Operation of a digital seismic network on Mount St. Helens volcano and observations of long period seismic events that originate under the volcano
A 9 station digital seismic array was operated on Mount St. Helens volcano in Washington State during 1981. One of the stations was placed inside the crater of the volcano, six were located on the flanks of the volcano within two km of the crater and two were approximately ten km from the crater. Four of the instruments recorded three components of motion and the remaining five recorded only the vertical component. A one day experiment was carried out during which the crater monitoring seismometer was complimented by the addition of two ink recording instruments. During the one day experiment six observers recorded times of rockfall, felt-earthquake occurrences, and changes in steam emissions from the dome in the crater. Using information obtained during the one day experiment seismic events recorded by the digital instruments were classified as earthquakes, rockfalls, helicopter noise and a type of event that is unique to volcanoes which is called long period. Waveforms of these long period events have a duration of up to 30 seconds and a spectrum that is peaked at approximately 2 Hz. The frequency at which the peak in the spectrum occurs is nearly the same at all stations which means that the unique waveform of long period events is due to a source effect, not a path effect. The peak frequency is fairly insensitive to the amplitude of the signal which means that the size of the source region is constant, independent of the signal amplitude. Long period events were not felt and were accompanied by no visible changes inside the crater which lead to the conclusion that they are some sort of seismic disturbance generated inside the Volcano.
- Research Organization:
- Oregon State Univ., Corvallis
- OSTI ID:
- 5376390
- Journal Information:
- Geophys. Res. Lett.; (United States), Vol. 9:9
- Country of Publication:
- United States
- Language:
- English
Similar Records
Observations of volcanic tremor at Mount St. Helens volcano
Operation of a digital seismic network on Mount St. Helens volcano and observations of long-period seismic events that originate under the volcano