MODELING GRAVITY VARIATIONS CONSISTENT WITH GROUND DEFORMATION IN THECAMPI-FLEGREI CALDERA (ITALY)

In recent years (1970-72 and 1982-84) two inflation episodes took plac e in the Campi Flegrei caldera (Italy), characterized by significant g round uplift and gravity variations. An elastic half-space model with vertical density stratification is employed to compute the displacemen t field and the gravity Variations produced by the deformation of buri ed layers, following the inflation of a spherically symmetric deformat ion source. Contributions to gravity variations are produced by dilati on/contraction of the medium, by the displacements of density interfac es (the foe surface and subsurface layers) and of source boundaries an d, possibly, by new mass input from remote distances into the source v olume. Three cases were examined in detail: In case I, the magma chamb er is identified as the deformation source and volume and pressure inc rease in the magma chamber is due to input of new magma from remote di stances; in case Il deformation is due to magma differentiation within the magma chamber (deformation source with constant mass); in case II I the geothermal system is identified as the deformation source and a pressure increase, possibly driven by the exsolution of high temperatu re and high pressure volatiles in the magma chamber, is assumed to pla y a dominant role. From the comparison between measured and computed g ravity residuals (free-air-corrected gravity variations) we can assess that, in case I, an inflation source with constant density would pred ict gravity residuals compatible with observations, whereas an expansi on at constant mass (case II) would predict gravity residuals much low er than observed. The resolving power of gravity data however prevents accurate assessment of the density of the emplaced material. In case III, the pervasive density increase of the geothermal fluids induced b y pressure increase is assumed to be the main source of gravity variat ions. The average porosity value required for this model to match both the ground deformation and the gravity residuals is found to be simil ar to 10%, a value which is compatible with measured porosity values a t Campi Flegrei in deep wells. The subsidence phases following both in flation episodes and the gravity residuals during subsidence lead us t o consider case III as more plausible, even if a suitable combination of cases I and LII cannot be discarded. (C) 1998 Elsevier Science B.V.