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NSF PR 00-55 - August 17, 2000
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State-of-the-Art Measuring Techniques Show California
Fault Slips Freely
Less Chance of Major Quake
Pushing the frontiers of today's techniques for monitoring
earthquake fault activity, a geophysicist funded by
the National Science Foundation (NSF) and affiliated
with the University of California at Berkeley, assessed
movement along California's northern Hayward fault
and found less chance of a major quake originating
on that segment than previously thought.
With the help of radar interferometry and data from
global positioning satellites (GPS), and analysis
of repeating microquakes six miles below the surface,
Roland Bürgmann and his colleagues concluded that
the deep portions of the fault steadily slip at about
the same rate as does the surface. This means the
rocks deep below the surface aren't locked and building
up strain that could be released in a catastrophic
quake.
"Our research shows no evidence of locking at any depth,
which means the threat from one of our worst hazards,
right in our backyard, is much reduced," said Bürgmann.
"However, other hazards - from the southern Hayward
fault, the San Andreas fault and other nearby faults
- leave the need to build reinforced homes and the
need to be prepared just as high as before." Bürgmann
and his colleagues at UC Berkeley, the Lawrence Berkeley
National Laboratory, the Jet Propulsion Laboratory
in Pasadena, Calif., and UC Davis report their findings
in the Aug. 18 issue of Science magazine.
"We're very excited about the applications of this
research, "says Robin Reichlin, program director in
NSF's division of earth sciences, which funded the
research. "Space-based techniques are poised to revolutionize
our view of Earth's crust, and the dynamic processes
that lead to volcanic eruptions and earthquakes."
The Hayward fault, considered one of the most dangerous
faults in California, stretches more than 60 miles
from San Pablo Bay in the north to below Fremont in
the south, and is a branch of the more famous San
Andreas fault that extends much of the length of California.
Putting information together from several sources,
the scientists estimated that the northern Hayward
fault slips underground at a rate of about 5 to 7
millimeters per year, essentially the same rate as
at the surface. The similar rates indicate that the
fault is slipping freely without locking. Over long
periods, and counting the slippage that occurs during
and after earthquakes, the entire Hayward fault moves
on average about 10 millimeters per year.
Most faults outside California do not slip freely,
but lock at depth. Bürgmann said what may allow the
northern Hayward fault - and some other state faults
- to move freely is a greenish rock that underlies
much of central and northern California and could
serve as a lubricant: serpentinite, often called serpentine.
Serpentinite, the official state rock, is soft and
fractures easily.
The work was supported by the National Science Foundation,
the Solid Earth and Natural Hazards program of the
National Aeronautics and Space Administration, and
the U.S. Geological Survey's NEHRP program.
For illustration of the radar image around the Hayward
Fault see: http://www.berkeley.edu/news/media/download/2000/08/pub5.html
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