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Geophysical exploration of the Campi Flegrei (Southern Italy) Caldera interiors: Data, methods and results
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Doppiavoce, Napoli, Italy, (June 2006)

Arrival times and waveforms of first and secondary arrivals (reflected and converted waves) produced by active sources (on land and offshore explosions) and/or natural sources (microearthquakes), can provide significant seismic information about the smooth variations of the velocity and attenuation of seismic waves and about the morphology of the most important reflectors. The project 'Integrated seismic methods applied to the investigation of the active volcano structure: an application to the Campi Flegrei caldera' aimed both at developing and using advanced techniques in order to model active and passive seismic data recorded in the Campi Flegrei region, and at the com- parison of the obtained velocity and attenuation measurements with labora- tory data. In particular, spatial variation of the elastic (P and S velocity values) and anelastic (P and S quality factors) properties, jointly with recent informa- tion about the caldera thermal state, can provide parameters necessary to a preliminary thermo-mechanical modeling of the active physical processes. Structural complexity of the upper crust in volcanic regions is due to mechanisms of magma feeding and volcanic structure building. Moreover, variations in rocks rheology and elastic parameters of the propagation medium can be produced by an anomalous thermal field. Thus, a reliable model representing the distribution of elastic/anelastic properties in the propagation medium is needed both to understand and to reproduce the mechanisms which drive the magma rising up and to monitor accurately and continuously the seismic activity occurring during eruptive quiescence periods and crisis episodes. Complexities of seismograms recorded in volcanic regions are usually produced by strong heterogeneities of the propagation medium, sometimes jointly to seismic source complexity (e.g. volcanic tremor, low frequency earthquakes), and by the small depth and magnitude of the seismic events. Since '80s, the availability of digital, reliable and dense seismic networks enable to record high quality data in volcanic regions, allows to interpret the seismograms complexities. Arrival times, amplitudes, polarisations and waveforms of the most important seismic phases can be analysed to discriminate source and propagation effects. From a methodological point of view, since the early '80s, travel time tomography of P and S first arrivals from local earthquakes has provided new information on the active volcanoes structure. In the last '80s-early '90s, the joint analysis of active (explosive sources located in boreholes) and passive (local microearthquakes) seismic data has been introduced in order to better resolve the strongly non-linear inverse problem of the joint determination of the propagation medium (seismic waves velocity in a 3D-grid nodes) and hypocentral parameters (earthquake location and origin time). The project has concerned the development and implementation of innovative techniques of analysis and interpretation of active/passive seismic data recorded in the Campi Flegrei. Thanks to bradiseismic crisis occurred in the early '80s, the scientific background and the general geological/geophysical knowledge are improved. The main goals of the project was the definition of a 3D detailed crustal model of the volcanic region and the identification and detection of the possible deep reservoir of magma feeding. The project was based on the analysis and interpretation of different seismic data set. The first one was a data set of several hundreds of microearthquakes recorded during the last bradiseismic crisis occurred on 1984. The second was the DSS (Deep Seismic Soundings) seismic refraction data acquired during the surveys conducted on 1980 and 1985 in the region. The latest was a data set acquired in the framework of the MareVes97 offshore seismic survey, an experiment devoted to the definition of the structure of the Somma-Vesuvio volcanic complex. Moreover, during the project development has been planned and carried out a new experiment (called SERAPIS, SEismic Reflection/Refraction Acquisition Project for Imaging complex volcanic Structures) based on off-shore seismic energization and data acquisition on the sea-bottom. The experiment was performed in September, 2001 during which the vessel NADIR of IFREMER (equipped with 12, 16-liters airgun) produced more than 5000 airgun shots recorded at a sea-bottom seismograph array of 62 OBS and 50 station installed on-land. The data set acquired during the SERAPIS experiment integrated the above mentioned data set and has been successfully used to infer 3D images of the volcanic structures of Campi Flegrei and Neapolitan bay. The importance of studying the structure of Campi Flegrei is related to the high volcanic risk associated even to a minor eruption because CF is one of the main unrest calderas in the world where several hundred thousands people live within its borders. 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 3 km depth) (b) the presence of a few km thick, inner basin characterized by low Vp, high Vp/Vs and high Qp; (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 remained 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. The main objective of the project was the detailed reconstruction of the 3D structure of the Campi Flegrei caldera through the inversion of passive and active data sets and the understanding of the relationship between the elastic/anelastic properties and the physical, mechanical and thermal properties of rocks forming the caldera edifice. In the enclosed CD-Rom you will find appendixes A and B.
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