SEATTLE (AP) - Small earthquakes rattled Mount St. Helens at the
rate of one or two a minute Monday, and seismologists were working
to determine the significance of some of the most intense seismic
activity in nearly 20 years.
Carbon dioxide and sulfur gas samples collected above the volcano
- which erupted to devastating effect in 1980 - will help scientists
figure out what is going on beneath the 925-foot-high dome of
hardened lava within the mountain's gaping crater. They want to know
whether the quakes are the result of water seeping into the mountain
or magma moving under its crater.
In either case, scientists will continue watching from the
Cascade Volcano Observatory operated by the U.S. Geological Survey
in Vancouver, Wash., about 50 miles away.
"But if it's magma, we'll be a lot more nervous," said the
observatory's chief scientist Jeff Wynn.
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(AP) The north side of Mount St. Helens is
seen looking into the crater from a helicopter on
Sunday,...
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A
helicopter carried scientists and instruments over the crater Monday
to assess the gases and ground deformation that would indicate
pressure building below the dome.
Measurements of ground movement "will tell us whether there's any
new magma coming into the system," said Seth Moran, a seismologist
at the observatory. That data will not be immediately available.
Swarms of small earthquakes began Thursday and increased in
frequency and magnitude until Sunday, when there were more than 10
events with a magnitude of 2 to 2.8. The quakes are at depths less
than one mile below the lava dome.
By Monday, the pace was unchanged and the magnitude range had
narrowed to between 0.5 and 2-plus, Wynn said.
"Since this morning, the energy releases have been slowly but
steadily ramping up," he said. "We don't know what that means. ...
That kind of energy hasn't been seen since 1986," when the
mountain's last lava-dome-building eruption occurred.
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(AP) Mount St. Helens is seen from a
helicopter on Sunday, Sept. 26, 2004 in Mount St.
Helens, Wash....
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Moran
said there was potential for explosions within the crater that could
throw rocks as far as the rim.
The USGS issued a notice of volcanic unrest on Sunday, citing "an
increased likelihood of a hazardous event." U.S. Forest Service
officials closed hiking trails above the tree line at 4,800 feet on
the 8,364-foot mountain, though the visitor's center and most other
trails at the Mount St. Helens National Monument remained open.
St. Helens' May 18, 1980, eruption killed 57 people, leveled
hundreds of square miles of forests and dumped volcanic ash across
the Northwest.
Sunday's activity was the most in a 24-hour period since the 1986
eruption, said survey geologist Willie Scott. Earthquake swarms in
1998 and 2001 did not result in any surface activity.
If there is an explosion, Scott said concern would be focused
within the crater and on the upper flanks of the volcano. A
five-mile area, primarily north of the volcano, could receive flows
of mud and rock debris.
On Monday, a helicopter lowered a geophysicist onto the lava dome
to replace a failed instrument used to measure tiny movements that
indicate whether the dome is swelling, Wynn said.
While the chopper was near the dome, the pilot was in radio
contact with Bobbie Myers, another geophysicist who during the 1980
blast learned to detect subtle changes in seismic monitors.
"She's known to be able to predict explosive events up to a
couple of minutes ahead of time," Wynn said.
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On the Net:
http://vulcan.wr.usgs.gov/Volcanoes/Cascades/CurrentActivity/%und_
http://www.pnsn.org/HELENS/welcome.html
