She's gonna blow! I'll be in Seattle in a couple of weeks, maybe I'll
get to see the fallout. I'll also be MTB'ing on Orcas Island if anyone
has some tips I'd appreciate it.
U.S. Geological Survey, Vancouver, Washington
University of Washington, Pacific Northwest Seismograph Network,
Seattle, Washington
Mount St. Helens Notice of Volcanic Unrest
September 26, 2004 3:00 P.M., PDT
Seismic activity at Mount St. Helens has changed significantly during
the past 24 hours and the changes make us believe that there is an
increased likelihood of a hazardous event, which warrants release of
this Notice of Volcanic Unrest. The swarm of very small, shallow
earthquakes (less than Magnitude 1) that began on the morning of 23
September peaked about mid-day on 24 September and slowly declined
through yesterday morning. However, since then the character of the
swarm has changed to include more than ten larger earthquakes (Magnitude
2-2.8), the most in a 24-hr period since the eruption of October 1986.
In addition, some of the earthquakes are of a type that suggests the
involvement of pressurized fluids (water and steam) or perhaps magma.
The events are still occurring at shallow depths (less than one mile)
below the lava dome that formed in the crater between 1980 and 1986.
The cause and outcome of the earthquake swarm are uncertain at this
time. Several causes are possible, but most point toward an increased
probability of explosions from the lava dome if the level of current
unrest continues or escalates. During such explosions the dome and
crater floor are at greatest risk from ballistic projectiles, but the
rim of the crater and flanks of the volcano could also be at risk.
Explosions would also be expected to produce ash clouds that drift
downwind at altitudes up to several thousand feet above the crater rim.
Landslides and debris flows from the crater that are large enough to
reach the Pumice Plain are also possible. Such events occurred at Mount
St. Helens between 1989 and 1991.
We continue to monitor the situation closely and will issue additional
updates as warranted, whether activity escalates or returns to
background levels.
Daily updates of earthquake data and other information can be found on
the WORLD WIDE WEB at URL:
http://vulcan.wr.usgs.gov/Volcanoes/Cascades/CurrentActivity/current_updates.html
and
http://www.pnsn.org/HELENS/welcome.html
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