[Geysers] Mount St. Helens Notice of Volcanic Unrest (Frangos)

[V]

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Perhaps some of you can see if you notice any 
changes in YNP!  

Vicky 


Mount St. Helens Notice of Volcanic Unrest 
September 26, 2004 3:00 P.M., PDT 

http://www.pnsn.org/NEWS/PRESS_RELEASES/MSH_09_2004.html

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

Webicorder and spectrograph archives for this sequence - from the PNSN

Mount St. Helens Swarm Activity 
Special information statement of Sep 24, 2004 10am PDT 
U.S. Geological Survey Cascades Volcano Observatory, Vancouver,
Washington 
University of Washington Dept. of Earth and Space Sciences, Seattle,
Washington

Since about 2 am PDT on the morning of 09/23 an earthquake swarm has
been occurring at MSH. Through 5 P.M. PDT about 200 small (less than
magnitude 1) earthquakes have been located at Mount St. Helens and many
smaller events have also been recorded through this morning. The
earthquakes are occurring at shallow depths (less than 1 kilometer, or
1/2 mile) mostly under the lava dome that formed between 1980 and 1986.
Such earthquakes are common at St. Helens, but a swarm with this many
earthquakes has not been recorded for several years, most recently on
November 3-4, 2001. The probability of small landslides and debris
flows in the crater may be enhanced during these periods. Such events
could affect areas several kilometers (miles) north of the crater on
the Pumice Plain. The probability of small steam explosions that hurl
rocks a few hundred meters (yards) may also be increased during periods
with increased shallow earthquakes. The cause of such shallow swarms is
uncertain, but may reflect increased ground water levels with the onset
of autumn rain.

Prior to the 2001 swarm, the last period of increased earthquake
activity at Mount St. Helens occurred in the spring and summer of 1998
when hundreds of earthquakes per month, most smaller than M=2, were
detected at depths as great as 9 kilometers (6 miles). An intrusion of
magma, or molten rock, deep under the volcano and resulting increased
gas pressure in the conduit that leads to the lava dome likely caused
this increase in earthquakes. The current swarm is different in that
the events are typically much smaller and shallower. We see no evidence
that an intrusion of magma similar to that of 1998 is underway. We
continue to monitor the situation closely and will issue additional
Updates as warranted.

Daily updates of earthquake data and other information can be found on
the WEB at URL: http://vulcan.wr.usgs.gov/Volcanoes/MSH/CurrentActivity

Webicorder and spectrograph archives for this sequence - from the PNSN


		
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