[RC] OT--Mount St. Helens Notice of Volcanic Unrest - DarkHorseGoddess
Mount St. Helens Notice of Volcanic Unrest >September 26, 2004 3:00
P.M., PDT > >U.S. Geological Survey Cascades Volcano Observatory,
Vancouver, Washington >University of Washington Dept. of Earth and Space
Sciences, Seattle, >Washington >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
