The strongest earthquake to shake the Campi Flegrei region in four decades did not catch geologists entirely by surprise. The September 27, 2023, magnitude 4.2 tremor is the latest symptom of a restless supervolcano that has been inflating and deflating for years. What changed this time is the number.
The Phlegraean Fields supervolcano sits west of Naples, inside a densely populated corner of the Campania region. Its name means “burning fields,” and for good reason. The ground here has been swelling since the 1950s, a process called bradyseism. As magma pushes upward from the chamber below, the earth rises. When pressure drops, it sinks. This constant motion stresses the crust, and stressed crust breaks into earthquakes.
Monday’s quake was the largest since the early 1980s, when a seismic crisis forced the evacuation of parts of the town of Pozzuoli. That crisis did not lead to an eruption. Neither did a similar event in 1970. Experts say an eruption is not imminent now. But the pattern is not comforting.
The supervolcano is part of a larger volcanic complex shaped by the collision of the Eurasian and African tectonic plates. They converge at a rate of several millimeters per year. That may not sound like much, but over millions of years it has built the Apennine Mountains and filled Italy’s geological record with violent events. The country has suffered more than 1,300 destructive earthquakes in the past millennium. Roughly 500 of those hit the central Mediterranean Sea.
Most seismic activity in Italy runs along the Alpine and Apennine mountain ranges. The Campi Flegrei system sits in the middle of that tectonic meat grinder. It is not a single volcano with a cone. It is a caldera, a collapsed depression roughly 12 kilometers wide, pocked with craters, fumaroles, and hot springs. The magma chamber beneath it is shallow. That means the ground above it can move quickly when pressure shifts.
The September 27 earthquake has prompted experts to call for building inspections in the immediate area. The concern is not that the volcano will blow tomorrow. The concern is that the frequent shaking may have weakened structures over time. Many buildings in the region are old. Some are built directly on unstable volcanic tuff. If a larger earthquake hits — and the history of the area suggests it can — those buildings may not hold.
Italy’s earthquake record is long and lethal. The 1908 Messina earthquake killed tens of thousands. The 2009 L’Aquila earthquake killed more than 300. The 2016 Amatrice earthquake killed nearly 300 more. Each of those happened in the Apennines, along the same fault system that feeds the Campi Flegrei. The difference is that those were pure tectonic quakes. The Campi Flegrei quakes are volcanic in origin. That does not make them weaker. It makes them harder to predict.
The Phlegraean Fields last erupted in 1538, creating a small cone called Monte Nuovo. That eruption was preceded by decades of ground uplift and seismic swarms. The current uplift phase began in 2005. The ground in Pozzuoli has risen by more than a meter since then. The number of earthquakes has increased steadily. The magnitude 4.2 event on September 27 is the strongest in that sequence so far.
Seismologists monitor the caldera with networks of GPS stations, tiltmeters, and gas sensors. They track the composition of gases escaping from fumaroles. When the ratio of carbon dioxide to sulfur dioxide changes, it can signal magma moving upward. Those readings have been watched closely for months.
For now, the warning is about buildings, not evacuation. The supervolcano is complex. It is dynamic. It is not quiet. And the ground beneath Naples keeps reminding everyone that it is still alive.
