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Mount St. Helens, Washington -
Information Statement, July 1, 1998


U. S. Geological Survey Cascades Volcano Observatory, Vancouver, Washington
University of Washington Geophysics Program, Seattle, Washington
JULY 1, 1998


MOUNT ST. HELENS INFORMATION STATEMENT

This statement updates the Information Statement of June 2, which described an increase of earthquakes in May at Mount St. Helens. The number of well-located earthquakes has increased from an average ~60 well-located events per month last winter, to 165 events in May, to 318 in June. June's earthquakes were very small, only 11 events larger than magnitude 1, the largest M=1.8. Even though the number of earthquakes increased in June, the average magnitude was so small (a person standing in the crater would not feel most of these earthquakes) that total seismic-energy release for the month was about the same as that of May. These earthquakes continue to occur chiefly in two clusters directly beneath the lava dome in the crater. One cluster is in the depth range of 2 to 5 kilometers (1 to 3 miles) below the dome, the other in the depth range of 7 to 9 kilometers (4 to 6 miles). Almost no events locate in the very shallow region 0 to 2 km below the dome. To better portray earthquake data, plots of number of events and seismic energy release are now updated daily (see Internet sites listed below).

In response to the increased level of seismicity, USGS scientists increased monitoring in June. An airborne survey of volcanic gases revealed the presence of magmatic carbon dioxide. Under high pressure deep within Earth's crust, carbon dioxide is dissolved in magma. As magma ascends, the pressure drops and carbon dioxide forms bubbles that separate from the magma. The carbon dioxide, which is far less dense than magma, rises through fractures in the rocks beneath the volcano and escapes to the atmosphere. Winds then mix and disperse the carbon dioxide. But because it is heavier than air, carbon dioxide can concentrate in surface depressions in the dome or crater floor, especially under calm conditions, and pose an asphyxiation hazard. Poorly ventilated cavities such as caves in the mass of snow and ice that is accumulating behind the dome could also be hazardous.

A network of surveying targets was established on the lava dome, crater floor, and lower flanks of the volcano to detect any ground movements that might occur in response to changes beneath the volcano. No significant movement occurred at any of the targets on the north flank of the dome between the first measurements on June 5 and a follow-up survey on June 29. Additional measurements of survey targets will be repeated periodically for the foreseeable future.

The increase in earthquakes and the release of carbon dioxide within the St. Helens system probably reflect replenishment of the magma reservoir, whose top is about 7 kilometers (4 miles) below the crater. This process will eventually prepare the volcano for renewed magmatic eruptions. Scientists don't know how much replenishment has occurred, or how much is necessary for renewed magmatic eruptions. However, such eruptions are unlikely without a significant increase in precursory activity. Owing to the recent unrest, the probability of small steam explosions from the dome, like those that occurred between 1989 and 1991, has increased slightly over the past month. Our concern for these will be heightened greatly if shallow seismicity increases.

We continue to monitor the situation closely and will issue additional information as warranted.

Maps and temporal plots of the Mount St. Helens activity may be found on the WORLD WIDE WEB at URL:

http://vulcan.wr.usgs.gov/Volcanoes/MSH/CurrentActivity
(CVO Menu - Monthly Summaries)
and

http://www.geophys.washington.edu/SEIS/PNSN/HELENS
(University of Washington - Seismic Updates)


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[Mount St. Helens Current Activity Menu]
[Mount St. Helens Monitoring Menu]



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07/01/98, Lyn Topinka