Internal structure of the Phlegraean Fields at the origin of bradyseism revealed

A new study published in Nature Communications offers the most detailed picture yet of the underground structures that control the Campi Flegrei volcanic system, one of the most dangerous and densely populated in Europe. The research was led by Professor Aldo Zollo and features Dr. Grazia De Landro, both of the Seismology Laboratory of the Department of Physics at Federico II University, as first author.

The study is the result of a major international scientific collaboration involving Stanford University (USA), Université Grenoble Alpes (France) and the geophysical technology company ALomax Scientific.

Using a combination of high-resolution seismic imaging, rock physics, and characterization of samples taken by laboratory coring, as well as sophisticated geophysical models, the research team revealed that it is not magma, but pressurized fluids deep within the Phlegraean Fields that are driving the current instability.

Scientists confirmed the presence of three main underground structures and, for the first time, identified how their spatial and temporal dynamics are closely linked to the evolution of seismic activity:

• A fibrous surface layer (1-2 km deep), which acts as a natural seal, capable of folding under stress, but can eventually give way, causing surface micro-earthquakes.
• A gas-enriched intermediate reservoir (2-4 km depth), containing a mixture of pressurized water, steam and carbon dioxide, which exerts strong thrusts on the overlying rocks.
• A deep, stable basement consisting of carbonate rocks with a concave downward shape, subject to thermo-metamorphic processes that can release non-magmatic gas.

This structural configuration explains the seismicity pattern and ground uplift observed in the last decade. The absence of magma in the top 4.5 km depth suggests that the current hazard is dominated by pore pressure buildup in the gas reservoir and activation of pre-existing faults.

"Our laboratory work shows that the sealed reservoir rock deforms exponentially as pore pressure increases as water transitions from vapor to liquid state," explains Tiziana Vanorio of Stanford University. "The natural seal covering the reservoir imparts ductility to the entire system, but it also represents the first structure that is destined to fail under increasing pressure inside the reservoir."

"This study exemplifies how the synergy between geophysics, geology and numerical modeling can solve a long-standing geological conundrum," says Jean Virieux of Université Grenoble Alpes. "The results set a new benchmark for volcanic monitoring and multi-hazard risk assessment."

"Using state-of-the-art seismic analysis tools, we were able to trace the outgassing column and stress transmission pathways with unprecedented clarity," says Anthony Lomax, founder of ALomax Scientific. "This level of resolution changes the rules of the game for real-time risk mitigation."

"As a researcher born and raised in Naples, contributing to the safety of my city through science has been both a professional and personal mission," comments Grazia De Landro, first author of the study. "This work demonstrates how basic research can have a real impact on risk assessment and community resilience."

According to Professor Zollo, these findings are key to improving warning systems and risk models, which must consider the possibility that multiple natural hazards-such as ground deformation, earthquakes, gas emissions and volcanic eruptions-could occur both simultaneously and at different times in time. Such in-depth understanding could improve land management efficiency in the future.

"Understanding the internal mechanisms of deep structures in the Phlegraean Fields allows us to design a flexible and dynamic hazard mitigation strategy," Zollo concludes. "This approach is based on multi-hazard scenarios, constrained by real-time data from monitoring networks, with the ultimate goal of improving protection for the hundreds of thousands of people living above this active volcano."