Seismic signals from 5 seismometers on the flanks of Mount St. Helens, one in
the crater, and 10 others within 40 kilometers of the volcano are radioed to the
Geophysics Laboratory at the University of Washington. Signals from several of
these stations also are radioed to and recorded at the Cascades Volcano
Observatory. This detailed seismic network enables seismologists to distinguish
between different types of volcanic earthquakes and surface events. Although
the seismic precursors to the May 18, 1980, eruption did not specify the time of
its onset, seismologists have learned to recognize certain characteristic
patterns of seismic activity that precede and accompany the subsequent eruptions.
By plotting the cumulative seismic strain energy release of various types of
seismic disturbances versus time, eruptions have been predicted from a few hours
to several days in advance.
Earthquake data from the Mount St. Helens seismic network are stored on computer files at the University of Washington in Seattle. Seismologists review and classify the seismograms, or "seismic signatures," from several local stations each day; during periods of high earthquake activity, seismologists monitor the records 24 hours a day. The following major types of seismograms have been recognized at Mount St. Helens:
The rate of activity of the various categories of seismic events is used to assist in predicting volcanic activity at Mount St. Helens. An increasing number of shallow volcanic earthquakes were observed several days to 2 weeks before each dome-building eruption from 1980 through 1982. As the number of As the number of earthquakes increase, total seismic energy release is calculated and plotted against time. The observation of a sudden upward turn in this smoothly accelerating curve a few hours before the eruption begins is the basis for relatively short-term predictions. Once the eruption is underway, shallow volcanic earthquakes cease, and surface events from rockfalls dominate the records. Increased seismic activity also preceded the post-May 18 explosive eruptions of 1980. Each of the seismic precursors were of a slightly different character, but two categories were recognized: shallow volcanic earthquake precursors and harmonic tremor precursors. harmonic tremor is a nearly continuous train of vibrations lasting from a few minutes to several hours. An increase in shallow earthquakes preceded the July and October 1980 eruptions when a lava dome was present in the crater. Harmonic tremor preceded the May 25, June 12, and August 7, 1980, eruptions when the vent was open and no dome existed. Having learned from the seismic buildup preceding the May 25 explosive eruption, seismologists were able to issue warnings of the subsequent explosive events by at least 2 hours.
Although a characteristic pattern of seismic activity has accompanied most
eruptions, there have been departures from the pattern. The explosive eruptions
of 1980 were followed by swarms of deep earthquakes (deeper than 5 kilometers).
In contrast, none of the subsequent dome-building eruptions were followed by
deep earthquakes that cound be related to the eruptions. The March 19, 1982,
dome-building eruption began with an explosion and was preceded rather than
followed by both deep and shallow earthquakes. The continuous dome-growth
eruption that began in early February 1983 was preceded by an unusually small
increase in shallow earthquake activity. Earthquakes and surface events
occurred daily reflecting the continuous dome-growth activity in 1983.
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