[1] The Campi Flegrei (CF) Caldera experiences dramatic ground deformations unsurpassed anywhere in the world. The source responsible for this phenomenon is still debated. With the aim of exploring the structure of the caldera as well as the role of hydrothermal fluids on velocity changes, a multidisciplinary approach dealing with three-dimensional delay time tomography and rock physics characterization has been followed. Selected seismic data were modeled by using a tomographic method based on an accurate finite difference travel time computation which simultaneously inverts P wave and S wave first-arrival times for both velocity model parameters and hypocenter locations. The retrieved P wave and S wave velocity images as well as the deduced V p /V s images were interpreted by using experimental measurements of rock physical properties on CF samples to take into account steam/water phase transition mechanisms affecting P wave and S wave velocities. Also, modeling of petrophysical properties for site-relevant rocks constrains the role of overpressured fluids on velocity. A flat and low V p /V s anomaly lies at 4 km depth under the city of Pozzuoli. Earthquakes are located at the top of this anomaly. This anomaly implies the presence of fractured overpressured gas-bearing formations and excludes the presence of melted rocks. At shallow depth, a high V p /V s anomaly located at 1 km suggests the presence of rocks containing fluids in the liquid phase. Finally, maps of the V p *V s product show a high V p *V s horseshoe-shaped anomaly located at 2 km depth. It is consistent with gravity data and well data and might constitute the on-land remainder of the caldera rim, detected below sea level by tomography using active source seismic data.Citation: Vanorio, T., J. Virieux, P. Capuano, and G. Russo (2005), Three-dimensional seismic tomography from P wave and S wave microearthquake travel times and rock physics characterization of the Campi Flegrei Caldera,

A two-dimensional active seismic experiment was performed on Mount Vesuvius: Explosive charges were set off at three sites, and the seismic signal along a dense line of 82 seismometers was recorded. A high-velocity basement, formed by Mesozoic carbonates, was identified 2 to 3 kilometers beneath the volcano. A slower ( P -wave velocity V P ≃ 3.4 to 3.8 kilometers per second) and shallower high-velocity zone underlies the central part of the volcano. Large-amplitude late arrivals with a dominant horizontal wave motion and low-frequency content were identified as a P to S phase converted at a depth of about 10 kilometers at the top of a low-velocity zone ( V P < 3 kilometers per second), which might represent a melting zone.

Summary 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 VP/VS 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 VP/VS 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.

An extended marine, active seismic survey has been performed on September, 2001 in the gulfs of Naples and\ud Pozzuoli by recording about 5000 shots at a network of 62 sea bottom and 72 on shore seismographs. 3-D images of the\ud shallow caldera structure are obtained from the tomographic inversion of about 77000 first P arrival times using the Benz\ud et al. [1996] tomographic technique. The buried rim of the Campi Flegrei caldera is clearly detected at about 800 –\ud 2000 m depth, as an anular high P-velocity and high density body. It has a diameter of about 8– 12 km and a height of\ud 1 – 2 km. According to stratigraphic and sonic log data from deep boreholes and tomographic P velocities, the rim is\ud likely formed by solidified lavas and/or tuffs with interbedded lava. This study confirms the existence for a\ud depressed limestone basement beneath the caldera at less than 4 km depth, while no evidence are found for shallower\ud magmatic bodies

Hydrogeological and geophysical investigations demonstrated the existence of an epikarstic zone in a carbonate aquifer of Southern Italy, about 10 m thick. Nevertheless, the hydrogeological behaviour of the epikarst is different from that schematized by several authors. In the test site, the contrast in permeability at the bottom of the epikarst does not cause retention of percolation and storage of water in a perched temporary aquifer within the uppermost portion of the carbonate medium. Because of the high fracture density and good interconnection of openings within the underlying limestone, the percolation is diffuse also below the epikarstic zone, as well as the groundwater flow. The ‘funnelling’ effect into larger shafts does not play an important role on the hydrogeological behaviour of the aquifer.