Bulletin of the Seismological Society of America
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Bulletin of the Seismological Society of America; December 1983; v. 73; no. 6A; p. 1797-1813
© 1983 Seismological Society of America
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Seismic signals preceding the explosive eruption of Mount St. Helens, Washington, on 18 May 1980

ANTHONY QAMAR*, WILLIAM ST. LAWRENCE, JOHNNIE N. MOORE and GEORGE KENDRICK

GEOLOGY DEPARTMENT UNIVERSITY OF MONTANA, MISSOULA, MONTANA 59812
POLAR ALPINE SERVICES, BERKELEY, CALIFORNIA 94709
UTAH INTERNATIONAL, MISSOULA, MONTANA 59801

Abstract

The intense seismic activity which preceded the 18 May 1980 eruption of Mount St. Helens, Washington, released 2 to 3 x 1018 ergs/day in earthquakes that did not correlate temporally with phreatic eruptions which occurred during the same period. Although the b value and amplitude ratios (long-period/short-period) of the earthquakes vary with time, there are no definitive precursors to the 18 May earthquake and eruption. A Mogi type II frequency-magnitude relation, with critical magnitude Mc = 4.6, constrains the characteristic dimension of the highly stressed region under Mount St. Helens to approximately 3 km, preceding the eruption. A major increase in seismic energy release and a decrease in b value around 1 April 1980 may indicate the first major influx of magma into the upper portion of the volcano.

Seismic waves from low-frequency volcanic earthquake have large periods at all epicentral distances. Recordings of volcanic earthquakes from 2 to 4 April 1980 at sites 4 to 9 km from Mount St. Helens show two predominant periods of 0.55 and 1.0 sec. We speculate that seismic signals from the low-frequency volcanic earthquakes have a tectonic origin, but may be modified by pressure oscillations in nearby magma.

Footnotes

* Present address: Geophysics Program, University of Washington, Seattle, Washington 98195.






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