On deformation sources in volcanic areas: modeling the Campi Flegrei (Italy) 1982-84 unrest

Abstract

Deformation sources in volcanic areas are generally modeled in terms of pressurized tri-axial ellipsoids or other cavities with simple geometrical shapes embedded in homogeneous half-spaces. However, the assumption of a particular source mechanism and the neglect of medium heterogeneities bias significantly the estimate of source parameters. Leveling and EDM data, collected during the 1982-84 unrest episode at Campi Flegrei (Italy), are employed to retrieve source parameters according to a Bayesian inversion procedure, considering the heterogeneous elastic structure of the volcanic area. We describe a general deformation source in terms of a suitable moment tensor, through 3D finite element computations. Best fitting moment tensors are found to be incompatible with any pressurized ellipsoid. Taking into account the deflation of a deeper magma reservoir, which accompanies the inflation of the shallower moment source, data fit improves considerably but the retrieved moment tensor of the shallow source is found to be incompatible with pressurized ellipsoids, still. Looking for alternative physical models of the deformation source, we find that the best fit moment tensor can be best interpreted in terms of a mixed-mode (shear and tensile) dislocation at 5.5 km depth, striking EW and dipping by 30 to the North. Gravity changes are found to be compatible with the intrusion of 60·106 m3 of volatile rich magma with density 2000 kg/m3.