I loved Naples. It is a lively Mediterranean city where there is always something going on. The people are amazing. I was told (a long time ago) by someone whose wife was from there that they were visiting the city, and his wife went somewhere and had told him in no uncertain terms to stay with the car. Which he did, but not at the car -stretching legs, walking a bit, always within sight of the car. When he came back, sure enough, the car had been robbed – unseen by the watching wanderer. You have to admire them. When I was there I was told which particular street to avoid if I valued my life, and had no problems – perhaps because I did not bring a car! The city is sandwiched between two volcanoes: Vesuvius, an excellent tourist introduction to what a volcano should look like, and Campi Flegrei, looking nothing like that. As Hendrik pointed out to us many years ago, the danger is in the hidden one, in typical Neapolitan fashion looking for an opportunity to erupt unseen while in full sight, like that unknown Neapolitan robbing the car.

The most recent eruption in Campi Flegrei started off-shore before migrating to land. It produced a nice new mount on the coast, did a lot of damage to the nearby area, wiped out an old harbour and caused half the population of Naples to flee towards it to have a good look – while it was still erupting. This all happened in 1538, and is described in a recent post. This was the first eruption (as far as we know) in 3000 years, was relatively small and was preceded by months (or years or even decades) of earthquake swarms. There had been plenty of warning. The first recorded earthquake swarm was in 1470-1472. After that, there was inflation, just a few meters (!) at first but eventually reaching 14 meters. There were a few more earthquake swarms, but they accelerated about 18 months before the eruption and became damaging in the months before the eruption.

After 1538, very little happened. There were major earthquake swarms in 1582 and 1594, but that did not lead to eruptions. Nothing significant was reported for centuries afterwards.

Of course, there were other problems. The area suffers from uplift and deflation -bradyseism- of up to 12 meters – the old roman town is now that far below sea level. One of the Roman towns in the area was at Pozzuoli, in the heart of Campi Flegrei. The unstable leveling was possibly one reason why it didn’t age well. A new city was build further down the coast, away from the unstable ground. This was called ‘New City’ of NeaPolis, which became known as Naples. (Actually it was ‘new’ compared to a hill side settlement much closer to modern Naples, but it is a nice story.)

The instability came back to Pozzuoli in 1950, after centuries of rest, when there was 70cm of uplift over two years. And in 1969 locals again began to notice uplift on the shoreline, affecting bridges and wharfs. Measurements confirmed that between 1968 and 1970, there had been almost a meter of uplift. There were minor earthquake swarms in 1970/71, too small to be felt. Things quickly calmed down again. (There was a suggestion that some of the earthquakes that were detected later in the 1970’s were actually from fishing with explosives!) But in 1982, uplift resumed and in May 1983, earthquake activity restarted in earnest with a M3.4 event. Over the next two years, there were multiple swarms with events reaching M4.0. Uplift over this period reached 3 meters. People were evacuated. But again, things calmed down and by 1990, measurements showed some deflation. Obviously there was no problem. Never mind that this was first time since the 1538 eruption that there had been such uplift and earthquake swarms – Naples deals with the present, not the past, and whatever happened a decade before can not be important.

But it did not last. Subsidence came to an end in 2005, and over the 20 years since, activity has increased – slowly enough for people not to readily notice, but the INGV (Osservatorio Vesuviano, with a notable naming focus on only one of the volcanoes under its remit) was on the case. They have installed many monitoring stations and produce regular bulletins. Over the past few years, casual observers have also been able to notice the changes. Solfatara is more active. Earthquakes are becoming more frequent. The sea is falling.

Let’s first look at the region. The map below shows the Naples area with Vesuvius, Pompei (of course) and other famous locations. The red oval shows the (very) rough outline of the overlapping calderas of Campi Flegrei.

The next map zooms in on this caldera. It shows the location of the volcanic features (old eruptions) of this region, all of which happened after the last major explosion a very long time ago. The 1538 eruption, Monte Nuovo, is number 9 on the map.

