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September 23, 2004:
- Submitted at 6:00 pm PDT, September 23, 2004.
- Volcanoes in the Cascade Range are all at normal levels of background seismicity.
Other volcanoes include,Mount Baker, Glacier Peak, Mount Rainier, Mount Adams, and Mount St. Helens in Washington State; Mount Hood, Mount Jefferson, Three Sisters, Newberry, and Crater Lake, in Oregon; and Medicine Lake, Mount Shasta, and Lassen Peak in northern California.
- Since about 2 am PDT on the morning of 09/23 an earthquake swam has been occurring at Mount St. Helens. Through 5 P.M. PDT about 200 small (less than magnitude 1) earthquakes have been recorded at MSH. 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 cause of such shallow swarms is uncertain, but may reflect increased ground water levels with the onset of autumn rain. Any eruption is unlikely to occur without significant additional precursory activity.
- We continue to monitor the situation closely and will issue additional Updates as warranted.
September 24, 2004:
- Mount St. Helens Information Statement,
September 24, 2004, 10:00 AM, PDT
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"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."
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September 25, 2004:
- 9:30 AM PDT
- 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 of September 24, 2004.
- We continue to monitor the situation closely and will issue additional Updates as warranted.
September 26, 2004:
- Mount St. Helens Notice of Volcanic Unrest,
September 26, 2004, 3:00 P.M., PDT
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"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."
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September 27, 2004:
- 7:00 PM PDT
- Seismic activity has very slowly increased throughout the day. All earthquake locations are still shallow and beneath the dome. The largest earthquake recorded in the past 24 hours has been about a M1.5 and no M2 earthquakes have been detected in the past 24 hours. Cascades Volcano Observatory (CVO) crews installed GPS equipment today to monitor any ground movement on the dome, crater floor and the slopes of the volcano. Preliminary results from a gas flight late this afternoon did not detect any magmatic gas around the lava dome.
September 28, 2004:
- 6:30 PM PDT
- Throughout the day seismic activity has remained at a fairly constant, but high, rate of about 2 small (less than magnitude 2) earthquakes per minute. All earthquake locations are still shallow and in or below the lava dome. A USGS field crew continued their deployment of GPS equipment today in order to monitor any ground movement on the lava dome, crater floor, or lower slopes of the volcano. Tomorrow’s field work includes continued GPS deployments and another gas flight. USGS and PNSN personnel held a teleconference to discuss scientific issues and future needs. USGS and PNSN personnel also participated in a teleconference with Washington State and County emergency-management officials to discuss hazard and response issues.
September 29, 2004:
September 30, 2004:
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