The volcanic area of Campi Flegrei has been active in the recent past, as well illustrated by the historic eruption of Monte Nuovo in 1538, by the 1970-72 and 1982-84 bradyseismic upflit, by the fumarolic activity, and by hot springs. In September 2001, an extensive active-seismic investigation was carried out in the Gulfs of Naples and Pozzuoli, with the aim of investigating and reconstructing the shallow crustal structure of the Campi Flegrei caldera, and possibly identifying its feeding system at depth (the Serapis Project). The present study provides a joint analysis of the seismic reflection data and tomographic images based on the Serapis dataset. This is achieved by reflection seismic sections obtained by the 3D data gathering and through refined P-velocity images of the shallowest layer of Pozzuoli Gulf (z<1000 m). From the refined Vp model, the overall picture of the velocity distribution shows the presence of a complex arc-shaped anomaly that borders the bay offshore. The deeper part of the anomaly (beneath 700 m, with Vp > 3500 m/s) correlates with units made up of agglomerate tuff and interlayered lava, which form the southern edge of the caldera, which was itself probably formed following the two large ignimbritic eruptions that marked the evolutionary history of the area under study. The upper part of the anomaly that tends to split into two parallel arcs is correlated with dikes, volcanic mounds and hydrothermal alteration zones noted in previous shallow reflection seismic analyses. The depth of the transition between the upper and lower parts of the anomaly is characterized by an abrupt Vp increase on the one-dimensional (1D) profiles extracted from the 3D tomographic model and by the presence of a strong reflector located at about 0.6/0.7s Two Way Time (TWT) on Common Mid Point gathers. The moveout velocity analysis and stack of the P-P and P-S reflections at the layer bottom allowed to estimate relatively high Vp/Vs values (3.7 ±0.9) . This hypothesis has been tested by a theoretical rock physical modeling of the Vp/Vs ratio as a function of porosity suggesting that the shallow layer is likely formed by incoherent, water saturated, volcanic and marine sediments that filled Pozzuoli Bay during the post-caldera activity.

 

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 *Notice: This is an electronic version of an article published in Bulletin of Volcanology: complete citation information for the final version of the paper, as published in the print edition of Bulletin of Volcanology, is available on the Springer online delivery service, accessible via the journal's website at http://www.springer.com/earth+sciences/geology/journal/445