Real-time forecasts of unrest at Campi Flegrei, Southern Italy, 2013-2023.

Large volcanic calderas can take decades or more to return to activity after centuries in repose. Their reawakening frequently triggers several, intermittent episodes of unrest before magma finally erupts. The non-eruptive episodes can divide scientific opinion about the cause of unrest and its outcome. The Campi Flegrei caldera is a remarkable example. Fifteen kilometres across, it is Europe’s largest active caldera and supports a population of more than 360,000 people to the west of Naples in southern Italy. It has been episodically restless since 1950, for the first time in four centuries. Uplifts in 1950-52, 1969-72, 1982-84 and since 2004 have created a caldera-wide bulge that, close to its centre, has raised the coastal town of Pozzuoli by more than 4 m, while tens of thousands of small earthquakes have shaken the volcano to depths of 4 km.

The behaviour is consistent with repeated rupture of the crust. The most recent rupture developed between 2013 and 2023. The rate of local volcano-seismic earthquakes evolved from an exponential increase, through a constant rate to a hyperbolic increase with time. This is the classic pre-rupture sequence for elastic-brittle rock being extended under an approximately constant rate of supplied stress.

We used changes in the sequence to forecast the approach to rupture in real-time. Observations followed the forecast trend through the first nine months of 2023, by which time the increase in seismicity was expected to continue until early 2024. Instead, the rupture sequence culminated in October 2023, since when the rates of seismicity have decayed. The end of rupturing was brought forward by an increase in the number of larger magnitude earthquakes, which accelerated rates of stress loss compared with previous months. Uplift had also slowed by December 2024.  Each rupture sequence increases the amount of damage in the crust. Other factors being equal, therefore, if any new magma can reach the shallow crust in the near future, it is likely to meet a lower resistance to eruption than has been the case since unrest began in 1950.