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October 9, 2007


Sampling The San Andreas

First core samples from deep inside the fault zone promise new earthquake understanding

Elizabeth K. Wilson

For the first time, scientists have unfettered access to complex soil, rock, and fluid samples from deep inside California's San Andreas Fault that will help them better understand the underground molecular events associated with earthquakes.

Borrowing heavily from oil-drilling technology, engineers brought up 4-inch-diameter, 135-foot-long Earth cores from the San Andreas Fault Observatory at Depth (SAFOD) in Parkfield, Calif., in September (C&EN, Jan. 23, 2006, page 39). The notorious 800-mile-long San Andreas Fault bisects California and has produced some of the most devastating known earthquakes in the region, such as the 1989 Loma Prieta quake and the 1906 San Francisco quake.

NEW SAMPLE Scientists gather around freshly dug-up core samples from the San Andreas Fault.

"There's a lot of chemistry going on down there," U.S. Geological Survey geophysicist Stephen Hickman said at an Oct. 4 press conference at Stanford University, announcing the retrieval of the core samples from two-and-a-half miles down. "We're preparing to literally get our hands on the San Andreas Fault for the first time." He said geochemists will be able to study firsthand the reactions of minerals and fluids that occur directly at an earthquake source.

Large mysteries remain about the behavior of the San Andreas Fault. For example, some areas in the joints between tectonic plates along the fault slide past each other in a "creeping" fashion, while others lock and jolt violently. One mechanism to explain the creep phenomenon, postulated for decades, holds that it's promoted by the mineral serpentinite, which can react with water at high temperatures to produce the slippery, weak mineral talc. Slurries of ground rock that SAFOD brought up from the fault zone in the past have been found to contain talc (Nature, 448, 795, 2007).

The ability to examine undisturbed sections of the fault will now make possible a whole new level of understanding, Mark D. Zoback, earth sciences professor at Stanford, said at the press conference.

"I think it's very important they've got this core," said B. Mack Kennedy, a geochemist at Lawrence Berkeley National Laboratory. "Nobody's ever really sampled anything like this in situ before???something's going to be learned from that."

Lisa Wilson/C&EN
CORE DEMO USGS earthquake researcher William Ellsworth discusses a SAFOD fault core.

The Parkfield area is of particular interest to earthquake scientists because of the frequent, regular trembling of small earthquakes along the fault there. For the past several years, SAFOD scientists have been drilling down into the fault and are now setting up seismometers and accelerometers inside the hole to monitor seismic activity.

The core retrieval "is completely unprecedented," said Kaye Shedlock, director of the National Science Foundation's EarthScope program, which oversees earthquake-related projects, including SAFOD. "Exciting and transformative research is guaranteed from this."

Earthquake scientists around the world have been invited to a "sample party" at Stanford in December, where they'll get a chance to inspect the cores and request pieces of them for study. Zoback said he and his coworkers have already snagged several samples, which they're studying with techniques such as X-ray diffraction and transmission electron microscopy.

"This is a dream come true for hundreds of scientists," Zoback said.

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Chemical & Engineering News
ISSN 0009-2347
Copyright © 2009 American Chemical Society


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