The Alaska Volcano Observatory (AVO) reported on 21 January 2000 that investigations of recent seismic data had revealed evidence for small explosions at Shishaldin. Later detailed study of the seismic records showed that the activity may have begun in as early as late September. The numbers of explosions varied from several to over 200/day, but no steam or ash plumes were observed by airborne or ground observers. Also, no thermal anomaly was observed in satellite imagery, indicating that lava had not reached the surface. It was thought that the explosions were phreatic, caused by the flashing of water to steam; these events may represent a local hazard within a few hundred meters of the vent but do not pose a hazard to aircraft. Small explosions continued at a similar rate through 28 January.

Small low-frequency seismic events, present at Shishaldin since June 1999, gradually increased in amplitude after 28 January, with a noticeable increase during 2-3 February. Seismic data continued to show the presence of small phreatic explosions. Reports of steam plumes were received during the week ending on 2 February, with heights reaching as high as ~900 m above the summit. However, no thermal anomaly was observed in satellite imagery and no seismic tremor was identified; both were seen prior to the last eruptive episode in April and May 1999 (BGVN 24:03, 24:04, 24:08). Due to the increased activity, AVO raised the Level of Concern Color Code to Yellow on 3 February, indicating that the volcano is restless and an eruption may occur.

No appreciable number of seismic events were detected after 4 February; that was also the last day that small explosions were observed. Small low-frequency seismic events continued through 11 February, but at a slower rate and slightly lower amplitude. By 18 February seismic activity had declined significantly with no thermal anomalies or observations of unusual activity, so the hazard status was changed back to Green, indicating normal seismicity and surface activity.

Small low-frequency seismic events and very low-level tremor was recorded through 3 March, although at or below the levels observed in the months prior to the 19 April 1999 eruption. Low-level seismicity continued through the end of March. Vigorous steaming was reported in the second half of March, but no thermal anomaly observed in satellite imagery.

Geological Summary. The symmetrical glacier-covered Shishaldin in the Aleutian Islands is the westernmost of three large stratovolcanoes in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." Constructed atop an older glacially dissected edifice, it is largely basaltic in composition. Remnants of an older edifice are exposed on the W and NE sides at 1,500-1,800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is covered by massive aa lava flows. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. A steam plume often rises from the summit crater.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.