Explosion earthquakes of Mount Erebus, Antarctica

The characteristics of explosions of Mount Erebus, Antarctica are analyzed using the data of the seismic network operated by the International Mount Erebus Seismic Study (IMESS). About 70 earthquakes have been located by the network between 16 February and 29 June 1982. An aseismic zone is recognizable in the southeastern part of Mount Erebus on the vertical projection of hypocenter distributions. The aseismic zone seems to suggest the existence of a magma reservoir which is estimated to be located beneath the summit. Three types of explosions are recognizable by the seismic and the infrasound signals. The types are named α-, β- and γ-types and characterized as follows : 1) the α-type explosion has high infrasound frequency and high frequency spectra of explosion earthquakes, 2) the β-type has low infrasound frequency and low frequency spectra of explosion earthquakes and 3) the γ-type has low infrasound frequency and small elastic vibrations at the summit station. About 110 explosion earthquakes of both α- and β-types are counted and 14 clear γ-type events were recognized during the period

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

A PRELIMINARY REPORT OF EXPLOSION SEISMIC EXPERIMENTS IN MOUNT EREBUS, ANTARCTICA

To improve knowledge of the crustal structure, explosion seismic experiments were carried out on Mount Erebus (77.5°S, 167°E, 3794m) by the group of the "International Mount Erebus Seismic Study (IMESS)" in November and December 1984. Seven blasts at four different sites were made after a few test explosions. The seismic observation of IMESS had started in December 1980. The number of seismic stations was increased from three to ten by the end of 1983. These stations were linked by radio-telemetry to Scott Base of New Zealand, at 77°51\u2703"S, 166°45\u2745"E, about 38km south of the Erebus Summit and all data were centrally recorded on the 14-channel magnetic tape and some on paper at Scott Base using a quartz clock.Seven seismic stations were temporarily operated around the summit area of Mount Erebus for the explosion experiments in December 1984. A tentative P-wave structure model of the Erebus region was obtained as follows : 1) The thickness of the first layer of 2.9km/s is 0.5km. 2) The second layer has a velocity of 4.7km/s with a thickness of 1.5km, and the third layer has a 6.2km/s velocity with a thickness of 2.5km. 3) The lowest layer of the model has a velocity of 7.0km/s which was obtained from the explosion seismic experiments which had been carried out in the McMurdo Sound region