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. -- Mastin, 1994 |