Geophysical events


  • Anonymous


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 the explosions of October 16–18 and the brief period of lava dome growth that followed, activity at Mount St. Helens was limited to vapor emission and occasional seismic activity through early December.

Most early November seismic events were caused by rock slides from the crater walls. No significant local earthquakes or harmonic tremor were recorded until mid-November, when brief episodes of harmonic tremor began, barely within the detection limits of sensitive seismographs on and near the volcano. Intermittent low-level tremor continued through early December. Stronger tremor started on November 25 at 2054, gradually fading into background noise about 35 min later. Observers in a U.S. Forest Service aircraft reported a slightly brighter glow in the dome area after this event. A second burst of stronger tremor began November 27 at 2034, continuing for about an hour, and several more such episodes, lasting only a few minutes each, were detected through November 30.