Damaged crust beneath Campi Flegrei linked to uplift and seismicity

Campi Flegrei, a volcanic caldera just west of Naples, has shown persistent signs of unrest for nearly two decades. Since 2005, the ground, particularly in the town of Pozzuoli, has risen by approximately 1.1 to 1.2 m (3.6 to 3.9 feet), accompanied by frequent small earthquakes. While there is no clear evidence of magma rising toward the surface, scientists continue to investigate the underlying processes driving this prolonged activity.

A recent study by Gianmarco Buono and his colleagues at the National Institute of Geophysics and Volcanology (INGV) sheds new light on what might be happening underground.

The team studied rock samples from deep inside the caldera. They combined this with seismic data and computer models. What they found was a weakened layer of rock, sitting between 2.5 and 3 km (1.55 and 1.86 miles) underground. It likely became damaged long ago by rising magma. Today, this fragile zone may be driving the ongoing ground uplift and earthquakes.

Analysis of rock layers and weak zones

The study hinges on data from the SV1 geothermal well, drilled to a depth of 3 046 m (9 993 feet) near the caldera center. Temperatures at the bottom of the borehole were inferred to exceed 419°C (786.2°F), based on the melting of zinc placed during drilling operations.

From this well, researchers identified three main layers of rock based on how they respond to stress:

1. Upper layer (0.5-2.0 km (0.31-1.24 miles)): These rocks are soft and porous (20-28% porosity), made up of volcanic tuffs and sediments. They deform gradually and don’t crack easily. Their strength is moderate, with compressive strength around 15-50 megapascals (MPa).
2. Middle layer (2.0-2.5 km (1.24-1.55 miles)): Here, the rocks become less porous and stronger (up to 70 MPa). They start to crack more suddenly under pressure.
3. Lower layer (2.5-3.0 km (1.55-1.86 miles)): This is where things change as their compressive strength comes to 110 MPa. Despite being deep-seated, these rocks are weaker than the ones above. They’ve been heated and altered by hot fluids and magma in the past, which has left them fractured and chemically changed. They contain minerals like epidote, chlorite, and amphibole that form at temperatures above 360°C (680°F).

This weakened zone doesn’t behave like intact rock. It breaks more easily and can trap fluids. That makes it a likely spot where pressure can build up underground.

Seismic evidence of fractures and pressure buildup

Seismic data from local earthquakes recorded between 2005 and 2023 were used to generate 3D images of the subsurface. These showed a significant reduction in seismic wave velocity at depths of 2.5 to 3 km (1.55 to 1.86 miles), a characteristic often associated with fractured rock saturated with gas or fluids. The strongest earthquakes in recent years have occurred just above this layer, supporting the hypothesis that pressure accumulation within the weakened zone is triggering rock failure and seismic activity.

Below this level, at around 4 km (2.5 miles) depth, the rock changes again. The volcanic ash rocks give way to much harder carbonate rocks, which are denser (2 700 kg/m³ vs. 1 900 kg/m³) and stiffer (shear modulus of 30 GPa vs. 8.5 GPa). This boundary likely stops rising magma from moving any higher.

Modeling magma pathways

The researchers used computer simulations to see how magma might move as it rises from deep underground. They focused on dykes, which are narrow, vertical sheets of magma. In their models, these dykes started at a depth of 8 km (5 miles) and pushed upward through the crust.

The models showed that these dykes tend to stall around at a depth of 3–4 km (1.8-2.5 miles). This is right at the transition from soft tuffs to hard carbonates. Each time a dyke stops, it stresses and heats the surrounding rock, damaging it further. Over time, this repeated intrusion could have created the weak zone we see today.

To make the simulations realistic, the team included factors like:

1. Surface pressure reduction from the caldera collapse (5 MPa)
2. Magma density (2400–2600 kg/m³)
3. Dyke sizes of 0.003 to 0.008 km² (0.0012-0.0031 mi²) in cross-section

They also assumed that each dyke gradually “forgets” the stress from earlier ones, reducing its effect by 20% each time.

Unrest at Campi Flegrei linked to gas pressure rather than magma movement

While past unrest episodes at Campi Flegrei may have involved magma intrusion, the current activity appears to be driven primarily by the accumulation of gas in a shallow, weakened crustal layer between 2.5 and 3 km (1.55 to 1.86 miles) deep.

In this zone, fluid buildup leads to increased pressure. Once it surpasses the rock’s strength, fracturing occurs, triggering earthquakes and surface uplift.

The study used models of sill-shaped cracks to figure out how much pressure would be needed to cause the ground to deform. They found that it would take only 5 to 15 MPa of overpressure, which is still within the strength range of the weakened rocks in that layer.

Connecting past damage to current activity at Campi Flegrei

This study gives us a fresh perspective on what might be causing the unrest at Campi Flegrei. In this case, the crust has a kind of geological “memory.” Old intrusions have left a scarred, brittle zone that is now reacting to deep gas and fluid injections. That zone could keep generating uplift and quakes even if magma stays far below.

There’s also a chance that larger volumes of magma from deeper levels of 7 to 8 km (4.35–5 miles) could rise straight to the surface. In those cases, the weakened zone might not slow it down. Some past eruptions may have followed this more direct path.

By understanding this structure, scientists can make more sense of what current signals are telling us and get a clearer picture of what might happen next. It also reveals that even deep-seated magma can still affect the surface through old damage in the crust.

References:

¹ Weak Crust Owing Past Magmatic Intrusions Beneath Campi Flegrei Identified: The Engine for Bradyseismic Movements? – Gianmarco Buono, Francesco Maccaferri, et al. – American Geophysical Union Advances – DOI https://doi.org/10.1029/2024AV001611 – OPEN ACCESS

² CAMPI FLEGREI | A “weak layer” in the Earth’s crust has been highlighted to help understand the phenomenon of bradyseism – National Institute of Geophysics and Volcanology – May 6, 2025