High-resolution radon monitoring and hydrodynamics at Mount Vesuvius

A yearlong high-resolution radon survey has been carried on at Mount Vesuvi us, starting in May 1998. Radon activities were acquired by exposing charco al canisters and track-etch detectors. Sampling stations were deployed alon g two major summit faults and around the caldera bottom. Volcanically-relat ed earthquakes, with MD 2.5, may be discriminated from regional seismic eve nts since their cumulative radon anomalies are recorded from stations locat ed along all the above structural features. On the contrary, radon anomalie s correlated to regional earthquakes, with M-D greater than or equal to 4, are essentially recorded by the sampling sites located along the two summit faults (whose roots extend deeper into the Tertiary basement rocks that un derlay the volcano). Radon migration to the surface is ruled by convection within a porous medium of relatively low porosity (phi approximate to 10(-5 )), suggesting that fluid motion is strongly localised along fractures. It is suggested that fluid pressure build up, followed by fluid release and mi gration during incipient fracturing of the porous medium, precede the onset of volcanically- induced earthquakes.