According to a new study by researchers from UCL (University College London) and the Italian National Research Institute of Geophysics and Volcanology (INGV), the Campi Flegrei volcano in southern Italy has weakened and is more prone to eruption, making an eruption more likely.

The volcano, which last erupted in 1538, has been restless for over 70 years, with a biannual eruption of unrest in the 1950s, 1970s and 1980s, and a slower phase of unrest over the past decade. During these periods, tens of thousands of small earthquakes occurred, and the coastal city of Pozzuoli rose about 4 meters, about the height of a double-decker bus.

In a new study published in the journal Communication World and EnvironmentHe used a volcanic rupture model developed at UCL to interpret earthquake patterns and ground elevation, and concluded that parts of the volcano were stretched almost to the breaking point.

Lead author Professor Christopher Kilburn (UCL Earth Sciences) said: “Our new work confirms that Campi Flegrei is approaching rupture. However, this does not mean that an eruption is guaranteed. A rupture can open a crack in the crust, but still for an explosion to occur. The magma has to be pushed into the right place.”

“This is the first time we have applied our model based on rock failure physics to any volcano in real time.”

“We first used the model in 2017 and since then Campi Flegrei has behaved as we predicted, with the increase in small earthquakes indicating downward pressure.”

“We will now need to adjust our procedures to assess the possibility of opening new pathways for magma or gas to reach the surface.”

“This study is the first of its kind to predict the eruption of an active volcano. This marks a step forward in our goal of improving eruption forecasts around the world.”

Dr Nicola Alessandro Pino, of the Vesuvius Observatory representing INGV in Naples, said: “Our results show that parts of the volcano are weakening. This means that it can break even if the stress that broke it is less than before. During the last crisis 40 years ago.”

Campi Flegrei is the closest active volcano to London. It is not an open volcano because instead of turning into a traditional mountain, it is shaped like a slight depression (and therefore known as a caldera) 12-14 km (7.5-8.5 miles) in diameter. This explains why 360,000 people now live on its roof.

In the last ten years, the soil under Pozzuoli has risen by about 10 cm each year. Continuous small earthquakes were also recorded for the first time since the mid-1980s. More than 600 people were registered in April, the highest figure for the month.

The disturbance was caused by the movement of liquids about 3 km below the surface. Some of the fluids may be molten rock or magma, and some may be natural volcanic gas. The final stage of the unrest is likely due to igneous gas seeping through cavities in the rock and filling the 3km-thick crust like a sponge.

Earthquakes are caused by the displacement of faults (cracks) due to the stretching of the earth’s crust. The pattern of the 2020 earthquakes shows that the rock is responding inflexibly, collapsing instead of bending.

Dr Stefania Danesi from INGV Bologna said: “We can’t see what’s going on underground. Instead, we must decipher the clues that the volcano gives us, such as earthquakes and riots.”

The team explained in their paper that since the 1950s the impact of disturbances has been cumulative, meaning that relatively weak signals such as a slower ground rise and fewer earthquakes can come before a possible eruption. Such was the case with the 1994 eruption of the Rabaul caldera in Papua New Guinea, followed by small earthquakes one-tenth less frequent than the crisis a decade earlier.

The current tensile strength of Campi Flegrei (the maximum stress a material can withstand before it breaks when stretched) is probably about a third of what it was in 1984, the researchers said.

The team stressed that the explosion was not inevitable. Vesuvius Observatory’s Dr. Stefano Carlino explained: “This is the same for all volcanoes that have been silent for generations. Campi Flegrei may enter a new routine of gentle rise and fall, or simply come back to rest, as seen with similar volcanoes around the world. We can’t say for sure what will happen just yet. The important thing is to be ready for all consequences.”

Professor Kilburn and colleagues will now apply the UCL volcanic eruption model to other volcanoes that reawaken after a long period of time to establish more reliable criteria for determining the probability of an eruption. Eruptions are currently predicted based on statistics specific to each volcano rather than basic principles that can be applied to multiple volcanoes.