Ana Astort – Istituto Nazionale di Geofisica e Vulcanologia
Volcanic unrest refers to noticeable changes in key observables from their usual baseline level, such as variations in seismic activity, gas emission flux and signature, and ground deformation. These changes are the result of dynamic processes taking place below the surface, often complex and challenging to be fully understood.
One powerful way to trace these underground processes is through geodetic data, including GNSS (Global Navigation Satellite System) measurements and satellite Synthetic Aperture Radar (SAR) images. These techniques are capable of tracking even sub-millimeter changes on the volcano’s surface, revealing deformation patterns that occur during unrest. These data are crucial for real-time and near real-time monitoring, and with a sophisticated modelling can be used to constrain the pressure source(s) producing the deformation patterns observed. Insights from these models help reveal characteristics of the volcano’s internal structure by estimating the geometry, volume, and location of deformation pressure sources. However, interpreting the nature of these sources can be challenging, especially when they occur at intermediate depths, where it can be difficult to determine whether they have magmatic (normally deep) or hydrothermal (normally shallow) origin.
Our recent publication outlines the application of this approach to Campi Flegrei caldera.
Bradyseism and the current unrest at Campi Flegrei caldera (Italy)
Campi Flegrei caldera has a diameter of 15 km and is located about 30 km northwest of the Vesuvius volcano. It encompasses part of the city of Napoli and hosts about 33,600 people. Its last eruption happened in 1538 CE, during which the Monte Nuovo scoria cone was formed, rising to 123 m. Prior to this eruption, according to archaeological data collected in situ, at least 1500 years of ground subsidence was followed by a shorter uplift phase in the caldera. This particular ground deformation behaviour, known as bradyseism, involves alternation of subsidence and uplift phases, usually accompanied by shallow and low-magnitude seismic activity.
After the 1538 CE eruption, three episodes of unrest occurred during the 1950s, 1970s, and early 1980s, characterised by rapid uplift following longer periods of subsidence. In the town of Pozzuoli, located in the center of the caldera, measured uplifts were 0.7 m, 1.7 m, and 1.8 m, respectively. In 2005, a new period of unrest started, characterised by variations in gas emissions, increased seismicity, and ground uplift (Figure 1). Geochemical changes in the shallow hydrothermal system have been attributed to magmatic fluid circulation and CO₂ emissions. In 2012, a seismic swarm took place after several years with minor activity, and since 2014 the cumulated seismicity has increased exponentially. Moreover, since February 2023, several major earthquakes reached magnitudes 3.5 – 4.4 occurring at depths shallower than 3 km. The surface deformation is monitored by a dense network of 25 benchmarks managed by the INGV- Osservatorio Vesuviano, Naples, and shows ground uplift at increasing rates since 2007, culminating with a total vertical displacement of 1.33 m in Pozzuoli in November 2024.
Modelling approach
Campi Flegrei caldera is a popular site among volcanologists, and several aspects of the ongoing unrest have been investigated over the years. In particular, studies employing geodetic data and modelling consistently identify the primary deformation source at depths exceeding 3 km, often identified with a sill-like shape. However, discrepancies arise depending on the modelling approach, for example, whether a homogeneous or heterogeneous medium is assumed. Multidisciplinary geophysical and geochemical studies agree on the existence of a deeper source at about 8 km depth, commonly identified as magmatic. Still, a debate persists regarding the nature of the shallower source and its interaction with the deeper source, complicating interpretations of the ongoing uplift.
In our research, we used time series from the ground-based GNSS network and satellite SAR images collected from 2007 to late 2023, when noticeable deformation rates started (Figure 1). We modelled this data employing a 3D finite element model (FEM) that includes the integration of two seismic tomographies. This setup helps us get a more realistic representation of the underground elastic structure as a 3D heterogeneous medium.
We explored a two-source plumbing system, with a deeper sill source at 8 km depth, and searched for the shallower source between 2 km and 6 km without predefining its location and shape. We divided the entire deformation period into seven subperiods, based on secondary short-time signals within the main deformation pattern, as shown in Figure 1. This step-by-step analysis lets us track any changes in the inversion parameters (e.g. depth or volume variation of the source) over time.
Once we obtained the geodetic modelling results, to dig deeper into understanding what kind of process might be driving the changes beneath the caldera, we constrained our interpretation with petrological simulations.
Results and interpretation
Our first finding is that during the seven deformation subperiods between 2007 and 2023, the shallower source has been inflating, and the deeper reservoir has been deflating. The resulting shallower source is a spheroid that has been growing gradually and moving upward from about 6 to 4 km depth from 2007 to 2015. After 2015, this spheroid remained at 4 km and continued the inflation until late 2023, the end of our analysis.
By computing the volume changes of the two-source system from 2007 to 2023, the deeper sill deflated by a volume of about ten times less than the inflation of the shallower spheroid (104–105 m3/yr versus 106 m3/yr). This suggests that there could be a connection from the deeper to the shallower reservoirs. To explore this possibility, we combined our geodetic modelling results with petrological simulations. We considered the volume changes that might be linked to different magma-related processes, thus testing the plausibility of several scenarios explaining the ongoing unrest.
A key question is whether the deep-shallow transfer can be explained as purely due to fluid migration, or if magma is rising and contributing to inflation in the shallow region. To look into this, we considered five possible scenarios for the petrological simulations: (a) movement of only volatiles; (b) mix of volatiles and magma; (c) volatiles being released at the shallower source due to magma cooling and crystallisation; (d) magma rising and releasing excess of fluids (outgassing) as it moves up from the deeper source; and (e) a direct connection between the two sources, allowing magma to move from the deeper to the shallower reservoir. Figure 2 shows the schematic representation of these five scenarios.
Of these, the only likely scenarios are the last two. The first two, (a) and (b), are dismissed for not being able to achieve the volume expansion expected at the shallower source. The cooling and crystallisation process, (c), is also dismissed since it would likely take too long to explain the observed ground changes, much more than the actual unrest period. In both of the likely scenarios (d) and (e), we would expect magma to rise from below 8 km to recharge the deeper source, and part of this magma would travel to shallower levels (scenario d), potentially up to about 4 km deep (scenario e). Therefore, the main conclusion from these simulations is the direct involvement of rising magma in the ongoing unrest. It’s worth noting, though, that we can’t precisely determine how much magma might be rising into the shallower source. The same amount of volume change could be produced by either the release of gas from a large volume of magma at a shallower level or a smaller amount of magma rising further. But in any case, all signs point to magma moving up from 8 km deep and playing a direct role in the current unrest. Figure 3 shows a schema of the Campi Flegrei feeding system, based on this main conclusion.
To conclude, we used an advanced 3D modelling approach alongside petrological calculations to study the current unrest at the Campi Flegrei caldera. Although our findings are based on simulations and may be subjected to a degree of uncertainty, proving the magmatic nature of unrest as we did for the Campi Flegrei caldera may have implications for hazard assessment in disaster risk management.
Ana was born in Buenos Aires (Argentina). Currently, she is a postdoctoral researcher at Istituto Nazionale di Geofísica e Vulcanología – INGV in Rome (Italy). After her PhD and her first postdoc at the University of Buenos Aires, she started to work on the modelling the ground deformation at Campi Flegrei as part of the INGV LOVE-CF (Linking surface Observables to sub-Volcanic plumbing system: a multidisciplinary approach for Eruption forecasting at Campi Flegrei caldera) project. In the photo Ana is at the Serapeo Roman Market sited in Pozzuoli, located in the center of the Campi Flegrei caldera, talking with the director of the INGV – Osservatorio Vesuviano (Naples), Mauro Di Vito.
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