Sampling The San Andreas

FOR THE FIRST TIME, scientists have unfettered access to complex soil, rock, and fluid samples from deep inside California's San Andreas Fault. Analyses of these materials should help these investigators better understand the underground molecular events associated with earthquakes.

Borrowing heavily from oil-drilling technology, engineers bored down 2.5 miles to bring 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.

"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 coveted core samples. "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, Mark D. Zoback, a geophysics professor at Stanford, noted at the press conference. For example, some areas in the joints between tectonic plates along the fault slide past each other easily in a "creeping" fashion, while others lock and jolt violently. One proposed explanation for the creep phenomenon at great depths is that the movement is promoted by serpentine minerals, which can react with silica-bearing 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).

IN THE CORES, geologists already have identified two strands of soil and rock where creeping occurs and, along with those strands, the mineral serpentinite. Although the talc-lubed mechanism isn't expected to explain creep at the relatively shallow depths that SAFOD plumbed, the implication is that if serpentine minerals exist there, then there's probably more of the minerals deeper down. And that bolsters the case for talc's importance in the San Andreas Fault's behavior, said Diane Moore, a geologist at U.S. Geological Survey in Menlo Park, Calif., and coauthor of the Nature study.

Geologists also have hypothesized that either low-strength clays or high-pressure fluids in the fault zone are lubricating the fault's joints, allowing the fault to slip easily. That hypothesis has been difficult to study in the slurries of rock chips brought to the surface during SAFOD drilling. But now, the intact cores provide unique opportunity to study fluid flow in the fault zone, said geochemist Thomas Torgersen, a professor in the marine sciences department at University of Connecticut.

Once at the surface, and no longer being pressed on by rock two miles thick, the cores can expand and tiny cracks in the soil can widen, allowing fluids harbored within the rock to drain out and be collected.

Torgersen and his colleague Martin Stute at Lamont-Doherty Earth Observatory in Palisades, N.Y., got their samples as soon as the core reached the surface and put them under a vacuum. Over the next year, they'll study the isotopic compositions of elements like helium, neon, and argon in the fluid as it drains out. The resulting data will help the scientists get an idea of the relative age of the fluid at different points across the fault. This will help them determine the rate of fluid flow and, thus, whether the flows are possibly turning rock into low-friction clays or are pressurizing the fault.

The Parkfield area is of particular interest to earthquake scientists because of the frequent, regular small earthquakes along the fault there. SAFOD scientists have been drilling into the fault for the past several years 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," she said.

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 and others, including Moore, already are studying several core samples with techniques such as X-ray diffraction and scanning electron microscopy, helping to identify promising sections for scientists when they make their requests.

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