Campi Flegrei Earthquakes: A Closer Look at Seismic Activity

Located in southern Italy near Naples, Campi Flegrei is one of the world’s most active and closely monitored volcanic calderas. Recent seismic activity has raised concerns about potential hazards, making it essential to understand the underlying causes and implications of these earthquakes.

A detailed examination of seismic patterns, hydrothermal influences, and their connection to magma movement provides insight into this activity.

Geological Features Of Campi Flegrei

Campi Flegrei, a vast volcanic caldera, has been shaped by multiple eruptive events over the past 39,000 years. Unlike a singular stratovolcano, it consists of a network of craters, fumaroles, and hydrothermal fields. The caldera formed after two major eruptions—the Campanian Ignimbrite eruption around 39,000 years ago and the Neapolitan Yellow Tuff eruption around 15,000 years ago—each ejecting massive pyroclastic material and reshaping the landscape. These events left behind a depression that continues to experience ground deformation and seismic activity, indicating ongoing magmatic and hydrothermal processes.

Bradyseism, a phenomenon of slow, cyclical ground inflation and deflation caused by magma and hydrothermal fluid movement, has been recorded for centuries. Notable episodes in the 1970s and 1980s caused significant structural damage in Pozzuoli. Numerous craters, such as Solfatara and Astroni, highlight the caldera’s history of intermittent eruptions and geothermal activity, with some still emitting fumarolic gases and boiling mud.

Beneath the caldera, a system of faults and fractures facilitates fluid and gas movement, influencing seismicity and surface activity. Even minor pressure changes can trigger noticeable ground deformation. Geophysical surveys have identified a partially molten body at depths of 4–5 kilometers, playing a key role in ongoing unrest. This magmatic reservoir, combined with the hydrothermal system, contributes to persistent releases of carbon dioxide and sulfur-rich gases, visible at sites like the Pisciarelli fumarole field.

Seismicity And Caldera Activity

Seismic activity at Campi Flegrei stems from tectonic stress, magmatic movement, and hydrothermal circulation. Unlike fault-driven earthquakes, much of the seismicity here results from pressurized subsurface fluids and caldera floor deformation. Bradyseism reflects pressure accumulation and release, often manifesting as swarms of low-to-moderate magnitude earthquakes. Historical records describe significant ground deformation events, particularly in Pozzuoli.

Seismic events cluster along faults and fractures, pathways for magma and hydrothermal fluid migration. Increased seismicity has been recorded near Solfatara and Pisciarelli, where intense fumarolic emissions indicate subsurface unrest. These swarms tend to be shallow, originating within the upper 5 kilometers, linking them to pressurized fluids rather than deep magmatic intrusions. While individual tremors typically range from 1.0 to 3.5 in magnitude, their cumulative effect contributes to long-term ground deformation and structural instability.

Seismic monitoring shows earthquake frequency aligns with ground uplift phases, reinforcing the connection between seismicity and caldera inflation. During particularly active periods, such as the 1980s and the 2020s, earthquake rates surged, reflecting heightened subsurface pressure. These episodes are often accompanied by increased gas emissions, particularly carbon dioxide and sulfur dioxide, signaling greater crustal permeability. The caldera’s sensitivity to pressure shifts makes continuous monitoring essential for assessing potential escalation toward volcanic activity.

Hydrothermal Influences On Earthquake Swarms

The hydrothermal system beneath Campi Flegrei plays a key role in seismic activity, as pressurized fluids exert stress on the surrounding rock. Superheated water and gases, primarily carbon dioxide and sulfur compounds, circulate through fractures, dissolving minerals and weakening rock formations. This makes them more prone to failure when pressure fluctuates. Heat from underlying magma causes these fluids to expand, increasing pore pressure within fault zones and triggering earthquake swarms. Unlike tectonic earthquakes, these fluid-induced tremors are localized and exhibit irregular frequency patterns.

