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Time vs. # of Events & vs. Seismic Strain Energy Release
1980 (1 year)
Graphs show average number of events occurring
per day (blue) and average seismic strain energy release
(square root of seismic energy) per day (red).
The times and types of eruptions are shown at the top of each
figure with the length of the symbol roughly proportional to
the size of the eruption. Magmatic
explosive eruptions are those which had a significant amount of new,
juvenile magma ejected into an ash cloud. Phreatic explosions are
steam or gas explosions involving little or no magmatic material.
Dome growth eruptions are relatively quiet, non-explosive eruptions
of lava onto the lava dome.
This plot is for the explosive eruption period of 1980.
The first phreatice eruption occurred on Mar. 27, 1980 and the
cataclysmic avalanche, lateral blast and plinian ash eruption
occurred on May 18 1980. If compared with the
activity in the 1990s
one must realize that the scales are very different. The number of events
axis is more than a factor of two greater and the energy scale
is a factor of one thousand greater. In the 1980s the sensitivity of the
seismic network was much less than it is today and thus only larger
earthquakes were located. During the pre-cataclysmic eruption period
before May 18 over 600 earthquake greater than magnitude 3 were located
and 12 around magnitude 5. During this same period, both because of
network sensitivity but also because the seismic noise level was
so high that small earthquakes could not be well recorded, there
were only 6 events located with a magnitude less than 2.
After the major eruption on May 18 the seismic noise level was usually
much less and smaller earthquake could be more easily located, thus the
energy plot after May 18 is very small while the number of events continues
to have bursts of activity at moderate levels.
Back to:
UW Dept. of Earth and Space Sciences
PNW
Earthquake Info
University of Washington Dept. of Earth and Space Sciences, Box 351310, Seattle, WA 98195-1310
This is file SEIS/PNSN/HELENS/energynum.1980.html.
If you see any problems e-mail:
seis_web@ess.washington.edu
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