas found a weak fault and retrieved altered and deformed fault rock (inset).


SAFOD workers had two ways to get at fault stress. In one, they gauged the stress response of surrounding rock by sending sonic signals out from the hole. The changing orientation of the stress across the fault matched the pattern theorists had predicted for a weak fault—one that slips under relatively slight stress. When Colin Williams of the U.S. Geological Survey in Menlo Park, California, and USGS colleagues measured temperature and thermal conductivity down the hole, they found that the fault is not a source of heat. That's another sign of a weak fault. (Because of their high friction, strong faults generate lots of heat when they slip.) Other researchers had suggested that the San Andreas is weak (Science, 6 March 1992, p. 1210).

Why the weakness? So far, no one knows. Many geoscientists suspect that pressurized fluids—most likely salty water trapped within the fault zone—pry apart the opposite sides of the fault, reducing the amount of stress needed to make it slip. But “right now we see no evidence of overpressurization,” Zoback says. The SAFOD researchers did not detect any pressure surge when they hit the fault zone, and seismic waves passing along the fault to the drill hole showed none of the expected effects of overpressurization, he said.

To get to the bottom of how a weak, normally pressurized fault generates earthquakes, in 2007, SAFOD will drill short spurs off the main hole, targeting the fault patch that slips in small quakes. Researchers hope to learn how it does that and perhaps whether big quakes work the same way. The view from inside even a small earthquake machine, they say, is proving far more informative than the view from outside looking in.

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