Ongoing unrest at Campi Flegrei caldera: increasing potential for a phreatic/hydrothermal event within the Accademia-Solfatara area?

Explosive eruptions can occur with different, mechanisms, energy and magnitude, and consequently their impact may involve more or less large areas of the affected territory. Phreatic explosive eruptions are considered to be among those with the lowest magnitude and impact, although unfortunately they have recently caused casualties in several areas of the world. Their unpredictability and the impossibility, to date, of identifying precursor phenomena useful for specific monitoring of these events, greatly increases the volcanic hazard associated with them.

Volcanoes characterized by active surface geothermal systems are environments particularly prone to favor the possibility of phreatic and/or hydrothermal explosive events occurring. Many active calderas host widespread fumarolic and hydrothermal activity as a surface expression of the interaction between hot fluids of deep magmatic origin, surface aquifers, and fault or fracture zones in the shallow crust. The Campi Flegrei caldera has generated during its recent eruptive history phreatic explosive events, concentrated principally in its central sector. The Solfatara volcano was delineated as a result of the succession of phreatic events of varying energy that led to the formation of a maar/diatreme-type structure.

The Solfatara-Accademia area represents the most active hydrothermal sector of the Campi Flegrei caldera, characterized by intense fumarolic activity, shallow seismicity and localized deformation. Past phreatic and hydrothermal events, together with the behavior observed during recent unrest episodes, indicate that this sector may evolve through processes partly decoupled from caldera-scale dynamics, making it a key target for investigating shallow hydrothermal instability. Three-dimensional resistivity models obtained by magnetotelluric (MT/AMT) surveys, performed within INSIDE OUT project, in the framework of the INGV–MUR project Pianeta Dinamico, delineate a complex near-surface architecture characterized by laterally extensive conductive layers, interpreted as clay-rich, low-permeability caps, locally disrupted by resistive structures that connect deeper geothermal reservoirs to the surface. These features define preferential pathways for fluid and gas ascent and highlight strong lateral heterogeneities over short spatial scales.

Time-lapse MT observations reveal temporal variations in electrical resistivity within the uppermost crust, interpreted as changes in fluid circulation, gas flux and permeability conditions in the shallow hydrothermal system. Gravity monitoring showed largest gravity changes in time-span 2021-2025 located in Accademia-Solfatara area, while gravity time-series may suggest cycles of mass accumulation & discharge (with relevant amplitudes). These resistivity and gravity variations spatially correlate with zones of enhanced fumarolic activity, clusters of shallow earthquakes, and localized deformation anomalies in the Accademia sector. The observed patterns suggest a dynamic interplay between fluid ascent, diffusion and self-sealing processes within shallow cap layers, potentially leading to transient pressure build-up at shallow depths, complementing caldera-scale observations and improving the characterization of potentially unstable shallow zones in densely urbanized areas of Solfatara-Accademia.

Integration of structural observations, geophysical imaging and monitoring data, with particular emphasis on time-lapse magnetotelluric and gravity surveys indicate that the Solfatara-Accademia area represents a structurally controlled, shallow hydrothermal domain whose evolution may play a primary role in the development of localized phreatic or hydrothermal explosive activity as possible eruptive scenario for the investigated area.