- SEATTLE (AP) -- The land beneath California could spell trouble for Washington.
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- A scientist said the forces that generate
Puget Sound-area earthquakes may originate within the San Andreas Fault,
hundreds of kilometres to the south.
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- That, in turn, could mean much of western
Washington state is as prone to big-time quakes as San Francisco Bay or
the Los Angeles basin.
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- Craig Weaver, a University of Washington-based
scientist who heads the Seattle U.S. Geological Survey office, presented
his research Monday to the Specialty Conference on Geotechnical Earthquake
Engineering and Soil Dynamics.
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- Weaver noted long-held assumptions about
quake dangers in the increasingly urbanized Puget Sound region have been
shaken repeatedly in recent years.
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- He said his new model is based largely
on studies following the magnitude-7.2 quake that caused the death of more
than 6,300 people and billions of dollars wworth of damage in Kobe, Japan,
on Jan 17, 1995.
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- To verify it, he and other scientists
plan a detailed series of studies to detect earth-movement and evidence
of past quakes from northeastern California to northern Puget Sound.
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- I.M. Idriss, a geotechnical engineer
and civil-engineering professor at the University of California in Davis,
and Carlton Ho, an engineering professor at the University of Massachusetts
in Amherst, said the theory is plausible but far from proven.
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- If borne out, Ho said, "that has
some fairly significant earthquake implications for Seattle."
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- The Puget Sound region is susceptible
to three types of quakes. The most intense, up to magnitude nine or higher,
occur every 300-500 years in the Cascadia Subduction Zone roughly 130 kilometres
off the coast from Cape Mendocino, Calif., to Vancouver Island, where the
Juan de Fuca plate of the Earth's crust is drawn down, or subducted, beneath
the North American continental plate.
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- At least eight of these quakes have occurred
in the last 4,000 years. The last, dated by tree rings, soil deposits and
historical accounts of seismic sea waves, occurred Jan. 27, 1700.
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- The most well-understood quakes occur
deeper, where the downward edge of the Juan de Fuca plate bends and cracks
under intense pressure and heat about 50 kilometres beneath the Interstate
5 corridor.
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- That source has generated the two most
intense quakes in the region in the last 50 years, one 7.1 near Olympia
in 1949 and the other 6.5 south of Renton in 1965.
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- The least-understood and most recently
discovered faults in the area are found within the top 20 kilometres of
the Earth's crust.
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- A quake estimated at magnitude seven
altered much of the Puget Sound landscape 1,100 years ago. Jolts of about
three to 4.9 beneath Bremerton were felt in June 1997. Both were within
the Seattle Fault Zone, a disjointed east-west band about five kilometres
wide with three principal sets of fault lines fractured by two unnamed
north-south faults beneath the sound.
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- Weaver's "four-arc slice" model
begins with the San Andreas Fault that runs beneath California from near
the Mexican border east of the Salton Sea about 1,200 kilometres to Cape
Mendocino, Calif., and out to sea.
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- Movement along that fault, which separates
the Pacific and North American plates, pushes the Sierra Nevada mountains
northward by almost a centimetre each year.
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- "The question is what happens to
that centimetre," Weaver said.
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- He said clues may lie in a cluster of
quakes in the Klamath Mountains of southern Oregon, the virtual absence
of quakes throughout the Oregon Coast Range and a sudden increase north
of the Columbia River.
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- Northward crustal movement at a rate
about equal to that of the Sierra Nevadas has been detected at the Columbia
but in southwestern British Columbia the bedrock has been in place for
about 100 million years.
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- The northward movement of the Sierra
Nevadas, Weaver suggested, pushes and slightly turns the Klamath Mountains,
which in turn nudge the Coast Range northward.
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- At the end of the line, the Puget Sound
basin is squished against the older rock formations to the north and buckles,
"folding the crust over very long wavelengths" that produce shallow
east-west fractures like the Seattle Fault Zone, Weaver said.
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- If the theory pans out, said Weaver,
Ho and Idriss, Seattle may be raised from a zone-three earthquake rating
to zone four, the highest risk level and the same as San Francisco and
Los Angeles. San Diego, once rated in zone two, is now in zone four.
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- While those cities have had considerably
more frequent, deadly and damaging quakes than Seattle in the last century,
that's too short a period in which to draw seismic conclusions, Weaver
said.
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