In his hands, geophysicist Mark Zoback holds a piece of the San Andreas fault. The four-inch-diameter cylinder was retrieved from more than two miles underground and may hold the key to understanding the behavior of the infamous rift. “These are totally unique samples,” Zoback, MS ’73, PhD ’75, says. “They give us insight into the composition and properties of the fault, things that we have speculated about for decades.”
Zoback, the Benjamin M. Page Professor in Earth Sciences, is one of three principal investigators on the San Andreas Fault Observatory at Depth (SAFOD) project, which first broke ground in Parkfield, Calif., in 2004. Its ultimate goal is to establish the world’s first underground earthquake observatory. The core samples are the first ever retrieved from inside an active fault.
Over nine weeks this summer, and despite the odd equipment snafu, the San Andreas fault yielded its deepest secrets. But it wasn’t easy quarry. In all, geologists retrieved some 135 feet of rock cores, weighing in aggregate roughly a ton. The last of the samples were brought to the surface at around 3 a.m. on September 7, amid a thunder and lightning storm no less.
Already they’ve revealed some surprises. For one, sections composed of “fault gouge”—rock that’s been ground into a fine powder by the motion of the tectonic plates—were found to be wider than expected, as much as two meters across. And some of the cores have chunks of green serpentine floating in the gouge like raisins in bread pudding.
Geologists have long suspected that serpentine, either directly or through its ability to chemically alter into the smooth mineral talc, may allow the plates to inch gradually past each other, rather than moving in the fits and starts that can set off a temblor. Now they have evidence that the mineral is present in the fault itself. “It indicates very clearly that serpentine is playing a major role in where the San Andreas fault is and why it behaves as it does,” Zoback says.
These revelations have generated a tremendous amount of excitement in the international scientific community. Zoback and his collaborators at the U.S. Geological Survey expect hundreds, if not thousands, of requests for samples. “For scientists interested in how earthquakes work, these are like moon rocks,” he explains. In early December, the USGS hosted a party at its Menlo Park campus, where researchers from around the country got their first look at the cores and pitched their research proposals.
During the next phase of the SAFOD project, which begins in 2008, an array of extremely sensitive seismic instruments will be installed in the 2.5-mile-long underground tunnel to provide continuous data. “We’ll be right there next to the fault as these earthquakes are happening,” says Zoback, “observing as they nucleate, as they propagate and as they recur through time.”
The hope is that they may find some precursory activity that can reliably predict earthquakes. “We don’t know if earthquakes are predictable or not,” he says. “But if they are, it’s a tremendously important scientific goal.”
