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del.icio.usDiggRedditYahooMyWebGoogleWhat's this?

Science getting unprecedented look at
fault

San Andreas rock samples offer
quake info

By Betsy Mason, STAFF WRITER

Article Last Updated: 10/05/2007
06:39:59 AM PDT

Click photo to enlarge

Stephen
Hickman, of the U.S. Geological Survey, looks at rock samples
from two miles beneath the surface of the San Andreas Fault
during a news conference on the Stanford University campus in
Palo Alto, Calif., Thursday, Oct. 4, 2007. Hickman was part of
a team of scientists drilling a borehole into the San Andreas
fault to better understand how earthquakes are born. (AP
Photo/Paul Sakuma)

«

1

»

PALO ALTO — Rock drilled from the San Andreas fault two miles
below the surface is giving geologists an unprecedented look
at the inner workings of Californias most famous fault,
heralding a new era for earthquake science. The 135 feet of
four-inch diameter cylindrical cores promises to reveal the
answers to questions that scientists have grappled with for
decades, such as how earthquakes get started and what causes
them to stop.

For people like me, its sort of a Neil Armstrong moment to
actually hold the San Andreas fault in my hands, said Stanford
University geophysicist Mark Zoback, one of the leaders of the
$23 million drilling project, funded by the National Science
Foundation.

The rock samples, shown Thursday at a news conference at
Stanford, are the first ever from deep inside one of the
worlds major active faults that separate Earths tectonic
plates.

Scientists around the world are drooling over the
possibility of getting their hands on a piece of the cores.
Until now, scientists were forced to study old, dead faults
that have been uplifted by tectonic forces and exposed by
erosion.

The new rock core is a window into a fault that is still
actively moving. In a way its an autopsy, except that the
patient is still alive, said geophysicist Steve Hickman, a
project leader from the USGS in Menlo Park.

You can actually deduce why the patient is behaving the way
it is.

The drilling project, known as the San Andreas Fault
Observatory at Depth,

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began in 2004 on a well-studied stretch of the fault 23
miles northeast of Paso Robles near Parkfield.

In this area, the two sides of the fault are slowly and
gently creeping past each other, instead of staying locked for
long periods of time and then violently snapping past each
other in an earthquake as most of the 800 miles of the San
Andreas does.

There are many theories about why the slow-moving section
is different, but Zoback thinks the new rock samples may hold
the answer.

The cores were extracted from a patch within the creeping
section where small earthquakes, about magnitude 1.8, occur
frequently.

"What happens with these smaller earthquakes may tell us
what's happening on a larger scale," he said.

The Hayward fault also seems to creep in places, evidenced
by slowly deforming streets and curbs along the fault trace in
the East Bay hills, but it also has ruptured in major
earthquakes.

One possible explanation is that the upper part of the
fault is creeping while the deeper part is locked. Studying
the patches within the San Andreas faults creeping zone that
do experience small earthquakes could help scientists
understand the Hayward fault's split personality.

The cores are already yielding clues about the San Andreas
fault. The fault splits into smaller strands at depth, and the
team drilled across two of those strands.

Each strand was several feet wide and filled with gouge, a
mish mash of different types of rocks and minerals that have
been ground up by the motion of the fault.

One greenish mineral, known as serpentine, caught the
attention of the geologists right away. Scientists have long
speculated that serpentine which formed in the ocean crust and
was later thrust onto the continent tens of millions of years
ago.

They found the serpentine only in the fault gouge, not in
the surrounding rock. "Serpentine seems to have played a major
role in determining where the fault is," Zoback said.

Serpentine also reacts with hot fluids containing quartz to
produce soft, slippery talc which could be part of the reason
the fault creeps rather than sticks.

Many scientists have speculated that fluids also may be a
piece of the puzzle, lubricating the fault and allowing it to
slowly slide along. Now they may finally find out if they were
right.

By submitting the rock core in the lab to the pressure and
temperature experienced 2 miles deep in the earth's crust and
then watching how it reacts to movement, scientists will get
the next best thing to watching the actual fault.

The next step in the project is to install a slew of
instruments in the borehole across the fault to study the
small earthquakes that repeatedly occur there. Seismometers,
tiltmeters, accelerometers and instruments to monitor the
fluids will be part of a permanent observatory on the fault.

"I think this is the beginning of a very exciting phase,
and a lot of the great discoveries we anticipate will be
coming over for the next few years," Hickman said.

Betsy Mason covers science and the national laboratories.
Reach her at 925-952-5026 or [email-masked].

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