Mechanical and thermo-fluid behaviour during unrest at the Campi Flegrei caldera (Italy)

The Campi Flegrei caldera (CFc) is a resurgent, nested structure resulting from the two main collapses of the Campanian Ignimbrite (37 ka) and the Neapolitan Yellow Tuff (12 ka) eruptions. While the whole structure is affected by a broad subsidence, ongoing local resurgence and unrest occur inside the young, nested caldera structure. The caldera has shown signs of unrest during the past 30 years, with two uplift events that have generated a net displacement of 3.5 m, each followed by subsidence. The time evolution of both ground deformation and seismicity recorded in Campi Flegrei in the last 30 years (since 1969 up to the present) shows aspects not completely explainable by means of mechanical models. In particular, the occurrence of an intense seismic activity during uplift and its absence during subsidence lead us to infer that these two phases might be related to two variable mechanisms. The large amount of magma and the shallow convective fluids circulation needed to explain the very high temperature gradient (100°C/km) measured also in marginal areas of the caldera, suggest the presence of a thermo-dynamical system in supercritical conditions. We have carried out an analysis of the unrest episodes by means of 3D finite-element method, simulating the mechanical and fluid-dynamical response of a two phase medium (solid–fluid) to a sudden (stepwise) pressure or volume increase at a depth of 4 km. According to geological and geophysical constraints, in our scheme we have subdivided the caldera floor into a central and a peripheral zone. The central zone represents the resurgent block and has high permeability, while the peripheral zone is less permeable. We have performed a parametric analysis assuming both Young modulus and permeability of each zone as variables. The basic test for each solution was how well it simulates the time evolution of ground deformation during the last unrest episode (1982–1984). The results obtained clearly show that fluids diffusion accounts for some peculiar features of the ground deformation such as the variable behavior between the resurgent block and the peripheral part of the caldera floor. Subsidence is explained in terms of lateral diffusion of fluids instead of a regression of source processes. Consequently, no variation of shear stress occurs during this phase, providing a physical explanation to the absence of seismicity.