As pressure builds within the hydrothermal reservoir, fractures can open or close, redistributing stress throughout the caldera. This shifting pressure results in small-magnitude earthquake swarms propagating along existing weaknesses in the crust. Seismic records show many originate from depths of 2 to 4 kilometers, where hydrothermal activity is most intense. The Pisciarelli fumarole field, a site of continuous gas emissions and boiling mud, reflects these deep-seated processes. Changes in fumarolic activity, such as increased gas flux or temperature spikes, often coincide with seismic swarms, reinforcing the link between hydrothermal fluctuations and earthquake generation.

Gas chemistry provides further insight into hydrothermal dynamics. Elevated helium-3 concentrations suggest magmatic contributions, indicating periodic injections of heat and gas into shallower layers. This influx can destabilize the caldera’s plumbing system, increasing seismic activity. Variations in carbon dioxide-to-sulfur dioxide ratios during unrest reflect subsurface permeability changes. These chemical indicators, combined with seismic data, help assess the likelihood of future earthquake swarms and potential escalation.

Earthquake Swarm Characteristics

Seismic activity at Campi Flegrei often occurs as earthquake swarms—clusters of small to moderate tremors over a short period. These swarms offer valuable information about stress redistribution, fluid migration, and crustal deformation.

Depth Distribution

Most earthquake swarms originate from shallow depths, typically 2 to 5 kilometers beneath the surface. This corresponds to the region where hydrothermal fluids and gases interact with rock, creating pressure fluctuations. Seismic tomography shows these shallow earthquakes align with pre-existing fault structures, particularly near active fumaroles like Solfatara and Pisciarelli. Deeper seismic events, below 5 kilometers, are rarer but may indicate magma movement or deep hydrothermal reservoir pressurization.

Magnitude Variation

Earthquake swarms at Campi Flegrei generally range from 1.0 to 3.5 in magnitude. While individual tremors are not destructive, repeated ground deformation can impact infrastructure. During heightened unrest, such as the 1980s bradyseismic crises, some earthquakes exceeded magnitude 4.0, prompting evacuations in Pozzuoli. Magnitude variation within a swarm depends on factors like fluid pressure changes, fault slip behavior, and rock properties. Larger events may indicate increased stress accumulation, while smaller tremors suggest gradual pressure redistribution.

Frequency Patterns

Earthquake swarms follow irregular frequency patterns, with active periods interspersed with quieter intervals. These fluctuations correlate with ground deformation, gas emissions, and subsurface pressure changes. Some swarms last hours, while others persist for weeks, depending on the trigger. Long-term monitoring shows swarm frequency increases during caldera inflation, reflecting crustal stress buildup. In contrast, deflation phases often coincide with reduced seismicity. The clustering of these earthquakes provides critical clues about the caldera’s evolving state.

Relations To Magma Movement

The connection between seismic activity and magma movement at Campi Flegrei is complex. Unlike stratovolcanoes, where magma intrusions often precede eruptions, this caldera exhibits intricate interactions between its magmatic and hydrothermal systems. Magma does not always ascend directly but contributes to gradual pressure accumulation, leading to uplift, increased gas emissions, and localized seismicity. Distinguishing between routine unrest and signs of potential volcanic activity is challenging.

Geophysical surveys have identified a partially molten zone at depths of 4 to 5 kilometers, where magma remains in a semi-crystalline state. This reservoir periodically releases heat and volatiles into the hydrothermal system, influencing earthquake patterns. Seismic tomography shows that during heightened unrest, such as in the 1980s and the 2020s, shifts in this magmatic body coincide with increased seismic swarms and ground inflation. These changes suggest that while magma may not be directly intruding toward the surface, its thermal and gas contributions significantly affect subsurface conditions.

Helium-3 anomalies in fumarolic emissions further indicate magmatic gas injections into the hydrothermal system, reinforcing the link between deep magmatic processes and shallow seismic responses. Understanding these interactions is critical for assessing future volcanic hazards at Campi Flegrei.