Earthquake swarms

The earthquake activity after 2005 was initially minor, and shallow at 2 km depth or less. From 2012 it became more intense and since 2020 it has really taken off. The plot below comes from the monthly bulletins of the INGV and includes events up to April 2024 (so not this month’s swarms). It shows the rapid increase in number, and also the large increase in energy since 2023. The depth of the earthquakes remained initially around 2 km but more recently has also occurred at a depth of 4km. Much of this bradyseism, by the way, will be due to the sloshing around of underground heated water/steam. The hydrothermal system extends to 2-3 km below the surface and sees the effect of extra heat.

The swarm in October 2023 saw a M4.2 event, possibly (?) the largest Campi Flegrei earthquake for centuries. May 2024 saw this surpassed with an M4.4. (It was advertised as the ‘largest since 40 years’ but the largest one 40 years ago was ‘only’ an 4.0)

Earthquakes are mainly from three locations. One is an apparent fault in the middle of the bay, slightly curved. The second is the Starza fault along the coast, stretching from Pozzuoli eastwards. More recently, the earthquakes have been along only part of this fault, at the eastern end near Solfatara. Note that earthquakes in swarms can be hard to localize precisely.

What is the significance of the depth of the earthquakes? The events at 2 km occur at a calcium-rich cap layer. This is a thin rock cap later that separates the sedimentary layer below from the volcanic deposits (pyroclastics) above. Gasses and fluids rising from below get stuck against this cap: this hydrothermal stress bends the cap elastically and this causes the earthquakes. 2 km is about the depth where water can become steam, so it is likely that this earthquake zone is due to water vapour collecting below (and breaking through) this cap.

The sedimentary layer goes down to about 3km. Below this is the bedrock, affected by the heat from below, with another cap-rock separating the two. Since 2020, more of the earthquakes have been taking place around this deeper cap or even the bedrock below it. This may be mainly caused by CO2 degassing of magma. Magma intrusions have tended to stay within the bedrock and solidify there. The liquid magma is deeper down, at 7 to 8 km. (There is also a much deeper magma reservoir.) If magma is rising in significant quantities, it is likely that we will see earthquakes to this larger depth of 7-8 km, which we do not do at present. It is however possible that some magma did rise to 3 to 4 km in recent episodes: it has been proposed that the pressure increase was caused by the emplacement of new sills in 1950, 1970 and 1980. However, there was no proof that magma has collected at this depth – but see below.

Arise

The current activity is comparable to the events of 1983 and 1984, but with a much longer build-up to the activity. Since 2016 there has been uplift of around 1 meter at Pozzuoli (1.2 meters since 2005). The figure below shows uplift since 2021, accounting for half the total since 2016. It is focussed right at the coast, close to Solfatara. (It is interesting that the strong increase in activity in 2020 happened when inflation had recovered the highest surface level reached in 1984, although this may be just a coincidence.)

During the current inflation period, there has also been a slow change in the gas emissions at Solfatara. The fraction of CO2 over H2O has been steadily increasing, and there is also a bit more CO. This indicates that the gas emissions are coming from a reservoir that is getting a bit hotter, from 215C before 2005 to around 245C at present. There was a spike to 265C in 2020 – one wonders whether the reservoir caught covid. The temperature at which the gas reaches the surface has increased only a little, though. But the site is now considered too dangerous for tourists and it has been closed since 2019.

Plumbing

Seismic mapping has recently shown (a paper to be published in July – see references) that underneath Solfatara, the depth at which water becomes steam has grown shallower since 1984. This can be explained by an increasing temperature of the water, and provides independent evidence for this change.

The same seismic mapping has also shown that there are two somewhat shallow reservoirs within the caldera. One (‘A’ on the map) is located underneath the bay at some 2.5 km depth, and the other (‘B’) under land at some 3.5 km depth near Solfatara. After 1984, the reservoir underneath the bay appears to have cooled, while the one near Solfatara has grown hotter: there has been a change in the pathway used for the heating. Neither reservoir appears to contain significant amounts of magma, though. Think hot water.

