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Immediate Report of Victoria University Of Wellington Antarctic Expedition 1987-88: VUWAE 32

Proposed Programme

Proposed Programme

Erebus is a unique volcano. It is at higher latitude than any other active volcano above sea level, and is situated in an aseismic intraplate region. At present it has a negligible rate of lava eruption (c. 1 m3 per day) but it has maintained a liquid lava lake in its crater since before 1972. The phonolitic lava has higher silica content (56%) and theoretical viscosity (103 Pas at 1200°C) than any other persistent lava lake, yet it persists in unusually cold conditions at high altitude (3580 m).

Expeditions to the summit have consistently reported an average of several eruptions per day. These are accompanied by explosion earthquakes and as well, more than 100 volcanic earthquakes per day occur in the range ML-2 to +2. The foci are as deep as 15 km, and all the seismologists of the IMESS team (1980-1985) agreed that the foci of explosion earthquakes, identified by accompanying infrasound, extended down to 4 km below the active vent.

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The installation of TV monitoring of the lava lake in the 1986/87 season showed that surface explosions of the lava lake were the source of the explosion earthquakes, and suggested that the previously determined focal depths were an artifact of adopting model velocities which were lower than the true velocities in the volcano. Accurate determination of the true velocities is difficult because the onsets are emergent, but the seismic wave-forms of different explosions at the same station are often "identical" (cross correlation coefficients exceeding 0.85), and can be stacked to improve the signal to noise ratio. Volcanological observations around the world are just realising the importance of this, and I co-operated in discovering identical explosion earthquakes by computer search methods at Sakurajima volcano, Japan, in February 1988. The instrumentation at Erebus and recording facilities at Scott Base rival the best volcanological observations in the world.

The principal objectives of research for the 1988/89 season are to make the search for "identical" families of volcano-seismic events more efficient by installing a PC computer based digital seismic event recording system at Scott Base, and by using cross correlation, stack and residual software on all adequately recorded events in a routine manner, assemble enough data for reliable statistics and location of the families.