Southern Italy’s Campi Flegrei caldera, a volcanic region notorious for its restless behavior, has become the center of groundbreaking research that may offer a new approach to managing volcanic unrest and mitigating earthquake risks. Since 2022, this region has experienced an intensification of earthquake swarms, unsettling hundreds of thousands who live above its simmering geothermal reservoir. Now, a team of Stanford scientists has illuminated the underlying processes contributing to this cyclic seismic activity, revealing a novel mechanism that challenges long-standing beliefs about volcanic unrest.
Unlike common theories that attribute ground deformation and seismicity to magma ascending toward the surface, the researchers propose that water pressure buildup in a sealed geothermal reservoir beneath the town of Pozzuoli drives the observed geological phenomena. These findings arise from a multifaceted study involving subsurface imaging, laboratory experiments, and analysis of two major episodes of unrest in Campi Flegrei: the 1982–1984 uplift and earthquake sequence and the ongoing seismic activity from 2011 to 2024. The comprehensive approach provides a clearer understanding of how groundwater influences fault activity, the dynamics of deformation, and the generation of earthquakes.
At the heart of this study is the identification of overpressure within the geothermal reservoir as the key driver of unrest. According to senior author Tiziana Vanorio, an associate professor of Earth and planetary sciences at Stanford, fluid pressure rises as water and vapor accumulate under a fibrous, mineral-rich caprock that effectively seals the system. This caprock, composed of certain mineral fibers, behaves like a natural sealant; it’s capable of deforming under stress and healing fractures by mineral precipitation. Such processes create a closed environment in which pressure can build extensively over time, ultimately causing the rock to fracture and release this stored energy as seismic events.
Laboratory simulations conducted in Vanorio’s Rock Physics and Geomaterials Lab provided a crucial glimpse into the mechanisms of crack sealing. By replicating subsurface conditions with volcanic material and brine in a hydrothermal vessel heated to geothermal temperatures, the team observed rapid formation of mineral fibers and the self-healing of rock fractures. These experiments demonstrated that even minor breaks in the caprock patch up quickly, effectively trapping fluids and promoting pressure buildup. When this pressure reaches a critical threshold, sudden fracturing occurs, releasing water that rapidly flashes into steam, generating explosive bursts and characteristic booming sounds recorded in the region.
Tomographic imaging of the subsurface, developed by co-author Grazia De Landro, further confirmed this model by visualizing earthquake distributions and fault development over decades. The data reveal that seismicity begins in a relatively shallow layer, about one mile beneath the surface within the caprock itself, and progressively deepens over time—contrasting starkly with expectations under the magma-driven model where earthquakes would ascend from greater depths. This inversion of earthquake depth progression strongly supports the pressure-driven mechanism linked to water and vapor accumulation rather than ascending magma or volcanic gases.
The cyclic nature of Campi Flegrei’s unrest emerges from the interplay between groundwater recharge and the impermeable sealing characteristics of the volcanic caprock. Seasonal and long-term rainfall infiltrate the subsurface, incrementally increasing reservoir pressure. However, because the caprock limits fluid escape, pressure accumulates until fracture thresholds are met, producing sequences of uplift and seismic swarms. Following fracturing events, fluid discharge and steam escape decrease reservoir pressure, contributing to subsequent land subsidence observed after episodes of inflation.
These insights revolutionize previous volcanic hazard paradigms by shifting the focus from magma movement to hydrogeological controls on volcanic deformation and seismicity. The study emphasizes the importance of monitoring groundwater levels, precipitation patterns, and subsurface hydrology as vital indicators of unrest. Moreover, it suggests that strategic hydrological management—redirecting water runoff or deliberately reducing groundwater levels—could offer a proactive means to modulate pressure buildups in volcanic reservoirs, thereby decreasing seismic risks.
Vanorio highlights the empowerment this understanding brings to risk management strategies. While the deep magmatic “burner” fueling the system remains untouchable, human activity can influence the “fuel” supplied by fluids within the reservoir. Effectively managing this water could transform Earth science’s role from passive monitoring toward active prevention—akin to preventive medicine in public health—enabling authorities to intervene before destructive unrest unfolds.
The Campi Flegrei caldera, an eight-mile-wide depression formed by colossal eruptions tens of thousands of years ago, continues to “breathe.” This breathing refers to its episodic inflation and deflation and the persistent emission of gases and steam. Past unrest in the early 1980s saw the land rise dramatically, causing ship navigation to halt due to harbor shallowing and prompting the evacuation of tens of thousands of residents. Since that time, the region has remained under close surveillance due to ongoing seismicity and ground deformation threatening infrastructure and communities.
Despite years of investigation, distinguishing the precise triggers of this unrest has been challenging. The new research addresses this by integrating earthquake datasets spanning decades, demonstrating the reproducibility of underlying processes that govern reservoir pressurization and failure cycles. By dissecting these phenomena through combined rock physics and seismological methods, the scientists provide a quantitative framework explaining the system’s dynamics.
This interdisciplinary approach underscores the value of laboratory experiments in testing geophysical observations. As Vanorio analogizes, seismic tomography acts like a doorbell signaling “someone at the door” beneath the Earth’s surface but requires laboratory validation to identify and understand the visitor—in this case, the mechanisms generating and releasing pressure within the subsurface environment.
Looking ahead, this research could influence governmental policies in Campi Flegrei and similar volcanic regions worldwide. By incorporating fluid and pressure management into mitigation plans, authorities may reduce the severity of hazardous episodes, protect communities, and ensure more sustainable coexistence with dynamic volcanic landscapes.
The study represents a milestone in volcanic science—merging traditional geophysics, hydrogeology, and rock mechanics to unveil a hidden driver of volcanic unrest. This breakthrough refines the scientific narrative of Campi Flegrei and offers hopeful pathways toward managing volcanic hazards through informed hydrological interventions.
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Subject of Research: Geophysical mechanisms driving unrest and seismicity at the Campi Flegrei caldera, focusing on groundwater-induced pressure buildup in a sealed geothermal reservoir.
Article Title: The Recurrence of Geophysical Manifestations at the Campi Flegrei Caldera
News Publication Date: 2-May-2025
Web References: http://dx.doi.org/10.1126/sciadv.adt2067
Image Credits: Left image credit: Terme di Agnano; Right image credit: Tiziana Vanorio
Keywords: Campi Flegrei, geothermal reservoir, caprock, seismicity, volcanic unrest, groundwater pressure, fluid overpressure, rock physics, tomography, earthquake swarms, volcanic hazard mitigation
