EARTHQUAKE-INDUCED VARIATIONS IN THE COMPOSITION OF THERMAL WATERS ON VOLCANO ISLAND, ITALY

Luca Bolognesi (CNR - International Institute for Geothermal Research, Piazza Solferino 2, 56126 Pisa, Italy)

On Vulcano Island, in Southern Italy, numerous shallow water wells have been drilled for domestic use at the foot of the active crater. The thermal water in the well which supplies water to the camp-site Camping Sicilia is a mixture of reservoir water and shallow steam-heated groundwater of meteoric origin. During 1986-1988, the only variations observed in the available isotopic and chemical compositions of the Camping Sicilia well water, which changed from the almost pure reservoir component to the almost pure steam-heated meteoric component, occurred just before two sequences of tectonic earthquakes recorded near the island. After the seismic activity, the Camping Sicilia well returned to its pre-earthquake composition. These changes in composition could be explained by the dilatancy-fluid diffusion model. According to this model, the observed variations in the Camping Sicilia well water composition could be the consequences of the stress build-up prior to the seismic events, and stress reduction afterwards.