- SEATTLE - Geologists used
to answer with an emphatic "No" when asked if mega-earthquakes
like the one that hit Southeast Asia last week can trigger temblors on
the other side of the globe.
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- Today, some experts are not so sure.
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- Evidence is mounting that large earthquakes can rattle
geologic formations more than 1,000 miles away - and perhaps even set off
volcanic eruptions days, months or years later.
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- There's also an intriguing hint that major earthquakes
might occur in clusters: Nearly a third of the biggest quakes of the past
century struck during a 20-year span between 1950 and 1970.
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- After three decades of relative quiet, two massive quakes
came in quick succession this month: the magnitude 9 in Sumatra and a little-noticed
magnitude 8.1 off the coast of New Zealand three days earlier.
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- Do monster earthquakes beget more monster earthquakes?
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- Could the two recent events signal the start of a new
destructive cycle?
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- And is it possible the Sumatran quake jolted the geologic
plates off Washington's coast enough to hasten the day when they let loose,
unleashing what geologists predict will be a comparable catastrophe?
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- No one knows the answers to the first two questions,
which are hot topics of research and scholarly debate.
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- But scientists are fairly certain people in the Northwest
don't have any more to worry about now than they did two weeks ago.
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- "I would venture to say there's a minimal effect,
if any at all, on our region from the Sumatra earthquake," said Herb
Dragert, a research scientist for the Geological Survey of Canada.
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- Dragert and his American counterparts operate a network
of GPS sensors throughout the region. The instruments can detect even slight
movements of land masses, reflecting changes in the amount of stress at
the Cascadia subduction zone - the 600-mile-long offshore region where
the ocean floor is diving under the continental plate.
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- The measurements show no troublesome blips as a result
of the Sumatran quake, Dragert said.
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- "If we suddenly had a very large earthquake in Alaska,
which is much closer, and I saw displacement in my GPS instruments, then
I would begin to worry."
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- However, there could be ample cause for concern around
Indonesia. When the undersea plates there snapped apart, triggering the
earthquake, the dislocation almost certainly increased stress and strain
on adjacent geologic faults and plate boundaries. Geologists call the phenomenon
"contagion" because it raises the odds of subsequent earthquakes
like an influx of germs raises the risk of infection.
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- "It's very expected and quite dangerous," said
Brian Atwater, a U.S. Geological Survey researcher stationed at the University
of Washington. "It gives a certain sense of urgency to efforts to
get a warning system going around the Indian Ocean."
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- Scientists have long known about the contagion effect,
which can extend for 100 miles or so from the epicenter of a major quake.
It's the phenomenon that's responsible for the aftershocks that follow
many major quakes.
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- But most experts were stunned in 1992 when a magnitude-7.2
quake struck the Mojave Desert in Southern California and was almost immediately
followed by more than a dozen quakes as far away as Wyoming.
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- A similar thing happened in 2002, when a magnitude-7.9
earthquake in Denali, Alaska, triggered earthquakes and rearranged the
plumbing of geyser fields in Yellowstone National Park - 2,000 miles away.
The same event spawned a couple of small earthquakes under Mount Rainier
and set up sloshing waves that swamped houseboats on Lake Union in Seattle
and Lake Pontchartrain in Louisiana.
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- "As people around the world look more carefully,
they're seeing more examples of this kind of (long-distance) effect,"
said David Hill, a USGS geophysicist stationed at Menlo Park, Calif. "At
this point there's really no doubt that it happens."
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- Generally, the triggered earthquakes are smaller than
the original, though there's no reason to believe that larger earthquakes
couldn't be kicked off this way as well, said Hiroo Kanamori, a geophysicist
at the California Institute of Technology.
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- The effect seems to be caused by seismic waves that radiate
out from the epicenter of an earthquake, along the surface of the ground.
Imperceptible to people, these waves cover a lot of distance.
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- "The Earth ends up ringing like a bell," Dragert
said. "You have a surface wave that travels around the globe for hours
after the event, and if it passes through an area that is already critically
stressed, it can, indeed, trigger an earthquake."
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- That is, a fault or plate boundary must already be on
the verge of slipping or breaking for the surface waves to push it over
the edge.
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- There's still no detailed explanation for the way that
happens, though, Hill said.
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- "In a way, it's frustrating to be doing research
on this," he said, "because we can't do it in the lab and repeat
the experiment. We've got to wait for the Earth to do it, and then have
good recording networks in the field."
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- There's even less concrete data to show that distant
earthquakes can trigger volcanic eruptions, though the circumstantial evidence
is growing, Hill said. One analysis found a high number of volcanic eruptions
within a day or two of large earthquakes. Several volcanoes around the
world, including Pinatubo in the Philippines, have erupted within weeks
or months of major earthquakes.
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- Indonesia has many volcanoes, none of which has yet erupted
in he aftermath of last week's earthquake - but scientists will be watching
closely.
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- Last week The Washington Post reported lava was spewing
from a volcano on an island in the Andaman and Nicobar Islands, an Indian
archipelago off the coasts of Myanmar (formerly called Burma) and Indonesia.
Previously, the crater had emitted only gas.
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- Theories linking distant earthquakes to eruptions and
other earthquakes remain controversial. It's almost impossible to prove
what triggered an earthquake or eruption, Kanamori pointed out.
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- Researchers look mainly at the timing of events, then
do statistical analyses to show that they're probably linked, not just
random coincidences.
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- Hill does collect some hard data from strain meters buried
in 600-foot boreholes in California's Long Valley Caldera near Mono Lake.
The sensitive devices detect changes in the pressures pushing and pulling
on the rock, and have clearly shown effects from distant earthquakes, he
said.
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- The statistical jury remains out on the question of whether
the apparent cluster of major earthquakes in the middle of the century
is significant or simply a phantom.
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- It certainly looks compelling, Atwater said. Most of
the events are clustered around the Pacific Rim, from Alaska to Russia's
Kamchatka Peninsula to Chile.
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- However, the Cascadia subduction zone off the Northwest
coast was not triggered during that period, he pointed out. The last earthquake
there was a magnitude 9 in 1700.
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- Garry Rogers, a seismologist at the Geological Survey
of Canada in British Columbia, says major earthquakes are far too rare
and the historical record far too short to be able to draw any conclusion
about clusters or large-scale connections.
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- "In any random process, you will get clusters,"
he said.
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- Hill believes that more data will eventually solve the
mystery - and will probably reveal patterns and links no one understands
today.
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- "My own hunch is that there are lots of instances
of clusters that are, in fact, related physically," he said. "We
just don't know yet what the details might be."
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- © 2005 KRT Wire and wire service sources. All Rights
Reserved.
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