Mount St. Helens, Washington -
Information Statement, Aug 14, 1998

U.S. Geological Survey Cascades Volcano Observatory, Vancouver, Washington
University of Washington Dept. of Earth and Space Sciences, Seattle, Washington
AUGUST 14, 1998

MOUNT ST. HELENS INFORMATION STATEMENT

The rate of earthquake activity, which accelerated markedly from May through mid July, has returned to a level similar to that of last winter. The number of well located earthquakes in July was 445, compared to 318 in June, but most of July^Òs earthquakes occurred during the first three weeks of the month. The average rate for the past two weeks has been only about 4 well-located earthquakes per day. There have been several temporary increases in earthquake activity since the last dome-building eruption in October 1986. This recent episode was the most intense.

Airborne surveys of volcanic gases reveal that levels of magmatic carbon dioxide have decreased since June. However carbon dioxide is still present and measurable. The carbon dioxide is probably being released from magma that entered the magma reservoir during the past few months. The top of the magma chamber is about 7 kilometers (4 miles) below the crater. Because carbon dioxide is heavier than air, it can concentrate in surface depressions on 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 behind the dome, could also be hazardous.

We continue to monitor the situation closely and will issue additional Information Statements only if conditions change significantly. Daily updates of earthquake data and other information can be found on the WORLD WIDE WEB at URL:

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