Mount St. Helens, Washington
Seismic Studies 1980-1983

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:

1. deep earthquakes and those located away from the volcano, which produce high-frequency signatures and sharp arrivals similar to tectonic earthquakes,
2. shallow earthquakes, located under the dome at depths of less than 3 kilometers, which produce medium-to-low-frequency seismic arrivals,
3. surface events, such as gas and tephra events, rockfalls associated with dome growth, and snow and rock avalanches from the crater walls, which produce complicated signatures with no clear beginning or end, and
4. harmonic tremor, which is a long-lasting, very rhythmic signal whose origin is not well understood but which is often associated with active volcanoes.

Four major types of seismograms, or "seismic signatures," are recognized from seismometers in the vicinity of Mount St. Helens. Seismologists review and classify the seismic records daily.

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.