GNGTS 2022 - Atti del 40° Convegno Nazionale

92 GNGTS 2022 Sessione 1.2 THE DYNAMIC EVOLUTION OF LARGE CALDERAS: CURRENT UNREST AT CAMPI FLEGREI, SOUTHERN ITALY S. Carlino 1 , C. R.J. Kilburn 1,3 , S. Danesi 2 , N.A. Pino 1 1 INGV-Osservatorio Vesuviano, Naples, Italy 2 INGV-Sezione di Bologna, Italy 3 UCL Hazard Centre, Department of Earth Sciences, UCL, London, UK Campi Flegrei is the largest active caldera in Europe. About 12-15 km across, it extends west from the suburbs of Naples to the Tyrrhenian Sea. About a third is partially submerged beneath the Bay of Pozzuoli; the remaining two-thirds are home to more than 360,000 people. The volcano has been restless since 1950. It last erupted in 1538 after an interval of about 3,000 years (Smith et al. , 2011). Previous intervals have been as short as decades or centuries (Smith et al. , 2011), so that a return to eruption after nearly 500 years is a realistic possibility. Caldera- wide ground movement has raised the coastal town of Pozzuoli, near the centre of greatest uplift, by more than 4 m and twice triggered evacuations of about 40,000 people (Barberi et al. , 1984). The current uplift began in 2005 with a previously unseen style of behaviour. The movement has been attributed to the ascent of magma (D’Auria et al. , 2015) or of magmatic gas (Chiodini et al. , 2021) to depths of about 3 km. Both interpretations assume a change in the external conditions feeding the volcano’s magmatic system. We propose that the current uplift is instead being driven by internal changes in Campi Flegrei’s crust as sections of faults are partially reopened. The re-opening favours new pathways to the surface for magmatic fluids, ranging from escaping gases to magma itself. Campi Flegrei’s current unrest began in 1950 and has developed in four episodes (Del Gaudio et al. , 2010; Kilburn et al. , 2017; Troise et al. , 2019). Measured at Pozzuoli, the first three episodes produced comparable amounts of uplift over similar durations: c. 74 cm in 1950-52, 159 cm in 1970-72, and 175 cm in 1982-84. The fourth episode has continued for nearly four decades, first with twenty years of subsidence that lowered Pozzuoli by 93 cm, and then by seventeen years of uplift that by the end of February 2022 had returned the surface to its 1984 position. The amounts and timescales of the first three episodes are consistent with the intrusion of magmatic sills (Woo & Kilburn, 2010); those of the fourth, and ongoing, episode are consistent with the permeable flow of a combination of magmatic gas and meteoric water (Kilburn et al. , 2017; Troise et al. , 2019). We propose that the complete sequence describes the progressive stretching of the crust by repeated intrusions until its permeability had increased sufficiently for a major change in the flow of pore fluids. Our interpretation is supported by new data, which show that, by 2021, the regime of crustal deformation had changed from quasi-elastic (elastic with subordinate fracturing and faulting) to inelastic (dominated by fracturing and faulting). The change marks an evolution towards crustal rupture and has been observed at several volcanoes, including large calderas (Robertson and Kilburn, 2016; Kilburn, 2018). Rupture does not guarantee a magmatic eruption. However, it does favour an increase in the rate of escape of pressurized fluids that may trigger phreatic explosions. The presence of a new rupture also increases the possibility that any new magma reaching shallow depth will erupt, rather than intrude as had occurred between 1950 and 1984. As a result, it is unlikely that the volcano will replicate its behaviour during the past 70 years of unrest, so that new scenarios for emergency response must take account of its dynamical and structural evolution. References Barberi F., Corrado G., Innocenti F., Luongo. and G.; 1984: Phlegraean Fields 1982-1984: brief chronicle of a volcano emergency in a densely populated area . Bull. Volcanol. , 4: 175-185. Chiodini G., Caliro S., Avino R., Bini G., Giudicepietro F., De Cesare W., Ricciolino P., Aiuppa A.,