Campi Flegrei unrest episodes and possible evolution towards critical phenomena

Mechanical and thermal-fluid-dynamical contributions to unrest phenomena at active calderas are quantitatively evaluated, and a mixed mechanical-thermal-fluid-dynamical model is developed, to explain the unrest phenomena at Campi Flegrei caldera. Mechanical modelling involves the use of analytical and finite element formulations. Thermal-fluid-dynamical effects are considered in the framework of a one-dimensional approximation, solved by an analytical method. The results shed new light about the coupled effect of mechanically and thermally induced perturbations and variations in the geothermal fluid circulation regime. Unrest episodes at Campi Flegrei are interpreted in terms of the joint effect of a first, essentially mechanical, phase due to overpressure in a magma chamber and to a progressive amplification and final decay due to the circulating water. The elastic response of the rocks to the migrating front of overpressure is shown to be strongly conditioned by the caldera structure, which produces a concentration of strain in the inner part, and a stress concentration, causing the local seismicity, around the caldera borders. Thermal-fluid-dynamical effects are also shown to have important implications on the evolution of the volcanic system towards critical phenomena, which must be considered for a meaningful evaluation of volcanic hazard.