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Victoria University Antarctic Research Expedition Science and Logistics Reports 1978-79: VUWAE 23

Seismic, Audio, and Magnetic readings at Erebus Volcano, December 1978 (Ray Dibble)

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Seismic, Audio, and Magnetic readings at Erebus Volcano, December 1978 (Ray Dibble)

Volcanic earthquakes and eruptions from Erebus volcano (Plate XXVI) were detected by a geophone at Camp Cave, a low frequency microphone at the main crater rim, and a magnetic induction loop around the crater rim, and recorded on tape between 14 and 25 December 1978. Small earthquakes were as numerous as in December 1974, but larger ones (<1 kJ, including explosion earthquakes) were less frequent (Fig. 4). There was a weak and doubtful peak at 12-14 hours NZST in the occurrence of large earthquakes during the day, but not in the occurrence of small earthquakes (Fig. 5). Of well recorded earthquakes exceeding 10 J, 24% also appeared on the microphone channel, 5% on the magnetic channel, and 10% were accompanied by observed eruptions. The microphone events (which had dominant frequency 1-4 Hz) were of four distinct types: One corresponded with explosions of the Active Vent (Plate XXVIIA) but not with magnetic signals, and two other types (Plate XXVIIB & C, with rare-factional audio onsets) often corresponded with magnetic signals, but the fourth and most common type (Plate XXVIID), for which the air-wave velocity is confirmed, corresponded only with a particular type of earthquake with simple onset. Earthquakes accompanying the other audio types had fore-running vibrations beginning up to 45 s earlier. Lava bulged from the Active Vent during the forerunner before the explosion which occurred while W.F. Giggenbach was in the inner crater, but no signals corresponded with the 2-5 m drop and rise of Lava Lake level in the 15 minutes after the explosion. Occasional brief periods of volcanic tremor were recorded for the first time in December 1978.

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FIG. 4. Number of earthquakes of all types at Erebus volcano per day (in 5 dB ranges) versus seismic energy, for 1974 and 1978. The number of small earthquakes and slope (b) of the curve were similar in 1974 and 1978. The excess number of larger earthquakes (mostly observed explosions) was less in 1978 than 1974.

FIG. 4. Number of earthquakes of all types at Erebus volcano per day (in 5 dB ranges) versus seismic energy, for 1974 and 1978. The number of small earthquakes and slope (b) of the curve were similar in 1974 and 1978. The excess number of larger earthquakes (mostly observed explosions) was less in 1978 than 1974.

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FIG. 5. Average number of earthquakes each 2 hours of the day (NZST) at Erebus between the dates shown. Left: Earthquakes (mostly explosions) with seismic energy 10 kJ show a possible peak at 12-14 hours. Right: Earthquakes (mostly B-type) - 0.1 = 10 kJ show no consistent peak.

FIG. 5. Average number of earthquakes each 2 hours of the day (NZST) at Erebus between the dates shown.
Left: Earthquakes (mostly explosions) with seismic energy 10 kJ show a possible peak at 12-14 hours.
Right: Earthquakes (mostly B-type) - 0.1 = 10 kJ show no consistent peak.