San Andreas Fault
Paul Sakuma  /  AP
Mark Zoback holds up rock samples from two miles beneath the surface of the San Andreas Fault during a news conference at Stanford University in Palo Alto, Calif., Oct. 4, 2007.
updated 10/5/2007 7:51:36 AM ET 2007-10-05T11:51:36

A team of scientists on Thursday showed off the first rock samples taken from a borehole being drilled into the mighty San Andreas Fault to better understand how earthquakes are born.

The 4-inch-wide rock cores were pulled earlier this month from two miles beneath a seismically active section of the fault halfway between San Francisco and Los Angeles.

Researchers hope the core collection, weighing about a ton in total, will help answer questions about the fault's makeup and determine what happens during stress buildup at great depths.

"Now we have samples that we can literally hold in our hands and study in the lab," said Mark Zoback, a geophysicist at Stanford University and one of the project leaders.

As excited as scientists worldwide are about the rock cores, they likely won't help in earthquake prediction. That goal is still out of reach despite a century of research into earthquake physics.

Since 2004, a team of geophysicists and seismologists has been drilling in the town of Parkfield using a technique common in the oil industry. The site in Parkfield, the self-proclaimed "Earthquake Capital of the World," was chosen because it sits atop a creeping segment of the 800-mile San Andreas Fault. Creeping occurs when two sides of the fault gently slide past each other, triggering small temblors.

Last summer, scientists penetrated an active section of the fault for the first time and began the arduous process of extracting rock samples to the surface.

"These are kind of like moon rocks for people studying earthquake mechanics," said Stephen Hickman of the U.S. Geological Survey.

A preliminary analysis revealed the rupture zone contains very fine ground-up rock containing a greenish mineral called serpentine that may explain why the region of the fault creeps slowly to relieve stress. Scientists plan to study the mineral in the lab to find out the exact process by which it causes the fault to creep.

While the rock cores should reveal abundant clues about the middle section of the San Andreas, the findings may not apply to other regions of the fault. Seismologists are particularly concerned about the southernmost section, which has not popped since 1690 and is capable of producing devastating quakes.

Scientists next year plan to rig the borehole in Central California with sensors to try to catch an earthquake in the making. When completed, it will be the world's first underground earthquake observatory designed to study temblors up close.

The $25 million project is funded by the National Science Foundation, USGS and Stanford University.

Copyright 2007 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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