Proposed programme

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 per day have been recorded. The larger (ML cl) accompany the 3.6 ±2.7 strombolian eruptions per day. The largest observed eruption, of airwave energy 1.6×10E9 J, and seismic magnitude ML 2.4, occurred on 17 September 1984 during enhanced activity.

The logistic support available from Scott and McMurdo bases at the foot of the volcano have made it one of the easier volcanoes to study. Winter temperatures down to c-60±C are the main problem. Geophysical studies during summer began in 1974, and year round studies began in 1980, when Dr. P.R Kyle set up the International Mount Erebus Seismic Study (IMESS) with Dr. J. Kienle of University of Alaska. Prof. K. Kaminuma of NIPR, Tokyo, and Dr. R.R, Dibble of vuw. Seismic, infrasonic, and magnetic induction data were telemetered and recorded continuously at Scott Base. The principal objective was location of earthquake foci, but statistics of occurrence, tidal effects, spectra, energy, and time and amplitude relationships between different types of data for both earthquakes and eruptions were also studied. Two types of eruption occurred: a high frequency type from the exploding vent, and a low frequency type, possibly from the lava lake. In the latter type, the infrasonic trace often showed a gradual pressure onset, one second before the seismic onset at very near stations, caused perhaps by the lava lake ballooning before it erupted. IMESS terminated in 1986. A list of publications is attached.

It was found that eruption earthquakes had an apparent range of depths to 4 km, but that the infrasonic signals were more consistent with a surface origin. Although a possible explanation was time error in picking emergent seismic onsets, and inaccurate page 9 modelling of velocity structure, a more attractive option was the triggering of eruptions in the vents by earthquakes at depth, as suggested by recent studies at Sakurajima Volcano, Japan.

The principal objective of IMEEMS in 1986/87 was to find if the earthquake origin times were earlier than or equal to the times of visible eruption, as required by the triggering hypothesis, using video recordings of eruptions with accurate time display.