Elevated seismic activity observed at Mount St. Helens (MSH) during 1987-1989 continued through the first half of 1990 before gradually decaying to normal levels by the end of 1991. Average monthly rates decreased from 90 per month during the first half of 1990 to 15 per month in 1994. This decrease was most pronounced at depths below 3 km (1980-1996 Seismicity Plot). Before August 1990, more than half of all MSH earthquakes had depths > 3 km; after August 1990, the reverse was true. Earthquakes below 6.5 km defined an aseismic zone in map view, similar in location to one observed in post-May 18, 1980, deep earthquakes (Barker and Malone, 1991; Moran, 1994).

As in 1987-1989, earthquakes recorded during 1990-1994 were almost exclusively of the high-frequency type. Exceptions to this included signals associated with 5 confirmed steam-and-ash explosions and other explosion-like seismic events that were apparently not associated with actual explosions. The five explosions occurred during the winter months of 1990 and 1991 (see Mastin (1994) for a detailed description of the explosions). Products from these explosions were almost exclusively non-magmatic (Pallister et. al., 1992), thus we do not consider them to be eruptions. The largest blasts ejected 0.5-1.0 m diameter blocks of dome dacite up to 1 km from the dome, and associated ash plumes reached as high as 30,000 feet. The explosions had no apparent short-term seismic or other precursors, and were not predictable even in hindsight.

The occurrence of deep (> 3 km) earthquakes, the increase and subsequent decrease of seismic activity, and the occurrence of steam-and-ash explosions all suggest that a change occurred in the magmatic system during 1987-1991. Moran (1994) examined focal mechanisms from earthquakes occurring at depths of > 4 km, and found that stresses around the aseismic zone could be modeled as occurring in response to a pressure increase coming from within the aseismic zone. He interprets this to be the result of the sealing of the shallow conduit system after the last eruption in 1986, perhaps by cooling unerupted magma. Before the sealing, any pressure build-up in the magmatic system could be accomodated by magma transport to the surface; after the sealing, pressure accumulation could be accomodated only by brittle and plastic deformation of the country rock surrounding the magmatic system. If this interpretation is correct, then it implies that eruptions at MSH are unlikely in the near future, and any future eruption will first be preceded by considerable seismicity associated with the fracturing of the sealed conduit.

References

Barker, S. E., and S. D. Malone, Magmatic system geometry at Mount St. Helens modeled from the stress field associated with posteruptive earthquakes, Journal of Geophysical Research, v. 96, p. 11,883-11,894, 1991.

Ludwin, R. S., A. I. Qamar, S. D. Malone, C. Jonientz-Trisler, R. S. Crosson, R. Benson, and S. C. Moran, Earthquake hypocenters in Washington and northern Oregon, 1987-1989, and operation of the Washington Regional Seismograph Network, Wash. Div. Geol. and Earth Res. Information Circular 89, 40 pp., 1994.

Mastin, L. G., Shallow explosion-like seismicity and steam-and-ash emissions at Mount St. Helens, August 1989-June 1991: The explosive escape of magmatic gas following storms, Geological Society of America Bulletin, v. 106, p. 175-185, 1994.

Moran, S. C., Seismicity at Mount St. Helens, 1987-1992: Evidence for repressurization of an active magmatic system, Journal of Geophysical Research, v. 99, p. 4341-4354, 1994.

Pallister, J. S., R. P. Hoblitt, D. R. Crandell, and D. R. Mullineaux, Mount St. Helens a decade after the 1980 eruptions: Magmatic models, chemical cycles, and a revised hazards assessment, Bulletin of Volcanology, v. 54, p. 126-146, 1992.