Explosive tephra emissions at Mount St. Helens, 1989-1991; the violent escape of magmatic gas following storms?

From 24 August 1989 until 18 June 1991, Mount St. Helens produced at least 28 shallow, explosion-like seismic events with signatures similar to those produced by gas explosions on the dome during the mid 1980s. At least six were accompanied by violent emission of non-juvenile tephra, ejection of blocks of rock nearly 1 km from the vent, and avalanching of debris off the north side of the dome. Four produced no emission of tephra, and the remainder occurred when the crater could not be observed (although later crater visits found no new tephra deposits). All six confirmed emissions and most (although not all) other seismic events took place hours to days after storms. Over time periods ranging from 2 to 15 days, statistical goodness of fit tests give a probability of 1 X 10 (super -2) to 1 X 10 (super -5) that the precipitation prior to emissions was fortuitously greater than normal. Similar tests for precipitation prior to all 28 events give probabilities of 3 X 10 (super -2) to 1 X 10 (super -3) . The short delay between storms and emissions suggests that the events that follow storms originate at very shallow depth, probably within the dome itself. Although the exact causal mechanism is not known, it is speculated that slope instability or accelerated growth of cooling fractures following storms may have released gas trapped within or at the base of the dome. Of the events that did not follow storms, three were located seismically between 1.4 and 2.1 km depth. Thus the events as a group are not confined to shallow depth and probably record gas transport from a deeper, magmatic source. A deep source is also suggested by an unprecedented, 4-yr-long increase in background seismicity at 2- to 9-km depth that peaked in 1989 and 1990. Calculations indicate that the energy content of gas exsolved from the crystallizing conduit was probably sufficient to power the emissions.