Article:Quake scientists show off
rocks from depths of San Andr:/c/a/2007/10/04/MNASSK32I.DTL

Article:Quake scientists show
off rocks from depths of San Andr:/c/a/2007/10/04/MNASSK32I.DTL

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Quake scientists show off rocks from depths of San Andreas
Fault

David Perlman,
Chronicle Science Editor

Friday, October 5, 2007

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(10-04) 18:47 PDT STANFORD -- For the first time in
the history of earthquake studies, scientists have penetrated deep into
the very heart of an active earthquake zone and brought up a ton of rocks
as they seek to understand how and why quakes behave.

Their harvest comes from a borehole drilled more than 21/2 miles into
the San Andreas Fault just north of the tiny Monterey County village of
Parkfield and show clear evidence that hundreds of tiny quakes have been
jolting the ground there in just the past few months.

One hundred thirty-five feet of core samples were gathered across three
traces of the fault that is known for its constantly creeping motion and
for the hundreds - if not thousands - of tiny underground temblors that
barely register on instruments and certainly can't be felt by humans on
the surface.

"Now we can hold the San Andreas in our hands," said Mark D. Zoback, a
Stanford geophysicist. "We know what it's made of, and we can study how it
works."

Zoback and his colleagues, William Ellsworth and Stephen Hickman of the
U.S. Geological Survey in Menlo Park, are leaders of a project called the
San Andreas Fault Observatory at Depth, or SAFOD, which for years to come
will be analyzing the quake-shaken rocks.

"It's not unlike bringing rocks back from the moon," Ellsworth
said.

The newly acquired core samples are so uniquely precious they must be
kept shrink-wrapped and refrigerated, Ellsworth said as he touched a
length of the exposed rock with almost loving delicacy.

Parkfield, near the scientists' drill site, is long famed for the
frequent quakes with magnitude 6 and more that have struck that region for
more than a century. Parkfield is by far the most heavily instrumented
seismic research area in the nation.

The next step, the scientists said, will be to send a steel-encased
package containing an array of seismic sensors down the drill hole and
into the seismically active zone. The package will include miniaturized
seismometers, accelerometers and tiltmeters to monitor the underground
activity continuously. The instruments will remain underground for at
least a decade, sending their sensitive readings up to the surface by
fiber-optic cable. They will become the crucial heart of SAFOD, the
long-planned seismic observatory.

At a Stanford press conference Thursday, the excited project scientists
displayed three separate lengths of their 4-inch-wide core samples, each
revealing about five feet of alternating black and gray rock segments
flecked with tiny dull green patches of a mineral called serpentine. The
cores were recovered only recently - two in August and one on Sept. 7.

The serpentine in them, formed from magnesium, silica and water, often
contains the slippery mineral called talc, which Zoback explained might
well be a lubricant that helps the sides of a fault slide slowly past each
other as the fault creeps - and perhaps even accelerates the growth of
small quakes into much larger and damaging ones.

Some fragmented cuttings brought up earlier by the SAFOD drilling
project showed similar evidence of serpentine, and recent laboratory
studies showed the presence of slippery, watery, reddish-brown talc in the
rock. That find was reported by USGS scientist Diane Moore and her
colleagues in the journal Nature on Aug. 16.

Tiny samples of the drill cores will be distributed in December to
international teams of scientists assembled at the USGS in Menlo Park,
Hickman said.

Drilling at the site began in 2004, and the work has been interrupted
many times by storms, floods, drill rig breakdowns and even modest
quakes.

The scientists could tell that their drill had reached the highly
active seismic zone this summer because constant shaking from the tiny
deep quakes accumulated enough strength to warp the steel casing of the
core drill.

The microquakes that shiver the ground inside the fault zone where the
borehole ends are accompanied every once in a while by larger jolts -
still tiny by Loma Prieta or 1906 standards, but registering around
magnitude 2, Ellsworth said.

But the project is particularly important because it is tapping the San
Andreas itself - the 20 million-year-old fault that stretches for 800
miles along the length of California and marks the boundary between two
vast tectonic plates.

These are the immensely thick slabs of the Earth's crust called the
Pacific Plate and the North American Plate that have been sliding past
each other at nearly 2 inches a year for millions of years - fast enough
for Los Angeles to reach San Francisco someday if they last that long.

The SAFOD project is the first venture of a new national project called
EarthScope, a $200 million experiment that will deploy more than 100
smaller borehole seismic stations across the United States, with 800
Global Positioning System units tracking their readings.

Scientists will follow those with another 400 earthquake monitoring
stations at specific locations in the United States. They will be in
operation for two years at a time, and then be moved in relays until the
entire country has been surveyed for seismic activity. EarthScope is
funded by the National Science Foundation.

E-mail David Perlman at [email-masked].

This article appeared on page B - 1 of the
San Francisco Chronicle

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Quake scientists show off rocks
from depths of San Andreas Fault

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