Observations of volcanic tremor at Mt. St. Helens volcano
Digital recordings of ground motion during tremor episodes accompanying eruptions at Mt. St. Helens Volcano in the state of Washington on August 7 and October 16-18, 1980 are studied. The spectra of the vertical component waveforms contain at least two dominant peaks at 1.0 and 1.3 Hz for all events recorded during both eruptions that were studied. Spectra of horizontal ground motion show peaks at .9 and 1.1 Hz. the relative amplitudes of the two peaks changes between tremor episodes and during single tremor episodes and show no consistent relation to amplitude of ground motion. Spectra of long period earthquakes are very similar to those of tremor events suggesting that tremor is composed of many long period earthquakes that occur over a period of time. The path effect of tremor waveforms is small since there are no peaks in the spectra of waveforms recorded during tectonic earthquakes occurring in the vicinity of Mt. St. Helens. Amplitudes of ground motion varies between .11 ..mu..m and 4.7 ..mu..m. Seismic moment rates during the two eruptions are calculated using the model of Aki et al. (1977) and found to vary between 6 x 10/sup 18/ and 1 x 10/sup 20/ dynes cm sec/sup -1/ which are larger than values found by Aki et al. (1977) who studied amplitudes of shallow tremor events recorded during the October, 1963 eruption of Kilauea volcano in Hawaii. Study of tremor amplitudes recorded at Corvallis, Oregon leads to the conclusion that tremor accompanying the cataclysmic May 18, 1980 eruption was at least one order of magnitude larger in amplitude than tremor during August and October.
- Research Organization:
- Oregon State Univ., Corvallis (USA). School of Oceanography
- DOE Contract Number:
- AT06-82ER12017
- OSTI ID:
- 6576007
- Report Number(s):
- DOE/ER/12017-4; ON: DE83004902
- Country of Publication:
- United States
- Language:
- English
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