on August 7, 2025, 5:39 pm
Pressurised Fracture Beneath Campi Flegrei Responds to Earthquakes in Ongoing Unrest
By
David Freeman -
August 7, 2025
The Campi Flegrei caldera in southern Italy has remained under close scientific scrutiny for over a decade due to its slow but accelerating deformation and frequent low-magnitude earthquakes. Now, a newly published peer-reviewed study has introduced a development that raises serious concerns for one of the most complex and densely populated volcanic systems in Europe. For the first time, scientists have identified and confirmed the presence of a pressurised underground fracture beneath the town of Pozzuoli that is not only storing gas but actively responding to seismic activity. The implications of this finding are substantial, both in terms of understanding the internal mechanics of the caldera and the current condition of the system.
The fracture in question lies at an approximate depth of 3.6 kilometres, dipping west-southwest beneath the area of maximum uplift. It is nearly one kilometre in length, inclined, and located directly beneath the Solfatara and Pisciarelli fumarolic zones. These areas have seen a marked increase in degassing over the past two decades, including a significant rise in CO₂ emissions that has continued to accelerate. According to the study, this underground feature has remained active since at least 2018 and has now been confirmed to resonate during moderate seismic events. The term resonance in this context refers to very long-period (VLP) oscillations detected in the seismic signal, lasting up to two minutes in duration, with a dominant frequency of 0.114 Hz. This seismic behaviour was not previously observed at Campi Flegrei.
The new study provides high-resolution analysis of over one hundred seismic events between 2018 and 2025, using waveform similarity, moment tensor inversions, and resonance modelling to identify this previously undetected structure. It establishes that the fracture behaves like a resonator filled with gas or fluid, which begins vibrating in response to seismic waves passing through the system. This behaviour was recorded across multiple earthquake clusters within the caldera, but the resonance was strongest when activity occurred along the edges of the seismogenic zone. These responses were not isolated or random. The study presents consistent waveform patterns over several years, all pointing back to the same fracture source. The geometry, depth, and orientation of the structure have been derived through inversion methods and waveform analysis, offering a degree of clarity that has not previously been available for this region.
Importantly, the fracture is not simply a passive feature. It is storing pressure and interacting with both the shallow fault network and the deeper magmatic system. This level of mechanical coupling has been directly measured, not inferred. The June 30, 2025 magnitude 4.6 earthquake was the strongest recorded in the area since the current unrest began. It triggered a clear VLP signal originating from this fracture. The authors of the study link this response to direct energy transfer from the earthquake into the gas-filled structure, causing it to oscillate. This suggests that the fracture is not only mechanically active but also tuned to specific energy thresholds. It reacts to disturbances, which means it has the potential to influence broader behaviour within the system.
The overall structure of Campi Flegrei is known to be complex. The caldera spans roughly 12 kilometres in diameter and has formed through multiple collapse events over the past 15,000 years. The region experienced a significant bradyseismic crisis in 1982 to 1984, which involved rapid uplift and intense seismicity, but did not result in an eruption. Since then, the system entered a long period of deflation and relative quiet until around 2005, when uplift began again. By early 2025, the total measured ground uplift had reached approximately 1.4 metres. This deformation has been accompanied by an increase in earthquake frequency and magnitude. The rate of seismicity rose sharply in 2024 and continued through 2025, with more than one thousand earthquakes per month during peak periods. Most of these earthquakes were volcano-tectonic in nature and occurred at depths between 1.5 and 4 kilometres.
This increase in activity led to several studies re-evaluating the state of the system, and the newly published work provides one of the clearest indications to date that the pressure within the caldera is not just building, but moving. The presence of a fracture that connects the deeper magma source to the surface vent fields, while also resonating during earthquakes, confirms that the system is fully coupled across multiple layers. Mechanical stress, pressurised fluid pathways, and brittle fault zones are now interacting in a way that has not previously been confirmed at Campi Flegrei.
