Unrest at the Campi Flegrei caldera (Italy): A critical evaluation of source parameters from geodetic data inversion

We have analysed the deformation documented during unrest at the Campi Flegrei caldera, Italy, between 1981 and 2001. Via inverse modelling, we constrain the location, geometry and size of the source responsible for the continuing period of surface deformation. We present a critical re-evaluation of results from previously published models and for the first time invert post-1994 data to infer source parameters. Our evaluation is based on constraints from additional horizontal displacement data, mechanical properties of the country rocks, effects of volcanic surface loading and on other geophysical and geochemical observations. We invert leveling and tide-gauge data for a spherical point (Mogi-model) source, a penny-shaped crack and finally a prolate spheroid. Despite the good qualities of fit of both the Mogi-model and the penny-shaped source to the vertical displacement data, our critical evaluation of the implied source properties forces us to reject these models. We propose instead a vertical prolate spheroid located about 800 m East of Pozzuoli at a depth of 2.9 km (95% confidence bound 2.0 to 4.2 km) with an aspect ratio of 0.51 (95% bounds 0.37-0.69) as a more appropriate source model. This model best accounts for the criteria employed and the observed deformation between 1981 and 2001. Combined with results from the inversion of gravity change data (1982-1984) for the spheroidal source, we infer a hybrid nature of the source including both magmatic and hydrothermal components.