Weighing the risk at Campi Flegrei

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A 4.3 magnitude earthquake hit Campi Flegrei, just west of Naples, on 27 September. It was the region’s strongest shock in 40 years, and part of a seismic sequence that has been rattling Campi Flegrei for several weeks. The government has earmarked €52 million for risk assessment and prevention.

The seismic sequence is providing scientists with more data to study ground displacement at Campi Flegrei, a recurrent phenomenon called bradyseism. In the mid-1980s there was another period of ground uplift and intense seismic activity that eventually abated and led to a long period of ground subsidence. Then a new uplift started in 2003 and is ongoing. The ground at Campi Flegrei has risen by 1.15 metres since 2005. Activity has been intensifying for several months now, surpassing 1,000 earthquakes per month since last August.

Two hypotheses have been proposed to explain bradyseism. In the first one, the hydrothermal system, which lies between three and four kilometres below the surface, heats up because of magma intrusions from the reservoir, around 8 kilometres below. “We can think about the rocks in the crust as a sponge, whose pores are filled by hydrothermal fluids that can expand their volume”, explains Micol Todesco, volcanologist at the National Institute of Geophysics and Volcanology (INGV) in Bologna. The volume increase would in turn lead to the observed soil uplift.

In the second hypothesis, a column of hot gases ascends from the deep magma and reaches the hydrothermal system, heating it up. “Today the scientific community substantially agrees that the current unrest is driven by the degassing of deep magmatic fluids”, says Giuseppe De Natale, volcanologist at the INGV who directed the Osservatorio Vesuviano between 2013 and 2016.

Roberto Moretti, geochemist at the University of Campania Luigi Vanvitelli, points to the chemical composition of the gases that come out of the many fumaroles of Campi Flegrei. “Since 2000 the proportion of carbon dioxide with respect to water vapour has been steadily increasing,” Moretti says. Only magma at larger depth can efficiently store CO2 and thus emit gasses that are rich with it. Also, seismological data suggests that there is only one magma reservoir at larger depths.

This does not mean that magma intrusions at shallow depths have never happened at the caldera. “It is quite possible that the sudden uplift of 1982-1984, which was also accompanied by strong seismicity that led to the evacuation of the 40,000 people from Pozzuoli, was in fact caused by the intrusion of small batches of magma,” Todesco observes. But after that the intruded magma spread horizontally, forming a thin foil, and gradually cooled.

The fact that the cause of the current uplift is not ascending magma, does not mean there will not be an eruption. “Tension is building in the crust which could eventually lead to more and more rocks fracturing, as the rising seismicity suggests,” De Natale explains, warning that if the fractures join together, an eruption could follow. “It would likely start with a violent explosion of water and minerals, but it could evolve because the pressure relief in the hydrothermal system could liberate the magma stored deeper below.”

Christopher Kilburn, a volcanologist at the University College London, developed a model to monitor the shift from an elastic regime, where rocks can bend and stretch to accommodate the growing pressure, to an inelastic one, where rocks fracture more and more and slipping starts along the faults. “The model correlates the number of local earthquakes to the amount of ground movement,” Kilburn explains. At the beginning, ground deformation will cause a small number of earthquakes, but as the stress accumulates in the crust, the same amount of ground deformation, with time, will cause an acceleration in the number of earthquakes.

In 2017 Kilburn, along with De Natale and Stefano Carlino of INGV, applied this model to Campi Flegrei showing that stress had been building in the system since the 1950s. In a more recent paper by Kilburn and collaborators, the analysis was updated to show that Campi Flegrei has now entered the inelastic regime. But the model cannot predict whether the ground movement will continue at the current rate, or at all. “There's no guarantee that this time the unrest is going to continue all the way up to rupture and, even if it does, this does not mean that an eruption will follow,” Kilburn comments. “It may simply allow accumulated gas to seep out more quickly and this might bring the unrest to an end.”

De Natale is not convinced of Kilburn’s latest analysis. “Data on ground uplift can be interpreted in a different way, and I believe they indicate the crust has not entered yet the inelastic regime,” he says. “If the crust had really overcome the critical point, the eruption would be very likely and very close in time.”

In the short term, De Natale is more concerned by the increase in seismicity. On 12 October the government approved a plan to check the stability of private and public buildings, and to update by early 2024 the evacuation plans in case of an eruption. But the plan does not set a deadline for the survey of buildings, which De Natale says should be done urgently.