Time-space variation of the micro-seismicity at La Fossa (Vulcano, Aeolian Islands, Italy): new insights into seismic sources in a hydrothermal system

About 1000 microearthquakes, occurring from January 2004 to December 2006 at La Fossa crater of Vulcano (Aeolian Islands, Italy) and related to processes of the geothermal system, have been analysed. Three episodes of increasing number of these seismic events, that accompanied geothermal and geochemical anomalies, took place during the studied time span. Six classes of microearthquakes (monochromatic, low frequency, medium frequency, high frequency, mixed, tornillos events), characterised by different spectral content and various similarity of the waveforms, have been recognized. We noted that events belonging to each class were not uniformly time distributed. In particular, the occurrence rate of microearthquakes of some classes seems to be more related to geothermal and geochemical anomalies than others. The microearthquakes, mainly clustered below La Fossa crater area at depth 0.5-1.1 km b.s.l., were space-distributed according to the classes. We infer that microseismicity at Vulcano is related to two different mechanisms: i) fracturing processes of "poorly competent" rocks and ii) resonance of cracks (or conduits) filled with hydrothermal fluid. Finally, a gradual time variation of the waveform of the low-frequency events was observed. It was likely due to processes of heating and drying of the hydrothermal system.