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Contact: Rosemary Sullivant, JPL, (818) 354-0474
Robert Sanders, UC Berkeley, (510) 643-6998
FOR IMMEDIATE RELEASE
August 17, 2000
STUDY FINDS REDUCED QUAKE RISK ON NORTHERN CALIFORNIA FAULT
A geophysicist at the University of California, Berkeley,
has assessed movement along the northern Hayward fault and found
less chance of a major quake originating on that segment than
previously thought. The study uses new techniques for monitoring
earthquake fault activity, including technology developed by
NASA.
With the help of radar interferometry and data from global
positioning satellites (GPS), plus analysis of repeating
microquakes 10 kilometers (6 miles) below the surface, Dr. Roland
Burgmann, assistant professor of geology and geophysics at UC
Berkeley, and his colleagues concluded that the deep portions of
the fault steadily slip at about the same rate as 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."
Burgmann and his colleagues at UC Berkeley, the Lawrence Berkeley
National Laboratory, NASA's Jet Propulsion Laboratory in
Pasadena, Calif., and UC Davis report their findings in the Aug.
18 issue of Science magazine.
The Hayward fault, considered one of the most dangerous
faults in California, stretches more than 95 kilometers (60
miles) and is a branch of the more famous San Andreas fault that
extends much of the length of California. Last year a statewide
team of seismologists estimated a 32 percent chance of a major
quake originating somewhere on the Hayward fault in the next 30
years. A major quake is one of magnitude 6.7 or greater.
The segment of the Hayward fault from San Pablo Bay south to
the border between Berkeley and Oakland is referred to as the
northern Hayward fault. Until recently, it also was ranked high
in terms of the chance of a major quake. The latest assessment,
issued by the U.S. Geological Survey Working Group on California
Earthquake Probabilities last October, lowered this risk, in part
based on preliminary findings supplied by Bürgmann's team.
"We know the Hayward fault creeps at about 5 millimeters (.2
inches) per year at the surface, but we don't know how deep this
creep goes," Burgmann said. "We decided to use all the data that
exists to try to say how deep the creep goes, and whether the
fault is locked at depth."
The techniques Burgmann used to study activity along the
fault have just recently become available. Only within the past
few years has interferometric synthetic aperture radar from
satellites been used to measure ground motion along faults.
Detailed mathematical analysis can determine the surface
displacement that has occurred between successive orbits of the
satellite, even when the orbits are years apart. With data taken
in 1992 and 1997 by a pair of European satellites, ERS-1 and ERS-
2, plus analysis software developed at JPL, Bürgmann was able to
determine the surface creep within a few millimeters along the
northern Hayward fault. Image available at
http://www.jpl.nasa.gov/pictures/haywardfault .
"The global coverage of the European radar satellites allows
the same interferometry technique used in this study to be
applied to active faults in other parts of the world," said paper
co-author Dr. Eric Fielding, a JPL geophysicist. "There are few
places in the world that have the detailed ground information
that was available for this study, but radar satellites image
nearly everywhere. This allows us to study active faults in
regions such as Turkey, Iran and Tibet to learn more about how
faults behave. Because faults may behave differently at different
times, it is important to look at a wide variety of faults to
understand all of the possible types of behavior."
To check the interferometer measurements, Bürgmann used
regional GPS stations which supply regional slip rates but are
not close enough to the northern Hayward fault to give precise
slip rates for that segment.
In addition, seismologists at UC Berkeley and LBNL have just
recently discovered that repeating microquakes - quakes too small
to be felt but indicative of small patches of the fault suddenly
slipping deep underground - can reveal the amount of movement
below the surface. This technique was calibrated at a study site
on the San Andreas fault by Dr. Robert Nadeau, Berkeley
Seismological Laboratory, and Dr. Thomas McEvilly, UC Berkeley.
"They found that some of these microquakes were occurring at
exactly the same spot, and that the microquake clusters could be
used to infer how fast the fault is creeping near these stuck
fault patches deep underground," Burgmann said. "We found
clusters of repeating microquakes as deep as 6 miles under
Berkeley, which is evidence of structural creep far below the
surface."
Putting all this information together, he estimated that the
northern Hayward fault slips underground at a rate of about 5 to
7 millimeters (.2 to .3 inches) per year, essentially the same
rate as at the surface. The similar rates indicate that the fault
is slipping freely without locking, he said. Over long periods,
and counting earthquake slippage, the entire Hayward fault moves
on average about 10 millimeters (.4 inches) per year. The
northern segment moves less because it is pinned by the southern
segment, which is locked. In fact, though the entire fault moves
at about 10 millimeters (.4 inches) per year, surface creep along
the southern segment is only 5 millimeters (.2 inches) per year,
which means strain builds up that can only be released in an
earthquake.
Co-authors of the paper with Burgmann, Nadeau, McEvilly and
Fielding are graduate student David Schmidt, M. D'Alessio and
Mark Murray, all of the Berkeley Seismological Laboratory, and D.
Manaker of UC Davis. The work was supported by the National
Science Foundation, NASA's Solid Earth and Natural Hazards
program, and the U.S. Geological Survey. JPL is a division of the
California Institute of Technology in Pasadena.
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