INFRASONIC WAVE OBSERVATIONS AT FAIRBANKS, AK, OF AUGUSTINE VOLCANO ERUPTIONS DURING JAN, 2006

Infrasonic arrays at UAF have been used from 1965 to the present to monitor volcanic eruptions from around the world. A new eight-microphone, infrasonic array with a digital data recording system was installed in the forest north of the UAF campus in 2001 as a part of the world-wide CTBT/IMS monitoring network. The microphone array, termed I53US, is comprised of eight sensors arranged in an outer pentagon of 5 sensors with an aperture of 1.7 km and an inner triangle of three sensors with an aperture of 0.17 km. The pressure data from each sensor are digitized at 20 samples/second, time stamped with GPS time and recorded in the data base of the GI infrasound group.

The recent Augustine eruptions, from the 11th to the 28th of January 2006, have produced a series of ten infrasonic signals observed at the I53US array. The eruption times for the signals were provided by the Alaska Volcano Observatory at UAF using a Chaparral microphone present on Augustine Island 3 kilometers east of the summit. The bearing and distance of Augustine from I53US are respectively: 207.8 degrees and 675 km. The analysis of the signals is done with a least-squares detector/estimator that calculates, among other things, the horizontal trace-velocity and the azimuth of arrival of the signal. The average values of the trace-velocity and azimuth for all ten Augustine signals are: 0.336 +/- 0.0136 km/sec and 208.7 +/- 1.5 deg. respectively. The celerity for each signal was calculated using the range 675 km and the individual travel times to I53US. The average celerity for all ten eruption signals was 0.287 +/- 0.018 km/sec. Ray tracing studies, using wind speed and temperature profiles supplied by Dr. Doug Drob at the Naval Research Lab, have shown that both stratospheric and thermospheric ray paths are present in the data set.