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Scientists Issue Mount St. Helens Volcanic Advisory

Date:
October 1, 2004
Source:
Pacific Northwest Seismograph Network
Summary:
Seismic activity at Mount St. Helens has accelerated significantly recently, increasing scientists' level of concern that current unrest could culminate in an eruption.

Credit: Photo courtesy of Pacific Northwest Seismograph Network

Mount St. Helens Volcano Advisory (Alert Level Two) : September 29, 2004 10:40 A.M., PDT -- U.S. Geological Survey Cascades Volcano Observatory, Vancouver, Washington / University of Washington Dept. of Earth and Space Sciences, Seattle, Washington

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Over night, seismic activity at Mount St. Helens has accelerated significantly, which increases our level of concern that current unrest could culminate in an eruption. We are increasing the alert level to the second of three levels, which is similar to Color Code Orange of the alert system used by the Alaska Volcano Observatory and analogous to the National Weather Service's hazard watch. Earthquakes are occurring at about four per minute. The largest events are approaching Magnitude 2.5 and they are becoming more frequent. All are still at shallow levels in and below the lava dome that grew in the crater between 1980 and 1986. This suggests that the ongoing intense earthquake activity has weakened the dome, increasing the likelihood of explosions or perhaps the extrusion of lava from the dome. The cause and outcome of the accelerating unrest is uncertain. Explosions from the lava dome could occur suddenly and without further warning. 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 rise several thousand feet above the crater rim and drift downwind. During today, wind forecasts from the National Weather Service, combined with eruption models, show that ash clouds will move in a southeasterly direction and could dust areas tens of miles or more from the volcano with ash. Landslides and debris flows from the crater that are large enough to reach the Pumice Plain are also possible. If the current unrest is being driven by a small slug of magma at shallow depth, extrusion of lava could also occur.

At present there is no evidence that new gas-rich magma has ascended to shallow levels and could generate a large sustained eruption. But we are being especially vigilant to become aware of such evidence should it appear.

We continue to monitor the situation closely and will issue additional updates as warranted, whether activity escalates or returns to background levels.

Mount St. Helens Notice of Volcanic Unrest - Initial Statement: 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.

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


Story Source:

The above story is based on materials provided by Pacific Northwest Seismograph Network. Note: Materials may be edited for content and length.


Cite This Page:

Pacific Northwest Seismograph Network. "Scientists Issue Mount St. Helens Volcanic Advisory." ScienceDaily. ScienceDaily, 1 October 2004. <www.sciencedaily.com/releases/2004/10/041001090809.htm>.
Pacific Northwest Seismograph Network. (2004, October 1). Scientists Issue Mount St. Helens Volcanic Advisory. ScienceDaily. Retrieved October 24, 2014 from www.sciencedaily.com/releases/2004/10/041001090809.htm
Pacific Northwest Seismograph Network. "Scientists Issue Mount St. Helens Volcanic Advisory." ScienceDaily. www.sciencedaily.com/releases/2004/10/041001090809.htm (accessed October 24, 2014).

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