Precise, absolute earthquake location under Somma-Vesuvius volcano using a new three-dimensional velocity model

The Somma-Vesuvius volcanic complex and surroundings are characterized by topographic relief of over 1000 m and strong 3-D structural variations. This complexity has to be taken into account when monitoring the background volcano seismicity in order to obtain reliable estimates of the absolute epicentres, depths and focal mechanisms for events beneath the volcano. We have developed a 3-D P-wave velocity model for Vesuvius by interpolation of 2-D velocity sections obtained from non-linear tomographic inversion of the Tomoves 1994 and 1996 active seismic experiment data. The comparison of predicted and observed 3-D traveltime data from active and passive seismic data validate the 3-D interpolated model. We have relocated about 400 natural seismic events from 1989 to 1998 under Vesuvius using the new interpolated 3-D model with two different V-P/V-S ratios and a global search, 3-D location method. The solution quality, station residuals and hypocentre distribution for these 3-D locations have been compared with those for a representative layered model. A relatively high V-P/V-S ratio of 1.90 has been obtained. The highest-quality set of locations using the new 3-D model falls in a depth range of about 1-3.5 km below sea level, significantly shallower than the 2-6 km event depths 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 mechanisms for a subset of these events, although highly variable, give a weak indication of predominantly N-S to near-vertical directions for the tension axes, and ESE-WNW near-vertical directions for the compression axes.