Another significant detail revealed in the study is that the fracture’s resonance frequency has remained constant over the entire observation period. From 2018 to 2025, the dominant frequency of 0.114 Hz did not shift, suggesting that the structure’s geometry and internal fluid properties have remained stable. This stability means the fracture is not leaking or adjusting to reduce pressure. It is instead holding that pressure, and continuing to vibrate when triggered by seismic waves. The volume of the structure has also been estimated based on the seismic moment of the strongest VLP event, placing the resonator’s size at roughly 220,000 cubic metres. This represents less than ten percent of the daily CO₂ output measured at the surface, indicating that the fracture does not serve as the primary release mechanism for gas, but rather a constrained and potentially volatile storage zone.
The fracture’s orientation also adds context to its risk potential. It dips at approximately 60 degrees toward the southwest, and its upper tip sits just beneath the most uplifted zone near Solfatara. This location places it beneath the densest portion of the urban environment, where infrastructure and population density are highest. If stress continues to build within the structure, and the system continues to respond to earthquakes in this way, the risk of destabilisation cannot be dismissed.
The study also maps out six main earthquake clusters within the caldera, and confirms that the largest magnitude events tend to occur closer to this fracture. The resonance is most often triggered by earthquakes at the outer edges of the seismically active region, particularly those associated with faults dipping inward toward the centre of the caldera. These outer clusters only became active during the later stages of the ongoing unrest, suggesting a progression from internal deformation toward broader structural engagement. In simpler terms, the caldera is not only inflating from below but is also now activating stress zones along its rim, with the pressurised fracture in between acting as both a recorder and participant in the process.
The paper also draws attention to the overall configuration of the subsurface system. Seismic tomography and geodetic modelling have already identified a low-permeability caprock layer at around 1.5 to 2 kilometres depth, sitting above the proposed magma reservoir. This layer likely inhibits upward gas flow and forces fluids to accumulate and divert laterally. The fracture sits beneath this layer, suggesting it operates below the primary cap and acts as a bypass or pressure release structure under certain conditions. It may be one of the only vertical channels currently connecting the deeper pressurised zone to surface vent fields, which increases its significance in the system.
The fracture has also been linked to magnetotelluric data collected in recent years. A previous study identified a vertical zone of high resistivity beneath the Solfatara area, interpreted as a potential pathway for supercritical fluids. The location, geometry, and depth of that resistive body align closely with the fracture confirmed in this new study. That correlation adds further support to the idea that this is not an isolated cavity or short-lived anomaly, but a key structural feature of the modern caldera system.
While the study refrains from drawing forward-looking conclusions, the data it presents leaves little room for interpretation regarding the current state of activity. Campi Flegrei is under pressure. That pressure is being stored within an inclined fracture that is mechanically connected to seismic events and surface gas emissions. The fracture has remained stable in geometry and resonance over seven years, confirming that it is not releasing energy but absorbing and reacting to it. Earthquakes are now directly affecting this structure, and its presence confirms that deep and shallow systems are behaving as one.
The interaction between moderate seismic events and the resonance behaviour of this fracture introduces a new layer of complexity for monitoring and risk assessment at Campi Flegrei. Traditional warning signs have often focused on ground uplift and shallow earthquake frequency, but this new data indicates that subsurface resonance within a fluid-filled structure can act as a real-time indicator of internal stress response. The more often this resonance is triggered, the more strain is confirmed to be present in the structure.
This development arrives during a phase of peak unrest. Earthquake magnitudes are higher than previously recorded during this inflation cycle. Surface uplift is ongoing and has reached new highs. Gas emissions have surpassed 4,000 tons per day at times, with no sign of long-term decline. The fracture’s role in linking these signals makes it one of the most critical features currently active within the Campi Flegrei caldera.
This study, published July 31, 2025 in Communications Earth & Environment, provides the most detailed mapping yet of the mechanical and fluid behaviour of a pressurised fracture beneath Campi Flegrei. Its response to seismic energy, its long-term activity, and its location beneath populated areas confirm it is not a background feature of the system. It is part of the ongoing unrest, and its behaviour is now tied directly to the forces reshaping the caldera.
Source Paper:
Rapagnani, G., Cesca, S., Saccorotti, G., Petersen, G., Dahm, T., Bianco, F., & Grigoli, F. (2025). Coupled earthquakes and resonance processes during the uplift of Campi Flegrei caldera. Communications Earth & Environment, 6, Article 607. https://doi.org/10.1038/s43247-025-02604-7
ChristopherBlackwell
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