Seismic signals preceding the explosive eruption of Mount St. Helens, Washington, on 18 May 1980

Abstract The intense seismic activity which preceded the 18 May 1980 eruption of Mount St. Helens, Washington, released 2 to 3 × 10 18 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.