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Campi Flegrei is a densely inhabited volcanic area within a caldera, 12–14 km in diameter, immediately west of Naples, Italy. In 1970–1972 and in 1982–1984, Campi Flegrei experienced considerable ground surface deformation producing a bulge about 3.5 m high at its peak. In the two inflation episodes the point of maximum uplift did not change, coinciding with the caldera center, about which the displacements exhibited radial symmetry. The 1982–1984 uplift was accompanied by considerable seismic activity. Gravity changes have been interpreted as reflecting the occurrence of a subvertical magmatic intrusion. Volcanological, petrological, and geophysical data point to the existence of a shallow magma chamber just beneath the center of the caldera. We assume that the observed surface displacements are the response of an essentially elastic medium to a pressure increase in the magma chamber. Static deformation is calculated by using a finite‐element method, assuming axial symmetry around a vertical axis through the caldera center. The role played by various components of the model, geometry, boundary conditions, elastic properties, is analyzed. It is shown that models assuming uniform elasticity are inadequate. Therefore the major volcanotectonic features as well as the influence of temperature and pressure on the elastic properties are taken into account. The results of various more complex models are presented. It is suggested that a progressively fractured zone, whose size and shape depend also on the possible occurrence of magmatic intrusions, may explain the observed surface displacements and the other data collected at Campi Flegrei. It is suggested that the 1970–1972 uplift was possibly accompanied by larger‐scale changes in the stress field.

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Campi Flegrei is a densely inhabited volcanic area within a caldera, 12-14 km in diameter, immediately west of Naples, Italy. In 1970In -1972In and in 1982, Campi Flegrei experienced considerable ground surface deformation producing a bulge about 3.5 m high at its peak. In the two inflation episodes the point of maximum uplift did not change, coinciding with the caldera center, about which the displacements exhibited radial symmetry. The 1982-1984 uplift was accompanied by considerable seismic activity. Gravity changes have been interpreted as reflecting the occurrence of a subvertical magmatic intrusion. Volcanological, petrological, and geophysical data point to the existence of a shallow magma chamber just beneath the center of the caldera. We assume that the observed surface displacements are the response of an essentially elastic medium to a pressure increase in the magma chamber. Static deformation is calculated by using a finite-element method, assuming axial symmetry around a vertical axis through the caldera center. The role played by various components of the model, geometry, boundary conditions, elastic properties, is analyzed. It is shown that models assuming uniform elasticity are inadequate. Therefore the major volcanotectonic features as well as the influence of temperature and pressure on the elastic properties are taken into account. The results of various more complex models are presented. It is suggested that a progressively fractured zone, whose size and shape depend also on the possible occurrence of magmatic intrusions, may explain the observed surface displacements and the other data collected at Campi Flegrei. It is suggested that the 1970-1972 uplift was possibly accompanied by larger-scale changes in the stress field. INTRODUCTION Ground surface deformations in volcanic areas are usually interpreted to reflect changes of pressure in shallow magma reservoirs and/or the occurrence of intrusive events. Large deformations are often observed to occur in a very limited area, and their interpretation in terms of schematic models [Mogi, 1958; Dieterich and Decker, 1975] implies the assumption of unrealistically shallow chambers or intrusions and unreasonably large pressure increases [Walsh and Decker, 1971; Davis et al., 1974; Dieterich and Decker, 1975; Bianchi et al., 1984]. Therefore, more realistic models taking into account volcanological and structural features as well as the nonuniform response of country rocks (related to temperature and pressure gradients, possible fracturing, presence of fluids, etc.) are required. In the present paper, we shall proceed along this line, focusing our attention on the Campi Flegrei volcanic area. Campi Flegrei (C.F.) is a 35,000-year-old caldera, 12-14 km in diameter, located immediately west of Naples, Italy, in an area of regional tensional tectonics [Finetti and Morelli, 1974; Scandone, 1979]. In the last 10,000 years its activity has been characterized by a large number of explosive eruptions from different vents within the caldera. The most recen...

Abstract. This study was undertaken with the aim of contributing to the risk evaluation of Vesuvius. We calculate the surface displacements due to an increase in pressure in a shallow reservoir and assess the mechanical instability of the volcanic edifice and of the feeding system caused by pressure on the reservoir's walls and by regional stresses. We consider axisymmetric models that take into account topography, gravity, homogeneous and heterogeneous country rocks, spheroidal magma chambers with different aspect ratios and variable depths, and subvertical intrusions from the top of the chamber to various depths. We impose both symmetric and asymmetric regional stresses increasing with depth as boundary conditions. The models are static. The ground deformation and the stress distribution are calculated, in the framework of linear elasticity, by a numerical finite element method. The surface displacements refer to an overpressure of 10 MPa. This value is considered an upper limit for the fracture of surrounding rocks. We find the maximum vertical displacements to be of a few centimeters in the most favorable case and the displacement gradients to be at the lower limit of measurability. To evaluate the mechanical stability, we calculate the stress distributions of a prolate ellipsoidal reservoir within a heterogeneous medium. We consider a total hydrostatic magmatic pressure starting from the value of 50 MPa at the reservoir's top and three regional stress regimes from symmetric tensile to asymmetric tensile to strike-slip. In the last two cases we use three-dimensional models. The criteria adopted for instability are (1) the development of tensile tangential stress and (2) the Navier-Coulomb criterion, in compression. In no case have we found instability near or on the wall of the reservoir, whereas the slope of the volcanic edifice exhibits a shear failure instability, which increases with greater regional stress anisotropy.