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.

International audienceInduced seismicity from anthropogenic sources can be a significant nuisance to a local population and, in extreme cases, can lead to damage to vulnerable structures. One type of induced seismicity that is of particular recent concern, and which in some cases is a limit on development of a potentially-important clean energy source, is that associated with geothermal power production. A key requirement for the accurate assessment of seismic hazard (and eventually risk) is a ground-motion prediction equation (GMPE), predicting the level of earthquake shaking (in terms of, for example, peak ground acceleration) given an earthquake of a certain magnitude at a particular distance. For geothermal-related seismicity, practically no such models currently exist and consequently the evaluation of seismic hazard in the vicinity of geothermal power plants is associated with high uncertainty. Various ground-motion datasets of induced and natural seismicity (from Basel, Geysers, Hengill, Roswinkel, Soultz and Voerendaal) were compiled and processed and the moment magnitudes for all events recomputed homogeneously. These data are used to show that ground motions from induced and natural earthquakes cannot be statistically distinguished. Empirical GMPEs are derived from these data and it is shown that they have similar characteristics to some recent GMPEs for natural and mining-related seismicity but the standard deviations are higher. Subsequently stochastic models, to account for epistemic uncertainties, are developed based on a single corner frequency and with parameters constrained by the available data. Predicted ground motions from these models are fitted with functional forms to obtain easy-to-use GMPEs. These are associated with standard deviations derived from the empirical data to characterize the aleatory variability. As an example, we demonstrate the potential use of these models using data from Campi Flegrei