Magma reservoir at Mt. Vesuvius: size of the hot, partially molten, crust material detected deeper than 8 km.

One- and two-dimensional V-p models were obtained by TomoVes experiment, all characterized by low V-p in the uppermost 500 m and a sharp discontinuity at about 2-3 km beneath the volcano. Large amplitude late arrivals were identified as P- to S-phases converted at the top, between 8 and 10 km deep, of a low-velocity layer with a dramatic drop of Vs, from approximately 3.6 km/s to less than 1.0 km/s. Here, we synthesize the interpretation of Rayleigh wave dispersion measurements, made by several authors, to delineate the extent of such anomalous layer of hot, partially molten, crust material. Our non-linear inversion of broad-band dispersion measurements evidences a main feature of Somma-Vesuvius deep structure consisting of low V-s layers at about 8-10 and 20 km of depth. The depth of the shallower low-velocity layer and the V-s value above it are in agreement with TomoVes results, but the Vs velocity reduction is of about 10%. If we assume Vs equal to 1.0 km/s in our non-linear inversion, a thickness not greater than 0.35 km results. The volume occupied by this very low-velocity layer, sill-shaped, is compatible with the size of Mt. Vesuvius cone, but it develops above a much larger hot mass, which could be the parental source as the erupted products are only few percent of magma chamber.