Volcanic tremors and shocks associated with eruptions at Etna (Sicily), September 1980

Three strong ash eruptions, superimposed on an otherwise rather quiet state of volcanic activity, took place from the northeast crater of Etna during September 1980. The erup-tions lasted only a few hours and were associated with an increase in the amplitudes of volcanic tremors by a factor of 10 compared with the mean value of the preceding months. Special emphasis in the tremor analysis was given to the eruption of Septem-ber 26, 1980. The time function of the amplitude envelope of the tremors follows a mathematical function which resembles the behaviour of a mechanical oscillator with a resonant period of about 300 s. A detailed spectral analysis of the tremors before, during and after the eruption reveals factors which seem to be associated with the ascent and descent of the tremor sources in the magma columns. The turbulent eruptive magma flow apparently is concentrated in a few dykes and pipes. Their eigenvibrations during an erup-tion are observed as spectral peaks of small bandwidth, superimposed on the smoother spectrum without a volcanic eruption. An analysis of seismicity shows a maximum in August with epicenters clustering along a NW-SE direction.