American Geophysical Union, Fall Meeting, San Francisco, USA (invited), 2002
Zollo A., Virieux J., Makris J., Auger E., Boschi L., Capuano P., Chiarabba C., D’Auria L., De Franco R., Judenherc S., Michelini A., Musacchio G.
Campi Flegrei is one of the main unrest calderas in the world. Several hundred thousansd people live within its borders. This makes very high the volcanic risk associated even to a minor eruption. The caldera formed as a consequence of a huge ignimbrite eruption around 40000 year b.p. In the last ten thousand years, the volcanic activity has been characterized by the occurrence of explosive eruptions with a return period of thousands year. The last one occured in 1538, giving rise to an about 130 m, spatter cone.
The bottom of the caldera has been continously sinking with an average speed of about 1 cm per year, from 1538 till 1970. Two resurgency episodes occurred in 1970-1972 and 1982-1984 with a nearly simmetrical, up-lift with a maximum of about 3.8 m at the town of Pozzuoli. Then the ground has slowly sinking down and it did not recover its level in 1970. In the past the structure of the caldera has been mainly investigated by a few km deep drillings, earthquake seismic tomography, gravity and magnetic surveys and sporadic observations of teleseismic and wide angle seismic data. The whole geophysical information indicate the following features: a) the evidence of sharp temperature gradients at shallow depths (450 degrees measured at 3km depth) b) the presence of a few km thick, inner basin characterized by low $V_p$, high $V_p$/$V_s$ and high $Q_p$; c) the shape of this basin is consistent with the gravity low anomaly and appear to be the site where most of deformation is concentrated during the recent ground uplift episodes; d) the possible occurrence of a magmatic reservoir at about 4-5 km depth from teleseismic observations and extrapolation of thermal data. Relevant open questions still remain to be answered, mainly concerning the depth and lateral extension of the shallow magmatic reservoir and the possible existence of intra-crustal magmatic sill as it has been found in the nearby Mt.Vesuvius. With the aim to provide new insights on the caldera structure and location of its feeding system, a dense and extended active seismic survey has been performed during September 2001,in the gulfs of Naples and Pozzuoli in the framework of the so called SERAPIS project. 60 three-component stations have been installed on-land in the areas of Campi Flegrei, Mt.Vesuvius and on the islands of Ischia and Procida and 72 sea bottom seismographs (OBS) have been installed in the gulfs of Naples and Pozzuoli. A denser 2D network of 35 OBSs has been deployed in the bay of Pozzuoli aimed at detecting and modeling reflected/converted waves from the possible shallow to deep discontinuities beneath the Campi Flegrei caldera. About 5000 shots have been performed during the SERAPIS experiment, at an average spatial spacing of 125 m, for a total ship travel path of 620 km. We present preliminary results based on the processing and analysis of the SERAPIS and microearthquake data sets, with a particular focusing on the upper crustal structure by seismic tomography, reflection and receiver function methods.