Mount St. Helens Volcano Updates:
March-April 1982
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| Type of Notice and When Issued |
Statements |
March 5, 1982 - 0900 PST;
EXTENDED OUTLOOK ADVISORY
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Seismicity at Mount St. Helens increased around February 21 and has remained at
a level somewhat above background since that time. Approximately 100
earthquakes that occurred during this period have been located. These
earthquakes fall into two groups: (1) a "deep" group of very small earthquakes
with centers at 6- to 11-kilometer depths, and (2) a shallow group of somewhat
larger (magnitude 1 or less) earthquakes located at 3-4 kilometers us to the
surface. There is a pronounced lack of seismicity at a depth of 4-6 kilometers.
Although poor weather during the past week has hampered observations and
monitoring on the mountain itself, measurements made last week show only slow
ground deformation in the immediate vicinity of the dome and no significant
increase in gas emissions.
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March 12, 1982 - 0800 PST;
EXTENDED OUTLOOK ADVISORY
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Seismicity beneath Mount St. Helens continues at elevated levels, but
individual earthquakes are of low magnitude. Earthquakes have been occurring in
1-2-day-long episodes separated by 1-2-day intervals of decreased activity. The
earthquakes are occurring between the surface and a depth of about 6 miles.
Rates of ground deformation in the crater area have increased during the last
two weeks, and they are similar to patterns observed before previous
dome-building eruptions.
Based on rates of deformation, an eruption is likely within the next 3 weeks.
Deformation is confined to the crater areas, suggesting that renewed dome growth
will occur. The current seismic patterns differ from any observed before
1980-81 eruptions, however, and raise the possibility of more hazardous
variations in eruptive behavior. If there were to be any pyroclastic flows,
from either and explosive eruption or collapse of the steep north face of the
dome, the possibility of rapid snowmelt would be a concern.
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March 15, 1982 - 1900 PST;
ADVISORY
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Accelerating rates of ground deformation in the crater of Mount St. Helens
suggest that na eruption, most likely of the dome-building type, will probably
begin within 1 to 5 days. Deep earthquakes have almost ceased, and shallow
earthquakes continue at a moderate rate. A further increase in shallow
seismicity is likely before the eruption starts.
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March 16, 1982 - 0800 PST:
"Daily Update"
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Geologists and hydrologists form the Geological Survey performed on-site
monitoring in and around Mount St. helens on March 15, 1982. Measurements
showed accelerated rates of ground deformation on thrusts and cracks in the
crater. University of Washington-USGS reports seismicity remains at a slight
increase over the weekend's activity. Shallow earthquakes continue at a
moderate rate (see Mount St. Helens Advisory, 7:00 p.m., March 15, 1982).
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March 17, 1982 - 0800 PST;
"Daily Update"
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Good weather permitted on-site monitoring in and around Mount St. Helens by
geologists and hydrologists of the U.S. Geological Survey. Measurements showed
continued acceleration of ground deformation localized around the lava dome.
Overflights of the volcano the last two days have shown an increase in the
number of incandescent areas on the dome. University of Washington-USGS reports
seismicity remains at a moderate rate.
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March 18, 1982 - 0800 PST:
"Daily Update"
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Continued good weather enabled monitoring to be carried out in the crater of
Mount St. Helens for the third day in a row. Measurements performed by
geologists from the U.S. Geological Survey continued to document accelerating
ground deformation rates. Airborne gas monitoring around the volcano during the
past several days has shown that a moderate increase in the rate of SO2
emissions has occurred. The University of Washington-USGS seismic lab reports a
slight increase in the level of seismicity over that of yesterday, but it still
can be described as a moderate level.
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March 19, 1982 - 0900 PST;
ERUPTION ALERT
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Seismicity at Mount St. Helens has increased significantly during the past
day. This indicates that an eruption will begin soon, probably within the next
24 hours. The character of both the seismicity and deformation in the crater
area indicated the the most likely type of activity is dome growth.
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March 19, 1982 - 1933 PST;
ERUPTION ALERT
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Verbal statement that eruption had begun at 1927. (Significant increase in
seismicity and pilot and radar reports of an ash plume indicated start of
eruption.)
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March 19, 1982 - 2025 PST;
ERUPTION UPDATE
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The eruption has subsided for the time being, and an ash plume is blowing to
the SSE. and S. with a lesser amount to the SSW. The size of the eruption and
the amount of the ash appear to be like those of the summer 1980.
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March 21, 1982 - 0900 PST;
ERUPTION UPDATE
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There is a new lobe being added to the SE. side of the lava dome in the
crater of Mount St. Helens. The new extrusion began during the night.
Seismicity and tilt are now following patterns observed during other recent
periods of dome growth.
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March 24, 1982 - 1730 PST;
ERUPTION UPDATE
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Growth of the new dome lobe has slowed significantly over the past 2 days,
but rates of deformation on the north side of the dome have increased over the
same period. Until additional measurements are made, it would be premature to
declare this eruption over. Seismometers are recording decreasing numbers of
avalanches as growth of the new lobe slows.
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April 5, 1982 - 0100 PST;
ERUPTION UPDATE
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Seismicity still continues at moderate to high levels. Fluctuations in
seismicity correspond to pulses of gas and ash, the largest of which began at
1237 a.m. and rose to a maximum of about 32,000 feet. There does not appear to
be much ash in the plume, but minor ashfall was reported earlier tonight at
Packwood. No increases have been reported in river levels.
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April 5, 1982 - 1800 PST;
ERUPTION UPDATE
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Seismicity is continuing at a moderate to high level. The status of the
volcano remains unchanged since the previous advisory (1 a.m., April 5, 1982).
Some additional dome growth and (or) small-scale explosive activity is likely;
larger explosive activity is less likely but cannot be ruled out.
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April 6, 1982 - 0915 PST;
ERUPTION UPDATE
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Another new lobe is being added to the dome. This reduced the immediate
likelihood of larger explosive activity, but small, intermittent gas and ash
pulses may continue to occur.
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April 12, 1982 - 1515 PST;
ERUPTION UPDATE
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Seismicity, deformation and gas emissions at Mount St. Helens have returned
to low levels, indicating that the eruption that began on March 19, 1982 is over.
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