Campi Flegrei (Phlegraean Fields) is a volcanic area bordering the city of Naples, in southern Italy. The caldera of Campi Flegrei was shaped by two major volcanic eruptions, about forty-thousands and ten-thousands years ago.

A prominent volcanic unrest phenomenon of the caldera is the slow, yet continuous, uplift and subsidence of the ground, such that Baia, a major town of the Roman empire, is now submerged. This phenomenon goes by the name of bradyseism, the etymology of which is precisely slow earthquake. It occurs as a bell-shaped deformation of the inner caldera, which has a radius of a few kilometers, roughly centered at the town of Pozzuoli. The origin of the bradyseism has been attributed to the action of pressure changes caused by magma intrusions at shallow depth and/or to the response of the shallow hydro-thermal system to changes in pore pressure and fluid content. However, the causative relation between magma movements at depth and ground deformation is still debated.

In recent years, there has been a steady rise in ground level since 2011. This uplift accelerated in 2018, reaching rates of about 2 cm/month in 2024, and resulting in about 9000 recorded earthquakes, most of which since 2022, with peaks of about 1000 earthquakes per month.

National census data enable estimating, in the deforming area about 85,000 inhabitants and 15,000 buildings. Moreover, the Campi Flegrei area is borderline to the city of Naples, the third largest by population in Italy. The building stock, as for most of the country, is designed with now-obsolete seismic codes or without any seismic provision. In other words, buildings are not expected to withstand, with the same level of safety, the design ground motions from the most recent  building code.

The increasing seismicity observed over time sparked public concern, which lead issuing a national government decree allocating resources to consider seismic risk mitigation; e.g., buildings’ seismic retrofitting. Risk mitigation via reduction of seismic structural vulnerability requires the provision of design ground motion (in terms of ), which—in turn—is related to the magnitudes that can occur in the area, their exceedance probability, and the shaking propagation in the region. This was the question of the local government of the Naples’ metropolitan area to the University of Naples Federico II in late 2023.

In reaction to this request, at the end of 2023, the rector established a university task force on Campi Flegrei. The interdisciplinary group of earthquake engineers, physicists, and geologists from four  departments, investigated the magnitude potential, the exceedance probabilities of the reference magnitudes and evaluated the effectiveness of mitigation strategies for the resulting seismic risk.

To evaluate the magnitude potential in the area it was first necessary by geologist to consider fault mapping and geomorphological inference together with refined recent earthquakes’ relocation, to identify potential sources. Subsequently, geophysical stress-drop analysis of the recorded events, together with fracture mechanics enable establishing the magnitude potential of the sources. It emerged that the reference moment magnitudes for the largest earthquakes in the caldera are in the range 4.4-5.1, with the largest one being considered a conservative upper bound.

Because it cannot be excluded that larger earthquakes occur in the area, in the case of unknown faults or ruptures extending beyond the size of the known sources, it was needed to investigate the exceedance probabilities of the reference magnitudes. These probabilities were computing using the state-of the art of so-called operational earthquake forecasting. It was found that these probabilities are strongly time-variant and related to the observed seismicity, which in turn depends on the ongoing uplift.

An earthquake intensity propagation model, which was also developed based on the recent observed seismicity, enabled the earthquake engineering component to assess that the minimum magnitudes expected to cause exceedance of the ground motions retrofitted construction must withstand by design, are generally larger than the reference magnitudes, which means that structural retrofitting could be a viable risk mitigation strategy. To confirm this conclusion, also taking into account the possible exceedance of the reference magnitude, the fatality risk reduction in reinforced concrete buildings, implied by the safety levels of new constructions was also assessed. It was shown that the safety standards of the current building code would reduce the risk by more than seventy percent.

This study, the results of which were summarized in an article entitled Seismic Risk Mitigation at Campi Flegrei in Volcanic unrest, published in Nature Communications, represents an example of rapidly assembling the, so-called, best available science and most-updated data to develop a multidisciplinary effort to possibly aid public decision making in the interest of the exposed community.