We model the location, geometry and density of the source of the recent geological unrest at Campi Flegrei caldera (Italy) by inverting levelling, trilateration and gravity measurements collected between 1980 and 1995. The best fitting source for the 1980–84 inflation is a horizontal penny‐shaped crack with a density 142 to 1115 kg/m3. The source best fitting the deflation period (1990–95) is a vertical spheroid with density between 902 and 1015 kg/m3. These results exclude the intrusion of magma, and indicate the migration of fluid to and from the caldera hydrothermal system as the cause of ground deformation and consequent unrest.

No abstract

A two-dimensional active seismic experiment was performed on Mount Vesuvius: Explosive charges were set off at three sites, and the seismic signal along a dense line of 82 seismometers was recorded. A high-velocity basement, formed by Mesozoic carbonates, was identified 2 to 3 kilometers beneath the volcano. A slower ( P -wave velocity V P ≃ 3.4 to 3.8 kilometers per second) and shallower high-velocity zone underlies the central part of the volcano. Large-amplitude late arrivals with a dominant horizontal wave motion and low-frequency content were identified as a P to S phase converted at a depth of about 10 kilometers at the top of a low-velocity zone ( V P < 3 kilometers per second), which might represent a melting zone.

Campi Flegrei caldera, including the extremely urbanised city of Naples, is the most risky volcanic area in the World. The last eruption in the area (1538) occurred at the end of some decades of ground uplift, superimposed to secular subsidence. During the last four decades, it experienced a huge uplift phase, reaching about 3.5 m in 1985, when a subsidence phase started. Recent geodetic data demonstrate that such a subsidence phase has terminated, and a new uplift episode started in November 2004, with a low but increasing rate leading to about 0.04 m of uplift till the end of October 2006. A new indicator, based on the monitoring of maximum horizontal to vertical displacement ratio with continuous GPS, indicates that this uplift is likely to be associated with input of magmatic fluids from a shallow magma chamber. The method is promising to monitor magma intrusion processes, at this and other volcanoes.

Magmatic carbon dioxide (CO2) degassing has been documented before the 31 March 2000 eruption of Usu volcano, Hokkaido, Japan. Six months before the eruption, an increase in CO2 flux was detected on the summit caldera, from 120 (September 1998) to 340 metric tons per day (September 1999), followed by a sudden decrease to 39 metric tons per day in June 2000, 3 months after the eruption. The change in CO2 flux and seismic observations suggests that before the eruption, advective processes controlled gas migration toward the surface. The decrease in flux after the eruption at the summit caldera could be due to a rapid release of CO2 during the eruption from ascending dacitic dikes spreading away from the magma chamber beneath the caldera.

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.

In the last four decades, Campi Flegrei caldera has been the world's most active caldera characterized by intense unrest episodes involving huge ground deformation and seismicity, but, at the time of writing, has not culminated in an eruption. We present a careful review, with new analyses and interpretation, of all the data and recent research results. We deal with three main problems: the tentative reconstruction of the substructure; the modelling of unrest episodes to shed light on possible pre-eruptive scenarios; and the probabilistic estimation of the hazards from explosive pyroclastic products. The results show, for the first time at a volcano, that a very peculiar mechanism is generating episodes of unrest, involving mainly activation of the geothermal system from deeper magma reservoirs. The character and evolution of unrest episodes is strongly controlled by structural features, like the ring-fault system at the borders of the caldera collapse. The use of detailed volcanological, mathematical and statistical procedures also make it possible to obtain a detailed picture of eruptive hazards in the whole Neapolitan area. The complex behaviour of this caldera, involving interaction between magmatic and geothermal phenomena, sheds light on the dynamics of the most dangerous types of volcanoes in the world. et al. 2001), while the subsequent largest event, which generated the Neapolitan Yellow Tuff

No abstract