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 − 7 h). After the devastating eruption of May 18, smaller but significant explosions occurred on May 25, June 12, July 22, and August 7. A series of similar explosions October 16–18 deposited ash as far away as north central Oregon and destroyed the lava dome extruded after the August 7 eruption.

Minor activity—early October. Activity from late August through early October was limited to slow outward movement of the northern crater rampart, minor seismicity, and weak vapor emission. A gradual increase in activity began October 7 with barely detectable harmonic tremor that started a few minutes after midnight and lasted less than an hour. During the day on the 7th, a few plumes, containing no appreciable ash, rose to about 3-km altitude before drifting to the northeast. No harmonic tremor was recorded October 8, but shallow earthquakes of magnitudes 1.6 and 1.8 occurred at 1535 and 1537. Some October 8 plumes contained smaller amounts of ash. Several minor gas clouds, containing a little fine ash, were emitted October 10 between 0915 and 1100, accompanied by intermittent low-level harmonic tremor. The initial column on the 10th reached 4.5–6-km altitude and drifted to the north northeast. Two more gas plumes were emitted during the afternoon, again accompanied by minor seismicity. Gas emissions decreased in frequency and intensity October 11–13. Seismic records during poor weather on the 14th indicated that occasional minor gas emission continued. On October 15, some cracks that had not been present the preceding week were visible on the crater floor. Three small volcanic earthquakes were recorded, but no harmonic tremor was detected.