Summary
The Phlegraean Fields volcanic complex, located beneath the metropolitan area of Naples – a city of 900,000 inhabitants in Italy – has been rising increasingly since 2005, accompanied by a growing number of small earthquakes. This development has been attracting increasing attention in the densely populated region for years. Although such phases of uplift and subsidence have occurred there for over a thousand years, the relationship between ground uplift and seismic activity is complex and not yet fully understood. A recent study in the Nature journal Communications Earth and Environment now shows that the long-term trend in earthquake activity can be well explained by combining changes in stress within the Earth’s crust with the friction behaviour of geological faults. The additionally observed swarms of earthquakes can be explained, at least in part, by interactions between individual earthquakes and successfully modelled as superimposed sequences of aftershocks.
The research team led by PD Dr Sebastian Hainzl from the GFZ Helmholtz Centre for Geosciences has successfully replicated the observed earthquake patterns of recent decades by combining long- and short-term modelling approaches. To this end, they analysed earthquake catalogues and elevation measurements dating back to around 1905. A test showed that the model is also suitable for short-term, probabilistic predictions, particularly regarding the expected earthquake rate and maximum magnitude over periods ranging from weeks to months. The study thus provides an important new tool for better assessing seismic hazard in the Campi Flegrei area.
Uplift cycles in Campi Flegrei
Beneath the conurbation of the 900,000-strong metropolis of Naples lies a complex magmatic system comprising various reservoirs in the Earth’s crust and upper mantle. For centuries, magmatic and hydrothermal processes in Campi Flegrei have led to recurring phases of ground uplift and subsidence, which are frequently accompanied by increased seismic activity. Occasionally, these cycles culminate in volcanic eruptions that form new maars or cones. The last eruption occurred in 1538 at Monte Nuovo following uplift phases between 1400 and 1536 that resemble the current situation.
Since 2005, the ground has been rising again, by more than a metre so far, accompanied by an accelerating rate of shallow earthquakes, which is causing concern given the dense population. Similar phases of uplift also occurred in the 20th century – particularly in the years 1950–52, 1969–72 and 1982–84. In some cases, these even led to evacuation measures due to the increased seismic activity.
The long-term phenomenon of land uplift and seismicity is attributed to the increase in pressure within a gas-rich reservoir at a depth of 3–4 kilometres, rather than to magma intrusion.
New study examines the seismicity patterns of the region
In their latest study, PD Dr Sebastian Hainzl, a researcher in GFZ Section 2.1 ‘Physics of Earthquakes and Volcanoes’, Prof. Dr Torsten Dahm, head of the same section, and Dr Anna Tramelli from the Italian Istituto Nazionale di Geofisica e Vulcanologia (INGV), investigate the cause of the distinctive seismicity patterns in the region. Their analysis is based on modelling the data from earthquake catalogues and surface deformation. The latter originate from long-term leveling measurements dating back to 1905, as well as from short-term data from the RITE GPS station located at the same site. The seismicity data are drawn from various earthquake catalogues: since 2005 from the local catalogue of duration magnitudes provided by the Observatorio Vesuviano; over a longer timescale from the HORUS catalogue of homogenised moment magnitudes for earthquakes in Italy since 1960.
To investigate the mechanisms behind the observed phenomena, the research team compared and combined various modelling approaches.
Long-term developments: ground uplift and earthquakes
The study shows that seismic activity in the Campi Flegrei is closely linked to ground uplift. However, the earthquake rate is not simply proportional to the uplift rate; since 2005, it has shown a distinctly non-linear, accelerated response. The researchers demonstrate that whilst the Kaiser effect, known from rock mechanics, explains the fundamental relationship between uplift and seismicity over the last 100 years, it is not sufficient for a detailed description. The continuously accelerating seismic activity, i.e. the non-linear relationship between uplift and seismicity, can instead be explained by the friction and fracture behaviour known from laboratory experiments. This is where the so-called RS model (Rate-and-State model) comes into play.
Short-term developments: earthquake swarms
Whilst the long-term seismicity trend correlates with uplift and can be explained by stress build-up in the rock, this does not apply to the swarm earthquakes observed on a short timescale. These swarms do not correlate with land uplift and are likely linked to episodic fluid intrusions and earthquake interactions.
“We were able to show for the first time that these earthquake swarms can be explained, at least in part, by interactions between individual events and that they possess typical characteristics of tectonic aftershock sequences,” explains Sebastian Hainzl.
The standard model for the statistical description of earthquake interactions is the so-called Epidemic-Type Aftershock Sequence Model (ETAS).
Innovative modelling approach: combination of two models
To capture both the physics of long-term stress-induced changes and the statistical characteristics of short-term earthquake clustering, the researchers combined the RS model for describing the time-dependent background rate with the ETAS earthquake interaction model for the earthquake swarms.
“With this combination, we were able to successfully model the long-term occurrence of the observed larger earthquakes with magnitudes M>3 since 1960, as well as the more detailed observations of activity with smaller magnitudes M>0.5 since 2005,” summarises Hainzl.
Promising forecasting tool
Forecasting tests based on historical data show that the developed combined model enables probabilistic short-term forecasts of earthquake rates and maximum magnitudes. “This hybrid modelling approach therefore represents a promising tool for improving seismic hazard assessment in Campi Flegrei and possibly also in other volcanic systems,” says Hainzl.
Original publication
Hainzl, S., Dahm, T. & Tramelli, A. A deformation-driven earthquake interaction model for seismicity at Campi Flegrei. Commun Earth Environ 7 , 244 (2026). https://doi.org/10.1038/s43247-026-03296-3
Figures
Fig. 1:
View of Naples from the Hermitage of Camaldoli with Mt. Vesuvius, Gulf of Naples, Sorrento Peninsula and the Eastern edge of Campi Flegrei. Taken whilst setting up measuring stations.
Photo: Simone Cesca, GFZ
ALT: Urban area at the foot of a mountain, with further mountains and the sea in the background.
Fig. 2
Top left: Map of the region showing earthquakes of magnitude M>0.5 that have occurred since 2005. The graph below shows the subsidence and uplift measured at the RITE station since 1905, compared with M>3 earthquakes recorded since 1960. On the right are the results of the prediction test for the number of earthquakes and the magnitude of the largest earthquake within a one-week time window, with the starting point always shifted by one week. The hatched areas describe the confidence intervals of the probabilistic predictions.
Figures: from Commun Earth Environ 7, 244 (2026). https://doi.org/10.1038/s43247-026-03296-3
ALT: Top left: Map of the region; blue dots show where earthquakes of magnitude M>0.5 have occurred since 2005. Bottom left and right: Diagrams with various curves – measured data are compared with simulated data.
Scientific contact:
PD Dr Sebastian Hainzl
Section 2.1 Physics of Earthquakes and Volcanoes
GFZ Helmholtz Centre for Geosciences
Phone: +49 331 6462-1897
Email: hainzl@gfz.de
Prof. Dr Torsten Dahm
Head of Section 2.1 Physics of Earthquakes and Volcanoes
GFZ Helmholtz Centre for Geosciences
Phone: +49 331 6462-1200
Email: torsten.dahm@gfz.de
Nature Communications
Computational simulation/modeling
Not applicable
A deformation-driven earthquake interaction model for seismicity at Campi Flegrei
19-Feb-2026
No conflicts of interest