New eruption parameters and spectral relationships between seismic and infrasonic signals from Erebus volcano, Antarctica

Magnitude and energy of the September 1984 eruptions have been calculated from seismic, infrasonic and gravity signal data. For the large initial eruption at 13d 05h 07m UT, the local magnitude obtained from the Scott Base WWSS seismograms was 2.0. Some eruptions were reported as felt at McMurdo Base, 37km from the crater. If the eruption earthquakes were felt, the magnitude would be at least 3,but probably the air wave was felt. The largest magnitude from Rayleigh waves recorded on the tidal gravity meter at South Pole Station, 1390km from Erebus was 2.4 for the eruption at 13d 15h 47m. Assumptions as to the instrumental response are involved here. For infrasonic signals, the largest eruptions grossly overloaded the University of Alaska short period recorders at Windless Bight, 26.6km from the crater, but the air wave energy of medium eruptions which were just clipped was 2×10E7 J. The eruption at 17d 10h 11m was clipped for 80s, and had air wave energy of at least 1.6×10E9. The histogram of time intervals between the 110 eruptions during the first 7 days of initial high activity had a peak at 35-40 min. The distribution is asymmetrical, with mean at 87±72 min. Repose period analysis favours a poisson random distribution, with a loading time of 20 min. Spectrograms of telemetered signals typical of normal activity in previous years are presented, showing that the durations of the earthquake seismograms are clearly correlated with those of the infrasonograms from accompanying eruptions. Such agreement in duration would not be expected if the eruptions were triggered at a distance from the earthquakes by a seismic signal