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:
Volcanoes in the Cascade Range are all at normal levels of background seismicity, but Mount St. Helens is experiencing an increased number of earthquakes.
Seismicity at Mount St. Helens has declined significantly since yesterday. However, small, shallow earthquakes are still occurring at above typical background rates. We will continue to monitor the situation through the weekend. This message supersedes the Information Statement below.
U.S. Geological Survey, Vancouver, Washington
University of Washington, Pacific Northwest Seismograph Network, Seattle, Washington
Mount St. Helens Information Statement September 24, 2004
10:00 AM, PDT
Since about 2 am PDT on the morning of 09/23 an earthquake swarm has been occurring at Mount St. Helens. Through 5 P.M. PDT on 09/23 about 200 small (less than magnitude 1) earthquakes have been located at MSH, 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 MSH, but a swarm with this many earthquakes has not been recorded for several years, most recently on November 3-4, 2001. The cause of such shallow swarms is uncertain, but may reflect increased ground water levels with the onset of autumn rain. The probability of small steam explosions that hurl rocks a few hundred meters (yards) may also be increased during periods with increased shallow earthquakes.
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 WORLD WIDE WEB at URL:
