The seismicity of Mt Vesuvius

Over the last years, in order to improve the quality of the determination of earthquake locations as well as the reliability of the fault-plane solutions, three-component digital stations were temporarily added to the permanent seismic network of the Osservatorio Vesuviano operating in the Vesuvius area. In this work, the seismic behaviour of the Mt. Vesuvius volcano has been investigated by merging the data gathered during four seismic surveys carried out in 1986-87, 1989, 1993 and 1994. Low to moderate seismic activity, both in energy release and event number, is characteristic at present stage of the volcanic quiescence. Although several hundreds of events were catalogued during the periods of seismic monitoring, many of these events were recorded only by the summit stations and therefore considered to belong to the very shallow microseismicity affecting the volcanic edifice. Seismicity density maps, obtained by the use of a Bayesian approach, show that seismicity and seismic moment release are clustered around the crater axis, at depths that rarely exceed 5-6 km from the top of the volcanic cone (1200 m). These results, corroborated with the recent findings from high-resolution seismic tomography, indicate that stress is concentrated, in the volcanic structure, in a highly rigid core extending down to about 5 km depth below the sea level. Focal mechanisms were computed both using classical methods based on P-wave first motions and by a probability method using P-wave polarity and S-wave polarization data. The latter method allowed for an increased accuracy in the evaluation of the focal mechanisms of small events occurring in the Vesuvian area. The general pattern of the focal mechanisms suggests that seismicity is mainly related to the regional stress-field, in particular to the main fault system in the area.