Geophysical events

Abstract

Mount St. Helens Volcano, Cascade Range, southern Washington, U.S.A. (46.20°N, 122.18° W). All times are local (GMT − 7 h). A major eruption destroyed the summit of Mount St. Helens, projected ash into the stratosphere, devastated the north and northwest flanks, and killed dozens of people last May 18. The initial explosion was heard more than 350 km away. Substantial ashfalls occurred hundreds of kilometers downwind, closing roads, schools, and businesses, and threatening crops in the northwestern United States.Mount St. Helens had begun to erupt on March 27, after a week of local seismicity. During the first 10 days, small to moderate steam and ash explosions ejected ash to as much as 3 km above the summit. A little ash fell as far away as Spokane, Washington, 500 km to the northwest. Small debris flows and cold ash avalanches moved down the flanks of the volcano. By April 12, the two initially separate explosion vents had coalesced into a single crater that was at least 500 m long, 350 m wide, and 300 m deep. Both explosions and ash content decreased in size through mid‐April, then episodic explosions were replaced by continuous steaming late in the month. Explosions, similar to those of early to mid‐April, resumed on May 6 and continued until at least May 14. Several tens of earthquakes per day of magnitude 3 or greater continued to be recorded through May 17. Total seismic energy release remained relatively constant through late April, then declined slightly.