Mount St. Helens Volcano, Cascade Range, southern Washington, U.S.A. (46.20°N, 122.18°W). All times are local (GMT - 8 h). After a week of local seismicity, Mount St. Helens began to erupt steam and ash on March 27, the first eruption in the contiguous United States since the 1914–1917 activity of Mt. Lassen, California. Steam and ash explosions and earthquakes from Mount St. Helens were continuing on April 19, but no fresh magma had reached the surface.
Seismic activity was initiated at 1548 on March 20 by a magnitude 4.1 earthquake centered on or near Mount St. Helens at 5 km depth or less. Events of magnitudes 3.5 on March 22, 3.4 on March 23, and 4.2, 3.4, and 3.4 on March 24 punctuated smaller events that were occurring every few minutes by the 24th, at depths of less than 5 km. Seismologists from the U.S. Geological Survey and the University of Washington installed an array of portable seismographs to record the earthquake swarm. USGS personnel who flew over the volcano on March 24 saw no new snow-free patches or fumaroles, but they observed a number of snow avalanches triggered by earthquakes. Seismicity became more vigorous on March 25, when 11 earthquakes between magnitude 3.4 and 3.8 were recorded, and continued to increase the next day, when 14 shocks, ranging in magnitude from 3.4 to 4.0, accompanied the numerous smaller events that continued to occur.