The seismicity of Etnean area and different features of observed seismic sequences

A review of the seismic activity that occurred in the Etnean region from 1976 up to 1987 is presented with the aim of investigating the characteristics of different seismic sequences relating to a number of eruptive phenomena. Etnean earthquakes generally appear randomly distributed both in time and space, with a small average magnitude (2.5-3.0). However, the observed episodic occurrence of swarms has shown particular characteristics, as regards both the epicentral and depth distribution, which seem to correlate to the volcanic activity taking place on the volcano. The space distribution of swarms that occurred before or during various eruptive phenomena as well as that of swarms that occurred during non eruptive periods were analysed. It was possible to distinguish swarms of shallow earthquakes preceding the opening of eruptive fractures, from seismic sequences of deeper earthquakes that are interpreted as responsible for the refilling of the volcano feeding system, either during eruptive phenomena lasting several months or during periods of moderate quietness between two successive eruptions. Moreover, a tendency towards a clustering in the areas of eruptive activity was observed for the seismic sequences preceding and accompanying such phenomena.