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- Study Indicates Unexpected Earthquake Dangers
Lie
Beneath The Pacific Northwest
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- When it comes to damaging
earthquakes in the Pacific
Northwest, everyone worries about "The
Big One," a great thrust
earthquake caused by the rupture of a
huge offshore fault beneath the ocean,
but there are other kinds of
earthquake that may be just as dangerous.
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- In fact, the most damaging
earthquake in the U.S. Pacific
Northwest in this century was a
different type of shock called an "intraslab"
earthquake.
That magnitude 7.1 event occurred in 1949 beneath Olympia,
Washington
and caused over $100 million in damage.
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- A new study by Arizona State
University geologist Simon
M. Peacock and Kelin Wang, a geoscientist at
the Geological Survey of Canada's
Pacific Geoscience Centre, provides
firm support for a recent theory that
explains how intraslab
earthquakes work and confirms the hazard posed to
certain geographic
areas. The study appears in the October 29 issue of
Science.
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- Both great thrust
earthquakes and intraslab earthquakes
occur in "subduction
zones," where oceanic crust dives beneath
the edge of a continent.
Great thrust earthquakes occur at shallow depths
of 0 to 50 kilometers
along the sloping boundary between the descending
plate and the
continental margin. In contrast, intraslab earthquakes occur
within the
descending oceanic crust at depths of 50 to 300 kilometers beneath
the
surface and are caused by different processes.
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- "The risks posed by
intraslab earthquakes have not
been fully incorporated into seismic
hazard analysis," said Peacock.
"In many cases, such as the
Pacific Northwest, these less easily understood
earthquakes occur
closer to major population centers than the larger offshore
earthquakes. The historic record bears this out."
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- In 1996, Stephen H. Kirby and
colleagues at the U.S.
Geological Survey proposed a mechanism for
intraslab earthquakes which
is supported by the current study.
Basically, Kirby's theory proposes intraslab
earthquakes occur because
the intense heat and pressure in subduction zones
metamorphose (or
change) the descending oceanic crust into denser rocks.
These
mineralogical changes cause the subducting oceanic crust to liberate
water stored in the original minerals and to reactivate pre-existing
faults.
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- "In the absence of water, these faults would not
move because o
f the weight of the overlying rocks," said Peacock.
"The
liberated water essentially lubricates the fault - pumping up
the water
pressure causes the fault to slip."
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- Peacock and Wang tested Kirby,s
theory by comparing two
subduction zones in Japan, carefully
calculating the temperature of the
subducting oceanic crust, and
comparing their seismic and volcanic records.
Confirming Kirby's
theory, the results showed that the oceanic crust subducting
beneath
southwest Japan - a "warm" subduction zone should liberate
water at shallower depths and thus trigger only shallow intraslab
earthquakes
and less volcanic activity as compared to activity in the
cold subduction
zone beneath northeast Japan.
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- "Things happen much deeper
beneath northeast Japan
because the subducting crust is much colder and
water is released at much
greater depth" said Peacock. "It's
the water being released at
depth that generates these intraslab
earthquakes and subduction-zone volcanoes.
Intraslab earthquakes occur
at relatively shallow depths beneath southwest
Japan because the
subducting oceanic crust is warmer."
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- Like southwest Japan, the
Pacific Northwest (northern
California, Oregon, Washington, and
southern British Columbia) and southern
Mexico are underlain by warm
subduction zones.
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- "The Vancouver-Seattle-Tacoma area may be more at
risk
from an intraslab earthquake than from a larger earthquake along the
offshore trench," Peacock said. "This risk has only recently
been recognized. We're starting to realize that we have to worry about
a magnitude 7-7.5 intraslab earthquake located 50 km beneath Seattle or
Vancouver, as well as a magnitude 8 or 9 out on the coast."
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- "Large intraslab
earthquakes occur quite frequently.
Just last month, on September 30, a
magnitude 7.4 intraslab earthquake
shook Oaxaca, Mexico killing at
least 27 people."
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- Peacock, whose main field of expertise is metamorphic
petrology, points out that this study is an example of how specialized
scientific research can sometimes yield information with real significance
to everyday life. "For years I have studied metamorphism, a largely
academic subject. Now we've learned that there is a solid connection
between
metamorphic processes and earthquakes that have killed tens of
thousands
of people," he said.
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- Editor's Note: The original news release can be found
at http://clasdean.la.asu.edu/n
ews/subquake.htm
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- Note: This story has been adapted from a news release
issued by Arizona State University, College Of Liberal Arts & Sciences
for journalists and other members of the public. If you wish to quote from
any part of this story, please credit Arizona State University, College
Of Liberal Arts & Sciences as the original source. You may also wish
to include the following link in any citation: http://www.
sciencedaily.com/releases/1999/10/991029071538.htm
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