The Somma-Vesuvius volcanic complex and surroundings are characterised by significant topography and strong 3D structural variations. This medium complexity has to be taken into account when monitoring the background volcano seismicity in order to get reliable estimates of epicentre, depth and fault mechanism of events beneath the volcano.

We have developed a 3D, P-wave velocity model for Vesuvius by interpolation of 2D velocity sections obtained from non linear tomographic inversion of the Tomoves 1994 and 1996 active seismic experiment data. The comparison of predicted vs observed 3D travel time data from active and passive seismic data validate the 3D interpolated model.

We have relocated the natural seismic events from 1989-1998 under Vesuvius using the new interpolated 3D model with two different Vp/Vs ratios and a global-search, 3D location method. The solution quality, station residuals and hypocenter distribution for these 3D locations have been compared with those for a representative layered model.

A relatively high Vp/Vs ratio of 1.90 has been obtained. The highest quality set of locations using the new 3D model fall in a depth range of about 1 to 3.5 km below sea level, significantly shallower than has been determined in previous studies. The events are concentrated in the upper 2 km of the Mesozoic carbonate basement underlying the Somma-Vesuvius complex. The first-motion mechanism for a subset of these events, though highly varied, give a weak indication of predominantly N-S directions for the tension axes, and ESE-WNW directions for the pressure axes.

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