Abstract
<p>A high-resolution image of the compressional wave velocity and density structure in the shallow edifice of Mount Vesuvius has been derived from simultaneous inversion of travel times and hypocentral parameters of local earthquakes and from gravity inversion. The robustness of the tomography solution has been improved by adding to the earthquake data a set of land based shots, used for constraining the travel time residuals. The results give a high resolution image of the P-wave velocity structure with details down to 300-500 m. The relocated local seismicity appears to extend down to 5 km depth below the central crater, distributed into two clusters, and separated by an anomalously high Vp region positioned at around 1 km depth. A zone with high Vp/Vs ratio in the upper layers is interpreted as produced by the presence of intense fluid circulation alternatively to the interpretation in terms of a small magma chamber inferred by petrologic studies. In this shallower zone the seismicity has the minimum energy, whilst most of the high-energy quakes (up to Magnitude 3.6) occur in the cluster located at greater depth. The seismicity appears to be located along almost vertical cracks, delimited by a high velocity body located along past intrusive body, corresponding to remnants of Mt. Somma. In this framework a gravity data inversion has been performed to study the shallower part of the volcano. Gravity data have been inverted using a method suitable for the application to scattered data in presence of relevant topography based on a discretization of the investigated medium performed by establishing an approximation of the topography by a triangular mesh. The tomography results, the retrieved density distribution, and the pattern of relocated seismicity exclude the presence of significant shallow magma reservoirs close to the central conduit. These should be located at depth higher than that of the base of the hypocenter volume, as evidenced by previous studies.</p>
Highlights
Vesuvius [Zollo et al 1996, Auger et al 2001]. These results revealed the presence of a large low Pwave velocity body, looking like to a very large sill located at an average depth around 8 km, extending from Mt
All aimed at refining the details of the shallow geological structures evidenced by TOMOVES experiments, suffer from low resolution in the rock volume located beneath the crater area, mainly due to the quantity and quality of data used
Notwithstanding the above restrictions, these studies have clearly imaged the structure of the Mesozoic limestone basement underlying the volcano edifice and its surroundings, showing the presence of depressions around Vesuvius and indicating the presence of a high velocity body below the volcano
Summary
The main scientific objectives were: a) to record the seismic waveforms from artificial explosions, with dense space and time sampling of the seismic signal, to image the velocity structure and the distribution of the strong elastic scatterers in the upper layers under the central crater, with a very high resolution; b) to study the response of different sites on the flanks of Mt. Vesuvius, in order to optimize the installation of a permanent small aperture array for volcano monitoring. Seismic velocity and reflecting interfaces The array recordings of the three shots fired for the 1994 experiment (D1) reveal at a first visual analysis a low degree of spatial coherence, a low degree of coherence in the waveforms of late arrivals and a complex particle motion pattern (Figure 1b) This observation is consistent with the presence of strong scattering and diffraction effects. The small aperture array installed along the crater rim confirms this relevant result
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