THE 2006 ERUPTION OF AUGUSTINE VOLCANO: OVERVIEW

This overview represents the combined efforts of dozens of Alaska Volcano Observatory (AVO) volcanologists from the US Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the Alaska Division of Geological and Geophysical Surveys. Augustine Volcano, in southwestern Cook Inlet, Alaska awoke from 20 years of dormancy in mid-summer, 2005. The rate of small, shallow volcano-tectonic earthquakes increased five-fold, and Earthscope/PBO continuous GPS instruments recorded the onset of steady inflation. On December 12, a sulfur-rich low-level gas plume was emitted, the odor of which was strongly noted in local communities as far as 70 km downwind. A few days later a phreatic explosion dusted the flanks of the volcano with ash, and a subsequent FLIR-equipped overflight documented new areas of steaming and extensive areas with elevated temperatures at the summit. Unrest for the following month was characterized by infrequent ~10 minute bursts of multiple local earthquakes. Seismicity changed significantly on January 11, with the onset of a 12-hour swarm of repetitive earthquakes, terminated by two strong explosions. After two days of relative quiet there were 6 more strong explosions and intervening earthquakes over a period of about a day. On January 27, after another two weeks of quiet, there were four more strong explosions, a half-day hiatus, and then a weeklong period of intense seismic activity, where several-minute periods of shaking were separated by several-minute periods of relative quiescence. At the time of this writing (Feb. 7) seismic quiet has returned, although the eruption may not be over. We interpret the sequence of events to reflect 1) magma movement into the edifice starting around May, 2005; 2) degassing accompanied by phreatic explosions starting as early as mid-September and continuing through mid-December; 3) large vent-clearing explosions in mid-January, followed by 4) dome extrusion in latest January and early February. Observations have been by real-time telemetered seismic and GPS data (about half the stations have been destroyed so far), satellite data on both plumes and thermal features, and fixed-wing and helicopter overflights on good-visibility days between the winter storms.