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Immediate report of Victoria University of Wellington Antarctic Expedition 1988-89: VUWAE 33

Proposed Program

Proposed Program

Erebus is a unique volcano in its high latitude location within a tectonic plate, and its persistently active lava lake of phonolitic composition. In an aseismic region, more than 100 volcanic earthquakes have been recorded in a day. The larger (M=c1) accompany the 3.6 +/−2.7 strombolian eruptions observed per day during summer expeditions. The largest observed eruption (17/9/74) had air wave energy 1.6x10E9 joule, and seismic magnitude M=2.4.

Video recordings of the eruptions, begun in December 1986, show that they occur too early to be triggered by earthquakes at depths up to 4 km, as previously hypothesised, and are probably the source of the earthquakes, and at shallow depth. It was also found that similar explosions have similar seismic waveforms, as in earthquake families, and that when these are stacked to improve the quality of the seismic onsets, the apparent velocity is 4.0 km/s, which is much higher than that previously determined by minimising the time residuals during focal determination. The probable consequence is that the previously determined distribution of explosion earthquakes in a column down to 4 km depth is in error. The digital seismic recordings begun this season, allow the data page 8 base to be rapidly increased so as to prove or modify this result within the remaining two years of the study.

The infrasonic recordings of Erebus explosions have always had a controlling influence on the interpretation of the seismic data, and in 1984, the recordings from the Windless Bight Infrasonic Array were complete enough to describe the explosions and calculate the quantity and rate of gas release during the enhanced activity of Erebus. As well, the Windless Bight Array has recorded large eruptions from all over the world.

Disappointingly WBA became no-man's-land from 1985 to 1988, and although it is now agreed to be part of IMEEMS, approval to operate it past the end of 1989 has not been forthcoming. A tragedy I think!