SEISMIC AND INFRASOUND MONITORING OF ERUPTIONS AT AUGUSTINE VOLCANO, DECEMBER 2005-JANUARY 2006

This summary represents the combined efforts of more than a dozen Alaska Volcano Observatory scientists from the US Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the Alaska Division of Geological and Geophysical Surveys. Seismicity at Augustine Volcano increased in May 2005 following nearly 20 years at background levels. The island was monitored with a network of 7 short-period and one broadband stations. Steaming, rockfalls, and thermal activity increased in fall 2005, and seismicity rates reached 93 events per day. In December several steam explosions occurred. One of these showed displacement signals with 2-sec period on the broadband station. New instruments were added including a telemetered infrasound sensor and six portable non-telemetered broadband stations. Following a 12-hr swarm of several hundred earthquakes on Jan. 10, Augustine erupted explosively on Jan. 11, 13, 14, 17, 27, 28 and continuing. To keep track of the eruptions we measured parameters such as seismic durations, and amplitudes on the infrasound sensor and broadband seismometer. These are various ways to estimate the "size" of the eruptions, to compare with estimates of plume height, etc. The situation was challenging because 4 instruments on the island ceased to function as a result of eruption damage. For example, durations were typically twice as long (6-15 min) on station AUH (on island) as at OPT (3-11 min), 35 km to the north, but AUH ceased operating Jan. 28. The short-period stations saturated, whereas broadband station AUL stayed on scale, until it also ceased to operate on Jan. 28. Preliminary analyses showed that 3 explosive eruptions had long codas on flank stations AUE or AUW, suggesting pyroclastic flows near those stations. These same 3 events had high ratios of infrasound to seismic amplitude, indicating that these eruptions were gas-rich. On January 28 at 15:30 AST the activity changed from short-lived strong explosions at intervals of a few hours to sustained moderate explosions at intervals of a few minutes. Simultaneously the infrasound amplitude dropped by an order of magnitude. Together these observations suggest that the volcano entered a new and less explosive phase of activity. Eruptions are ongoing as of this writing, and seismic and infrasound measurements continue to help identify and quantify eruptions in near-real time.