A Ton of Interest in Rocks From San Andreas Fault

Scientists said yesterday that they have extracted a ton of rock from deep inside California's San Andreas fault, the first time anyone has collected samples from a geologically active fault zone where earthquakes are spawned.

Bored out from more than two miles below the surface, the rock samples include substantial amounts of the soft mineral serpentine, which the scientists said may play a significant role in the creation and physics of fault zones and earthquakes.

"Now we can hold the San Andreas fault in our hands," said Mark Zoback of Stanford University, a principal investigator of the project. "We know what it's made of. We can study how it works."

Some of the rocks were displayed at a news conference yesterday, where they were hailed as the scientific equivalent of moon rocks. Researchers said dozens of groups from around the country and the world are already working on the project, and requests for samples are expected from hundreds or thousands more.

The rocks, they said, are likely to hold important clues to how fault zones develop and behave, and ultimately to how and when earthquakes occur.

"We're seeing things we can't see from the earth's surface," said William Ellsworth of the U.S. Geological Survey, another principal investigator. "We'll be working to understand the rocks, their strength under stress, and that hasn't been possible before."

Steve Hickman, a geophysicist at the agency, said that worldwide there is "a just extraordinary amount of interest in these core samples." The ton of rock is contained in 135 feet of cylinders four inches in diameter.

The samples, which are part of the National Science Foundation-funded San Andreas Fault Observatory at Depth project, were brought to the surface last month through boreholes. They were extracted near Parkfield, Calif., where earthquakes are relatively common but usually not severe.

The rock samples include large amounts of shale, sandstone and serpentine, a remnant of ancient oceanic crust. The substance can degrade under high temperatures into talc -- the soft powder used on baby's bottoms -- which researchers also expect to find in the sample.

The San Andreas fault, like most of the especially active ones, is the collision point of two massive and slowly moving tectonic plates -- one underlying the Pacific Ocean and the other bearing the North American continent. Earthquakes occur when intense pressure that built up over decades or centuries at their meeting point is suddenly released -- making these boundary areas especially important to geologists.

The newly extracted rock samples largely consist of fine-grained powder called fault gouge, which is produced by the grinding of rocks. The serpentine is visible in the gouge -- floating like "raisins in raisin pudding," Hickman said.

The San Andreas deep study project began in central California in 2004 and has already resulted in important findings. In August, the journal Nature published findings from mineralogist Diane Moore that some early drilling had brought up rock cuttings that did, indeed, include talc.