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. 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 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.