"Graphs show average number of events occurring per day (blue) and average seismic strain energy release (square root of seismic energy) per day (red). The times and types of eruptions are shown at the top of each figure with the length of the symbol roughly proportional to the size of the eruption. Magmatic explosive eruptions are those which had a significant amount of new, juvenile magma ejected into an ash cloud. Phreatic explosions are steam or gas explosions involving little or no magmatic material. Dome growth eruptions are relatively quiet, non-explosive eruptions of lava onto the lava dome.
"This plot is for the explosive eruption period of 1980. The first phreatic eruption occurred on Mar. 27, 1980 and the cataclysmic avalanche, lateral blast and plinian ash eruption occurred on May 18 1980... In the 1980s the sensitivity of the seismic network was much less than it is today and thus only larger earthquakes were located. During the pre-cataclysmic eruption period before May 18 over 600 earthquake greater than magnitude 3 were located and 12 around magnitude 5. During this same period, both because of network sensitivity but also because the seismic noise level was so high that small earthquakes could not be well recorded, there were only 6 events located with a magnitude less than 2.
"After the major eruption on May 18 the seismic noise level was usually much less and smaller earthquake could be more easily located, thus the energy plot after May 18 is very small while the number of events continues to have bursts of activity at moderate levels. "
Half a lifetime ago (for me).