Infrasonic signals from explosive volcanic eruptions have been observed for decades at Fairbanks, Alaska and Windless Bight, Antarctica as well as at many other infrasonic arrays around the globe. If the eruptions are very large, such as that of Mt. Saint Helens in 1980, the infrasound that is generated by the explosion can propagate around the earth several times arriving at the infrasonic observatories from both the direct and the antipodal great circle paths. Numerous volcanoes in western Alaska, such as Trident (843 km) in1974, Augustine (688 km) in 1986, Redoubt (550 km) in 1989, have had explosive eruptions that produced strong infrasonic signals that were observed at Fairbanks with the infrasonic array. In Figure 1, the volcanic signals are shown for January 23, 1976 from successive eruptions of Mt. Augustine 688 km southwest of Fairbanks that occurred over a six hour period. These analogue chart records of the Augustine signals are from the original infrasonic array installed at Fairbanks in 1965.

Infrasound has been recorded in Fairbanks from eruptions of several volcanoes located at great distances such as El Chichon in Mexico (6617 km), Galunggung in Java (8297 km),Awu in Indonesia (9471 km),  and Manam in New  Guinea (9358 km). The identification of a particular volcano as the source of an explosion infrasonic wavetrain relies on the correct estimate of the signal’s back-azimuth great-circle-path, the existence of an active volcano along this path, knowledge from local observations of the eruption time and a reasonable value of the celerity of the received signal in the range from 0.275 to 0.305 km/sec. The size of an explosive eruption determines the longest period in the infrasound wavetrain spectrum while the distance to the source determines the duration of the wavetrain. The amplitude of the infrasonic signal increases with the cube root of the explosion energy and decreases with the range of the source.

Manam Volcano Infrasound

Following the installation of the infrasonic arrays in Fairbanks and in Antarctica, volcanic infrasound was observed successively at I55US and then at I53US from the same eruption of Manam volcano in Papua New Guinea on January 27, 2005. In Figure 2 for I53US and Figure 3 for I55US the volcanic infrasound wavetrains are shown from the Manam volcano eruption. At the bottom of each figure there is a phase-aligned overlay of all 8 microphone traces for the coherent signal. A strong signal of 0.3 Pa  p-t-p amplitude was found at I53US in the passband from 0.03 to 0.10 Hz on the 27th from 23:12 to 23:51 UT.  The trace velocity of the I53US signal was 0.330 + / - 0.007 km/sec the back azimuth was 252.5 + / - 1.3 degrees, the mean of the cross-correlation maxima was 0.860. The great-circle azimuth and distance from I53US to Manam Volcano are 247.5 degrees East and 9358 km respectively. The westward deflection of 5 degrees between the true azimuth of 247.5, and the measured signal azimuth of 252.5 deg. is consistent with the variance expected over a 9000 km propagation path. During the time period on the 27th when the Manam Volcano signal was received I53US there was also an infrasonic background of mountain associated waves (MAW) observed from an azimuth of 287 degrees. The detector out-put for trace-velocity, azimuth and MCCM value is shown in Figure 4 below for a five hour period at I53US from Jan 27 21:00 to Jan 28 02:00, 2005. The MAW signals were sporadic throughout the day. In the azimuth plot one can first see that the MAW signals from 287.5 deg persist until the volcano signal from an azimuth of 251.7 deg. becomes the dominant signal at 23:13 UT.  The MCCM value for the volcanic signal was 0.904 while that of the MAW signals was lower at 0.793. The correlation detector MCCM locked on to the stronger volcanic signal at 23:13 UT as shown in Figure 4.

There was a very clear signal from Manam Volcano at I55US in Antarctica on January 27, 2005 at 22:07 UT. The trace-velocity of this signal from Manam volcano, as shown in Figure 3 below, was 0.311 + / - 0.01 km/sec. The back azimuth of the signal was 335.4 + / - 2.5 deg. The I55H3 channel data was not available. The mean correlation for the signal was MCCM = 0.964. The great circle distance from I55US to the Manam Volcano (lat. 4 deg. S; long. 145 deg. East) is 8303 Km at a back azimuth of 336.6 deg. The eruption time at Manam Island was 14:00 UT on Jan 27, 2005. The celerity for the I55US Manam volcanic signal is 0.284 km/sec. This is approximately the same as the celerity of 0.282 km/sec for the I53US signal. Both of the great circle signals paths from Manam Island toward I53US in Alaska and toward I55US in Antarctica were toward the East from the volcano. The trace velocity, azimuth and MCCM values for the I55US Manam Volcano signal are shown in Figure 5 below. The Manam signal at I55US was detected in the MCCM analysis for a period of one hour and 56 minutes. The mean signal azimuth was 333.7 degrees. This gives a westward deviation from the true great circle path of 2.9 degrees.

Mt. Erebus Volcano Infrasound

At Windless Bight on the Ross Ice Shelf there is an active volcano, Mt. Erebus, on Ross Island just 25 kilometers north of the I55US infrasonic array. There have been hundreds of small infrasound-producing events in the lava lake in the crater of Erebus over the past few years. These small eruptions of Erebus produce signals in the passband from 0.1 to 10 Hz that usually last less than one minute with amplitudes up to 1.5 Pascal. During the period from April 2002 to March 2003 there have been 182 Erebus impulsive signals at I55US with a mean azimuth of 336.5 deg +/- 0.39 deg and a mean trace-velocity of 320.5 m/sec +/- 6.32 m/sec. These impulsive signals from Erebus are coherent across the 1.7 km aperture of the 8 sensor I55US array. An example is shown in Figure 6 below of a typical signal at I55US from Erebus small eruptions in the crater lava lake. Following the colossal eruption in 1980 of the Mt. Saint Helens in Washington State, long period and long duration infrasound signals were received at the array in Windless Bight from the volcano by both the direct and the antipodal great-circle propagation paths. A third signal was then received in the following days after the St. Helens’s infrasonic signal circled the earth a second time.

Recent Eruptions of Augustine Volcano in Alaska

There have been a series of eruption events of Augustine volcano in Cook Inlet Alaska on January 11, 13, 14, 17 of 2006 that produced infrasonic signals detected at I53US in Fairbanks. The time of the eruption in each case was determined from an acoustic signal recorded by the Alaska Volcanic Observatory of UAF operating a Chaparral microphone that is installed on the base of the volcano about 3 km from the volcano crater. A very large amplitude acoustic signal was recorded on Augustine Island by the microphone at 16:58:57 UT on Jan. 17, 2006. Augustine volcano is located at a distance of 675 km from Fairbanks at a bearing of 207.8 degrees. The infrasonic signal received at I53US on Jan. 17, 2006 at 17:39 UT from the seventh Augustine eruption is shown in Figure 7 below. The peak-to-peak amplitude of the signal is about 0.3 Pa. The background microbarom signals and the long period wind-noise oscillations were excluded from the signal by bandpass filtering from [0.03 to 0.10 Hz]. The values of the trace-velocity, azimuth of arrival, coherence (MCCM) and the sigma tau (which is a measure of wavetrain planarity) are respectively: 0.350 km/sec, 209.1 degrees, 0.831 coherence, and 0.15 seconds. The travel time for this signal from Augustine to I53US was 40 minutes and 3 seconds. For the range of 675 km of Augustine from I53US this gives celerity for the eruption signal of 0.281 km/sec. The presence of an infrasonic microphone that is located on the base of Augustine volcano to detect the initial blast wave from the eruption gives excellent ground truth for the eruption times to within 10 seconds accuracy.