Seismic recordings of subterranean volcanic activity at Ruapehu during 1964 |
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| Authors: | R R Dibble |
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| Institution: | 1. Victoria University of Wellington, New Zealand
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| Abstract: | Volcanic vibrations from Ruapehu volcano which is situated in the centre of the North Island of New Zealand have been recorded on high magnification slow motion tape seismographs and drum seismographs at the Chateau Volcanological Observatory since December, 1960. In 1964, volcanic microtremor with a dominant frequency of 2 c/s commenced late in March, and reached a peak seismic power level of 100 KW for short periods in May. During the maximum phase, the tremor completely ceased for up to 20 minutes, and recommenced with an explosion or burst of strong tremor. The sequence and timing were very similar to the sequence of ash discharge-stoppage-explosion-ash discharge observed during the 1945 eruption of Ruapehu. In 1964 it is thought that the eruption of ash clouds was prevented by the Crater Lake, so that visible activity was limited to a few fumaroles, steam rising from the lake, and turbulence in the water. The lake temperature increased from about 25°C (a normal temperature) on 20 March to 50°C on 26 May. The corresponding rates of heat loss from the lake are 200 and 700 MW respectively, and an additional 150 MW or more was required to heat up the lake, giving a total average heat output of 850 MW for about 4 weeks. The corresponding average seismic power was about 3 KW, which is 0.0005 per cent of the thermal power. If this relationship is constant, the peak thermal power over a period of 6 minutes was of the order of 20,000 MW. The explosions initiating the tremor were mostly identical except in amplitude, and their magnitudes ranged up to 2.3, corresponding to 1014 erg. This was slightly greater than the energy conserved during the preceding stoppage of tremor. The dominant tremor frequency was normally 2.2 c/s and sometimes 1.2 c/s. Individual bands of frequency within the spectrum varied in power, but coherent migrations of frequency bands occurred on a few occasions. Near the end of the maximum phase, the entire spectrum migrated downwards by half an octave and the tremor power decreased to zero. Tremor power and frequency increased again, and a violent period of explosions and tremor of changing frequency occurred. Quite frequently the explosions initiating the tremor were multiple, and a few explosions occurred which were followed by tremor lasting only a matter of seconds. Explosions apparently unrelated to tremor were very rare and minor. The explosions were located by temporary installation of portable slow motion tape recorders around the volcano. The epicentre is very close to Crater Lake, but the depth cannot be determined from the data. Graphs of tremor power against time covering the active period from April to September, and graphs of cumulative energy against time, and frequency spectrum against time, for explosions and tremor are presented, and slow motion tape recordings will be played many times faster than the recording speed so that the tremor and explosion vibrations can be heard as audible noises. |
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