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-therm al-fluid-dynamical model is developed, to explain the unrest phenomena at C ampi Flegrei caldera. Mechanical modelling involves the use of analytical a nd finite element formulations. Thermal-fluid-dynamical effects are conside red in the framework of a one-dimensional approximation, solved by an analy tical method. The results shed new light about the coupled effect of mechan ically 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, aroun d the caldera borders. Thermal-fluid-dynamical effects are also shown to ha ve important implications on the evolution of the volcanic system towards c ritical phenomena, which must be considered for a meaningful evaluation of volcanic hazard. (C) 2001 Elsevier Science B.V. All rights reserved.