Scientists Get Hands-On Look at San Andreas Fault

Deep-Earth Rock Samples Aid Earthquake Studies

Scientists now have in their hands something they've never been able to study up close before: a deep-Earth sample of rock taken from directly inside the San Andreas Fault, according to news from the U.S. Geological Survey.

"Now we can hold the San Andreas Fault in our hands," said Mark Zoback, the Benjamin M. Page Professor in Earth Sciences at Stanford University and one of three principal researchers in the San Andreas Fault Observatory at Depth (SAFOD) project. "We know what it's made of. We can study how it works."

The other SAFOD researchers are William Ellsworth and Steve Hickman, both USGS geophysicists.

Launched in 2004, the SAFOD project aims to create the world's first underground earthquake observatory by placing seismic sensors deep into the San Andreas Fault borehole. The seismometers and tiltmeters will watch for the earliest signs of an impending earthquake from the area where earthquake energy first begins to accumulate. Preliminary results from tests made in 2006 have already detected earthquakes so small they have negative magnitudes.

Before now, scientists studying the faults that mark the boundaries between Earth's tectonic plates had to rely on samples from ancient faults exposed by millions of years of erosion, or on computer simulations and lab experiments designed to approximate possible conditions deep underground.

The samples from the San Andreas Fault amount to 135 feet of 4-inch-diameter rock cores, weighing a total of one ton, pulled up from a research borehole more than 2.5 miles deep.

"To an earthquake scientist, these cores are like the Apollo moon rocks," Hickman said. "Scientists from around the world are anxious to get their hands on them in the hope that they can help solve the mystery of how this major, active plate boundary works."