Infrasonic Recordings of Strombolian Eruptions of Erebus, Antarctica, March – December 1984, Covering the Jump in Activity on 13 September 1984
Continuous infrasonic recordings made at Windless Bight, 26 km from the summit crater of Erebus, between March and December 1984, show an abrupt change both in the average interval between explosions and in the maximum infrasonic amplitudes on 13 September. Before the change the average interval was 2.8 days, and the maximum infrasonic amplitude was 6.3 microbar peak, and afterwards the average interval was about 1.5 h, and the maximum amplitude on 17 September was 32 microbar peak. From 13 September to 8 December, when activity ceased for 13 days, about 1000 explosions occurred.
The b-value graphs of infrasonic magnitude in 5-day groups show flat peaks instead of the negative slope normal for earthquakes, and the peak occurs at higher magnitude during higher activity. Grouping all data gives an apparent b-value near 0.6, similar to that previously found for the larger earthquakes at Erebus.
The most frequent interval between the first 1000 explosions was 15–20 min (4.6%), but Poisson distributions of average interval 1.85 h for intervals less than 70 min, and 1.09 h for intervals exceeding 70 min, fit the data well. This indicates a recovery time of 70 min. Before 13 September, 1984, there were gaps in activity from 4 to 25 May and 4 July to 3 August. The penultimate explosion was on 8 September.
Most infrasonic signatures had a duration of 6–20 s, and consisted of 1 to 1.5 cycles with the wave period increasing with time from initial values of 4–10 s, independent of amplitude. A longer period may indicate a longer eruption. The infrasonic energy of the largest explosion (1984 September 17d 16h 57m UT) was 1–2.7 × 109 J and the total to 8 December exceeded 1.3 × 1011 J. Assuming infrasonic energy is that of volcanic gas expansion, the largest explosion released 1–5 × 104 m3 and the total to 8 December was at least 2 × 106 m3 of gas after adiabatic expansion to atmospheric pressure.
The study demonstrates that independent monitoring of explosive eruptions by infrasonic arrays can determine explosion energy and minimum estimates of erupted gas volumes at nearby volcanoes. The other unique advantage is a superior far-field detection ability which offers the greatest potential for systematically reporting volcanic explosion energy worldwide. Such arrays monitored automatically as events take place, could be used to warn commercial airliners against flying into ash clouds from otherwise unmonitored volcanoes.
KeywordsAverage Interval Apparent Velocity Lava Lake Small Array Wind Noise
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