What is bradyseism and why do earthquakes occur in Campi Flegrei?

Bradyseism : a word that has been part of the complex daily life of the Campi Flegrei for decades. Here, where the earth burns par excellence (in ancient Greek “ phlég? ” means “I burn”), for 80 thousand years we have had to deal with the instability of the soil: this is also why some point to the extreme anthropization of the area. Periods of uplift of the soil alternating with periods of subsidence: this is what is meant by “bradyseism”, a push from deep within that deforms the rocks, stresses them, until it reaches the breaking point. With the earthquake that represents the effect of the breaking of the rocks and the so-called seismic swarms that present a more or less long sequence of shocks.

The caldera and the volcanic field The Campi Flegrei are an active volcanic area west of Naples, which includes the municipalities of Bacoli, Monte di Procida, Pozzuoli, Quarto, Giugliano in Campania and part of the city of Naples. However, forget the classic idea of ​​a volcano, for example that of nearby Vesuvius, with a main active building that can erupt: here the caldera "embraces" several volcanic centers, some of which are also at sea, in the Gulf of Pozzuoli. Science speaks of an active volcanic field: some, with a colorful but scientifically imprecise expression, part of a "super volcano". And the caldera itself is the result of the repeated subsidence of a vast area caused by the collapse of the roof of the superficial magma reservoir following its emptying by at least two large eruptions, one that occurred 39 thousand years ago, and which went down in history as the infamous Campanian Ignimbrite, and another 15 thousand years ago. Processes of change still underway: according to experts, it is on the eastern external side of the Solfatara crater, affected by a significant change in activity since 2005, that the greatest probability of the opening of new eruptive vents exists.

What does “bradyseismic crisis” mean? Since 2005, the Campi Flegrei have been experiencing what is called a bradyseismic crisis: nothing profoundly different from other periods of recent instability, in particular those of 1969-72 and 1982-84, when many inhabitants of the area, especially those of the historic center of Pozzuoli and Rione Terra, were forced to abandon their homes. In particular, during the crisis of the 1980s, the uplift of the ground reached 9 cm per month, and even 1300 seismic events per month were exceeded. And today? What is happening? The uplift recorded at the Rione Terra Gnss station, considered the hotspot of bradyseism, is about 140 centimeters since 2005, of which about 22 since January 2024. Last February, 1813 earthquakes were recorded: certainly intense activity, which has led in recent months to two earthquakes of significant magnitude, 4.4, in May and in the night between yesterday and today.

The likelihood of an eruption

Phenomena that are part of the dynamics of the current crisis, as the INGV points out, and that do not currently have any correlations with an approach of the most feared scenario, the eruptive one, which would result in the evacuation of the red zone: in that case, precursor scenarios would be the magma rising to the surface, a circumstance that is not currently occurring . Probabilistic studies based on the past eruptive history of the Campi Flegrei say that the most probable eruptions are those of low magnitude (of the type of that of Monte Nuovo, in 1538). The most energetic events are considered less and less probable, up to Plinian type events, considered unlikely even if not with zero probability. The constant monitoring of geochemical parameters, in fact, shows the persistence of the warming trends, pressurization of the hydrothermal system and increase in the flow of gas, primarily carbon dioxide, on whose presence the attention of political decision makers had focused in recent weeks. And this is where the most important game is played: in the constant observation of what happens under the feet of the Phlegraean citizens through one of the most impressive seismic networks in the world, with a permanent network that includes 28 installation sites, and a mobile one, with more than 100 latest-generation stations. And the sea is also scanned, thanks to a multi-parametric network: it is called Medusa, it helps to understand what is happening even within the Gulf of Pozzuoli.