Fluid migration is the source of deformation at Campi Flegrei caldera (Italy).

Campi Flegrei, a densely populated volcanic caldera immediately west of Naples (Southern Italy), has been the site of significant unrest for the past 2000 years. The caldera floor of Campi Flegrei rose 1.7 m between 1968 and 1972, and subsided 0.22 m between 1972 and 1974. From 1982 to 1984 the caldera floor rose 1.8 m, then subsided again at an average rate of 5 cm/yr without an eruption or clear evidence of magma migration. Such episodic ground deformation, common to several calderas, is not well explained in terms of the simple inflation and deflation of a magma body. Alternative models proposed include hot fluid migration or intense magma degassing. In this work, we bound the location, geometry and density of the fluid reservoirs by inverting levelling, trilateration and gravity measurements collected between 1980-84, and 1990-95. Given the difference in density between silicate melts (~ 2500 kg/m3) and hydrothermal fluids (~ 1000 kg/m3), this approach can help to distinguish between possible sources of caldera unrest. We find that the best fitting source for the 1980-84 inflation episode is a horizontal penny-shaped crack, 2.8 to 4.0 km deep (95% bounds), radius between 1.1 and 2.7 km, and density 142 to 1115 kg/m3. The source best fitting the deflation period (1990-95) is a vertical spheroid, 1.8 to 2.4 km deep (95% bounds), aspect ratio of 0.35 to 0.62, and density between 849 and 1062 kg/m3. We can interpret both the uplift and the subsidence phase in terms of fluid flow to and from the caldera hydrothermal system. These results represent clear evidence that geothermal fluid migration plays a fundamental role in caldera unrest, and have important consequences for eruption forecast at these areas.