Abstract. We develop a model for describing water flow in a porous medium under the effect of thermal and pressure gradients. The model simulates geothermal systems in calderas. Given the boundary conditions and the fluid-dynamical properties of the medium, the model allows computation, in fluid-dynamical stationary states, of parameters characterizing the flow, such as flow velocity and temperature and pressure distributions at depth. The model is applied to investigate the effects of the local geothermal system on the unrest episodes at Campi Flegrei caldera. Using experimentally determined fluid-dynamical parameters for the caldera rocks, we show that changes of water flow in shallow aquifers under the effect of pressure and/or temperature variations within the geothermal system can be very important in the genesis and evolution of unrest crises. In particular, they can strongly amplify the effect of pressure increase in the magma chamber on ground uplift. They can also explain the timescales of evolution of ground movements in terms of transit times of the water front and of the connected temperature fronts due to advective transport. On such grounds an integrated mechanic-thermal fluid-dynamical model was built, allowing us to give a semiquantitative, global explanation to the genesis and evolution of unrest phenomena. Results obtained here can be generalized to other similar calderas.

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[1] The Campi Flegrei caldera is characterized by well known episodes of fast uplift, called ''bradyseism,'' the last of which produced $1.8 m of maximum vertical displacement in the period [1982][1983][1984]. Less known is a particular aspect of Campi Flegrei unrests, detected only in the last 20 years, namely, the occurrence of ''miniuplift'' episodes, characterized by relatively small positive vertical displacements (in the range 0.03-0.11 m). Miniuplift episodes occurred in 1989, 1994, and 2000 and, probably, one or two times in the period 1972-1982. These small ground uplifts were also accompanied by moderate seismicity. Each of them was followed by a short phase of relatively quick subsidence, after which the previous rate of subsidence of the period was resumed. An order-of-magnitude analysis of the forces, energy, and power that can cause the onset and determine the evolution of these events is derived on the basis of a thermal-fluiddynamical approach. It allows the quantitative correlation of the power input from the energy source with the rate of energy dissipation in the geologic system during the miniuplifts. In order to assess the likelihood of our fluid-dynamical model for these episodes, theoretical ground displacements have been computed. These displacements result from the changes in pressure as a function of depth predicted by the fluid-dynamical model for reasonable perturbations of the geothermal system, described as changes of the Péclet number. Theoretical results agree extremely well with the observed amounts of the miniuplifts.

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