Seismicity at Somma-Vesuvius and its implications for the 3D tomography of the volcano

A research project to define the feeding system of the Vesuvius volcano and the upper crustal structure of the area was started in 1993. The core of th.le project is the high resolution seismic tomography study by using explosive sources. Results of a preliminary 2D seismic profile have been discussed by Zollo et al. (1998) [J. Volcanol. Geotherm. Res., this volume]. The study of local seismicity at Vesuvius has many implications for the determination of the substructure of the volcanic area. The shape and size of the seismic volume and the study of the focal mechanisms put important constraints on the stress field of the area. The local seismicity can greatly improve the knowledge of the tridimensional velocity distribution of both P and S seismic waves inside the volcano, using passive tomographic techniques. In this paper we obtain a first three-dimensional tomographic image of the volcanic structure, obtained by inversion of first P and S arrival times of local earthquakes. The results show the existence of a sharp velocity contrast, along a lineation oriented NW-SE, cutting the crater along the line separating the relict of the Somma caldera from the southwest part of the volcano. This study confirms, also for S wave velocities, the absence of indications for a magma chamber In the first 4-5 Km below the sea level, as already evidenced, just for P waves, by 2D tomography. Furthermore, we discuss the improvement of resolution obtainable by deploying a small set of three component seismic stations to get a better coverage of the area. (C) 1998 Elsevier Science B.V. All rights reserved.