As magma moves into the reservoir during inflation, it must make room for itself by rupturing or crowding aside the solidified lava that surrounds the reservoir. Such underground ruptures produced seismic waves that travel through the volcano and are recorded by a network of seismometers placed on the volcano's surface. Ground motions sensed by the seismometer are converted into electronic signals, which are transmitted by radio and are recorded on seismographs located at the volcano observatory. The seismic data are analyzed to determine the time, location, depth, and magnitude of the earthquakes. Mapping the earthquake activity allows HVO scientists to track the subsurface movement of magma.
Seismologists review and classify the seismic records daily.
Signals from several of these stations are also radioed to and recorded at the Cascades Volcano Observatory. 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. 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.