Cascade Range Current Update

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/MSH/CurrentActivity
(CVO Menu - Monthly Summaries)

and

http://www.pnsn.org/HELENS/welcome.html
(University of Washington - Seismic Update)



Past Updates