The Campi Flegrei caldera volcano on the outskirts of the Italian metropolis of Naples is currently the most dangerous volcano in Europe. For eleven years, the „Phlegraean Fields“ have been on volcano warning level yellow, equivalent to: „Increased activity“. During the same period, both earthquake activity and ground uplift increased continuously. A recently published study by (Kilburn et al., 2023) received much attention, being interpreted by numerous media outlets as an announcement of a volcanic eruption. Today’s magnitude 3.6 earthquake, the strongest in 40 years, is likely to have heightened fears, although uncertainties prevail. About the current situation and legitimate reasons for concern.
Early Sunday morning, the earthquake was felt by many people inside the caldera, jolting them from their sleep. Magnitude 3.6 is the strongest earthquake in the volcanic caldera in decades, strong enough to cause damage and power outages. While investigations into this are ongoing, there is growing concern that recent earthquakes, which have become more frequent and stronger, may signal a volcanic eruption. Since 2020, earthquake activity has increased tenfold. Around 600 earthquakes are currently recorded per month, the most since the mid-1980s.
The cause of the earthquakes is the ever-increasing uplift of the ground. Above a magma chamber located at a depth of around four kilometres below Pozzuoli, the pressure is rising, causing the earth’s crust to bulge upwards. The ground has risen by about one meter in the last 10 years, currently at a rate of 20 centimetres per year. This volcanic behaviour is called bradyseismicity, or earthquakes that result from volcanic uplift. Already in the 70s and 80s, there were such phases. At their maximum, around 1200 earthquakes per month and uplift rates of up to four meters per year were recorded.
Magma raises the earth’s surface by one meter
While the current uplift rates are still well below those of that time, the earthquake activity at least seems to be getting closer. However, this apparent similarity is also due to the much better monitoring currently. Many of the current earthquakes would not have been registered in the 80s, so that the real number of earthquakes at that time was significantly higher.
Nevertheless, the ongoing uplift is having an impact. A new study by Christopher Kilburn (UCL Hazard Centre, Department of Earth Sciences, University College London) has investigated how the stress changes associated with uplift affect the top crust of the Earth above the magma chamber. In other words, the rock layer that currently lies between the magma and the Earth’s surface. Both the deformations in the 20th century and those currently ongoing were considered.
If a magma chamber presses on the uppermost layers of the earth’s crust from below, there are two ways in which deformation can take place:
- Ductile deformation: this involves bending the Earth’s crust without direct fracturing. A hill with a large diameter and very shallow slopes is formed.
- Brittle Deformation: The crust is fractured along existing zones of weakness (faults). The area of highest uplift is clearly delineated by terrain steps and cracks.
While ductile deformation usually occurs slowly and without earthquakes, brittle deformation is what happens as a result of earthquakes. The high earthquake activity during the strongest deformation phases in the 20th century indicated that the lower part of the earth’s crust is already fractured. Further fractures are also currently taking place there. Each earthquake is a new fracture. However, the uppermost crust, i.e. the upper one to two kilometres, continues to be only bent.
Uplift and earthquakes make the earth’s surface unstable
The study shows that this could also change soon. This is because the stability of the rock has now been greatly reduced by the ongoing uplift and earthquakes. This means that the probability is increasing that fractures will continue to the surface. When the critical point is reached and the uppermost crust becomes unstable, this can have two possible effects.
- Earthquake activity: Currently, earthquakes are limited to a very narrow strip at a depth of about 2 to 3 kilometres. Above, the crust is stable; below, the magma makes it too hot for earthquakes. The exception so far is microquakes below the Solfatara crater, which are caused by hydrothermal activity directly at the surface. If the upper crust becomes unstable, the fractures can propagate to the surface, making the earthquakes stronger (and more destructive).
- Volcanic activity: If the upper crust is stable, magma cannot easily reach the surface if it wants to. A possible volcanic eruption would be indicated by strong earthquakes, while the magma tries to break through the crust. If these fractures already exist due to the previous earthquakes, this warning sign is omitted. A volcanic eruption can happen more easily and without much warning, which also increases the risk for the population. In addition, gases and water can rise more easily, which increases the emission in the fumaroles and phreatic explosions can occur.
However, a fracture of the uppermost crust does not mean that a volcanic eruption will necessarily occur, but it does increase the danger posed by the volcano. If magma rises (we do not know if and where it is trying to do so at the moment), it can quickly reach the surface through existing cracks. But completely without warning this does not happen. This is because the fracture itself can also be seen as a warning sign, although not an immediate one. Surface rupture is usually accompanied by strong earthquake activity at known fault zones.
Fault zone of the last activity phase reactivated?
This brings us to the recent earthquake. In this earthquake, it is not only the magnitude that is striking but above all the epicenter. This lies outside the seismically most active zone, which is located between the Solfatara crater and the city centre of Pozzuoli. Instead, the epicentre was located in the Arco Felice district. This earthquake continues the trend of seismic activity becoming more extensive. Already before there were single microquakes in Arco Felice. There were also noticeable quakes near Bagnoli and north of Pozzuoli at the Astroni crater. But an epicentre in the direction of Arco Felice is clearly more exciting.
Between Arco Felice and Pozzuoli runs the western section of the La Starza fault system, recognizable by a cliff prominently overlooking the landscape. Along this fault system, the ground has shifted by about 40 meters in the last 10,000 years. This fault is thus one of the largest surface ruptures in the entire caldera. Earthquake activity between 1982 and 1984, the last phase of intense activity, also occurred primarily to the west of this fault system but, according to Kilburn (2023), triggered earthquakes only at depths of about 2 to 3 kilometres. The earth’s crust above remained stable but is particularly weakened here.
Today’s earthquake occurred in the immediate vicinity of the fault system, where earthquakes had already occurred 40 years ago and where, according to Kilburn (2023), the probability is greatest that a rupture will reach the surface again. The depth was about 2.6 kilometres, the same area where earthquakes occurred in the 1980s. Thus, propagation of the rupture to the surface did not occur initially. However, the earthquake at this location indicates a possible reactivation of the La Starza fault system. A new earthquake sequence as in the 80s thus becomes more likely. At the same time, La Starza also offers the potential for stronger earthquakes. Thus, the hazard potentially posed by earthquakes, which have been largely harmless so far, is increasing.
La Starza fault could break surface
If further earthquakes occur at this location, the probability of a stronger one and thus a rupture of the fault system up to the surface increases. In that case, magma that presses to the surface would have a clear path. And that La Starza can be a good path for magma is proven by the volcano’s most recent eruption in 1538, which formed what is now Monte Nuovo in the centre of Arco Felice, right on the western edge of the La Starza fault zone.
The situation at the Campi Flegrei Caldera volcano is thus increasingly tense. The continued increase in earthquake activity continued uplift, and the danger posed by a possible volcanic eruption is worrisome. Kilburn’s (2023) study is timely and provides guidance on potential warning signs and hazards that are extremely important for disaster preparedness in the Naples metropolitan region.
With today’s strongest earthquake in 40 years, there is also a growing risk that such a scenario could become a reality. Neither the study nor the current earthquake indicate an imminent volcanic eruption. But both give important indications of possible short-term and long-term dangers. Indications that are urgently needed in an emergency to warn millions of people in the danger zone in good time.