As always in volcanic regions, areas without earthquake are possible carriers of magma. The recent mapping has shown such a zone at a depth of 5 km not far from the shore line, which has risen to 4.5 km depth since 2019: this is zone ‘C’ on the map. Interestingly, this zone did not exist in 1984. A possible interpretation is that perhaps 15 years ago a bit of magma migrated up and formed this new sill. This may be what has been driving the sharp increase in seismic activity since 2019.

Current events

In the last few days, there has been a strong, though fairly brief, earthquake swarm. The swarm followed a minor swarm four days earlier and two swarms in April. The current series started just before 6pm (UTC) on May 20 and culminated in the evening with a strongest reported event at M4.4 – the precise value may be subject to change. More than 150 quakes were measured. After midnight the swarm calmed down, although some events were still continuing and around 4pm (UTC) on the 21st, they picked up again a bit with a minor burst. From a report in a local news site:

The seismic swarm that has been underway in the Campi Flegrei since yesterday ‘has slowed down its pace, but it is not over yet’, volcanologist Mauro Di Vito, director of the Vesuvian Observatory of the National Institute of Geophysics and Volcanology, told ANSA.

There are no signals that allow us to say whether and for how long the swarm will continue nor whether this seismic swarm will be followed by others, but ‘given the deformation of the ground that is affecting the area it is clear that we also expect other events’, he said.

‘In the meantime, we are working to collect new elements: ‘we are making measurements of all possible parameters’ and we are implementing the monitoring network in areas that have been less neglected so far.’

All phenomena linked to the condition of the magma are also monitored: ‘gas analyzes are highlighting an increase in temperatures and pressurisation of the upper hydrothermal system, with values ​​of the gas emitted equal to 4,500 tonnes of CO2 per day emitted by the sulphate system in locality Pisciarelli’. Similar measures have been extended in the gulf.

In the Vesuvian Observatory, meanwhile, researchers and technicians have been working for 24 hours now: ‘the staffing pace is becoming intense, there is great pressure and we would need more staff’. Many INGV researchers spontaneously went to help the workers in the Monitoring Room, where earthquakes continued to be recorded during the night with a frequency of 10 or 20 seconds.

Future

What will happen next? That is impossible to predict. INGV is reporting that no change is being seen in the inflation rate. That already suggests this is just part of the longer term activity. There is no sign of imminent changes.

Models of the number of earthquakes suggest that we are in a regime where strain is increasing. That may resolve itself in a rupture, which can be gentle (an opening forms in the crust which lets gasses escape, as may have happened in 1984) or instantaneous, i.e. where a fault gives way in a larger earthquake. The fault is not particularly long and therefore such an event would not be that strong, perhaps M5, but a shallow earthquake of such size in a densely populated area could still be damaging.

There is no indication of an impending eruption. But of course, this caldera has history. If we get a series of earthquake swarms which are propagating in depth and location, then a dike may well be forming. Otherwise, if a fracture grows in the crust, then the hydrothermal system may find its way more easily to the surface. As the activity is close to Solfatara, the likely escape path is there. In that case, phreatic explosions may occur. It seems best that Solfatara remains closed to tourists.

Should Pozzuoli be evacuated again? That is not for us to decide! But it is a difficult one. It can only be done once. If an evacuation is ordered and nothing happens, many people will not leave next time. To evacuate half a million people may take three days. In 1538, combined with modern instrumentation, this would have given plenty of time. So it may be best not to fire off the evacuation order until INGV can see the white in the eyes of a forming dike. In that case the biggest risk is phreatic explosions which can occur without warning.

But at the moment, it is time mainly to call for calm (also for the earthquakes) and get on with life. It is what Naples does best.

Albert, May 2024

Further reading

https://www.nature.com/articles/s43247-023-00842-1#:~:text=Uplift%20and%20subsidence%20since%201950,175%20cm%20in%201982–84.

https://www.sciencedirect.com/science/article/pii/S0012821X24001778?via%3Dihub