By Clare Baldwin

STANFORD, Calif., Oct 5 (Reuters) - Scientists have sampled rock from an active fault zone 2.3 miles (3.7 km) below the earth's surface for the first time, providing new clues about earthquakes and how they begin.

The samples are from the San Andreas Fault, which stretches 800 miles (1,287 km) along the coast of California, forming one of the world's most seismically active regions. The fault was responsible for the 1906 San Francisco earthquake and fire, the deadliest in U.S. history.

"It's sort of like a Neil Armstrong moment to actually hold the San Andreas Fault in our hands," said Mark Zoback, a professor of earth sciences at Stanford University who is a principal investigator of the project, referring to the first man to step on the moon.

The samples, drilled from a site near Parkfield in central California, weigh nearly a ton and are the first where scientists have been able to preserve the internal structure and mineralogy of rock from deep within an active fault.

Zoback and co-investigators William Ellsworth and Stephen Hickman, both geophysicists with the U.S. Geological Survey in Menlo Park, California, confirmed the presence of serpentine, a weak mineral long suggested as the reason the San Andreas Fault moves so easily.

Under certain conditions, serpentine can change into talc, the slippery mineral used in baby powder, and possibly smooth the way for the fault to shift. Serpentine can also dissolve and recrystallize, potentially allowing the fault to move under very small amounts of pressure.

"There is serpentine throughout the coast ranges of California and it may be the defining characteristic of why the San Andreas Fault is where it is," said Zoback. "It is a key factor in how the fault works."

Scientists also hypothesize that serpentine is a key components in the behavior of deep sea faults, like the one that caused the 2004 earthquake and Indian Ocean tsunami that killed more than 280,000 people.

If scientists can understand how serpentine behaves under different conditions, they may be able to determine where big earthquakes are likely to occur.

Similar projects to bore into earthquake zones are being planned for other regions, including New Zealand, Italy, and the sea floor off the island of Honshu, Japan.

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