Abstract

Sixty eruptions of the Erebus lava lake were recorded on videotape from a TV camera on the crater rim, and on a 9 station seismic net and 2 infrasonic microphones, between 16 December 1986 and 7 January 1987. The best 29 recordings have been analysed. Times can be read from all recordings to 0.04s, but onsets are seldom that sharp. Plots of seismic arrival time versus distance from the explosion site show that the seismic intercept time is 0.91±46s later than the time of the visible explosion. This implies that the explosions were the source of the seismic waves, rather than that earthquakes triggered the explosions as previously published. The apparent seismic velocity was 2.61±58km/s. The large standard deviation could be due to emergent onsets, or to time variation in vesiculation and seismic velocity in the lava. Explosions usually occur in the incandescent centre of the lava lake, and are preceded by an updoming for c.0.2s. Some occur around the edges of the lake aureole of c.40 m diameter. All explosions which threw out bombs were accompanied by earthquakes, but the few ash jet eruptions were aseismic. The study was terminated by withdrawal of NSF from the cooperative NSF/NIPR/VUK programme at Scott Base on 7 January 1987, but video surveillance continues.

An infrared temperature survey of 81 points was made of the crater walls and lava lake on 10 and 11 December 1986, using a Minolta IR-0510 infrared thermometer. The instrument provided by NIPR has a range of −50 to +1000°C. Rock temperatures ranged from −30 to +70°C, and the lava lake temperature ranged from 446 to 798°C, assuming an emissivity of 0.7. Stations were marked on polaroid photos to allow repeat observations.