There is a lot of discussion these days about bradyseism to the Phlegraean Fields, which he suffered a surge in the lifting speed, which reached around 2 cm/month (the average speed until now had been around 1 cm/month), after having reached 3 cm/month for a few weeks. Associated with these high speeds, as expected, seismicity has also increased, which is now practically continuous in the area even if the population only feels the strongest events.
Of the absolute urgency to control e secure buildings (or to evacuate them if they are dilapidated), which is also foreseen by the specific law passed (but not yet implemented, or at least not completely) we have already talked about on this blog. The aforementioned law was passed (as a legislative decree, then converted into law) in record time already on 12 October last, after the strong tremors of 27 September (M=4.2) and 2 October (M=4.0), which occurred shortly after a letter from me to the Prefect of Naples of 18 September 2023, in which I pointed out the very high seismic risk, imminent, and I suggested ordering a thorough check of the vulnerability of all the buildings in the maximum danger area: within a radius of at least a couple of km from Agnano-Solfatara-Pisciarelli. I also suggested, as a precaution, that temporarily evacuate that area while the buildings were being tested, to prevent particularly dilapidated buildings from being left exposed to possible strong earthquakes.
But already in 2018, when seismicity was at a very low level, I had notified the leaders of my Institute that, if the uplift continued, seismicity would significantly increase in frequency and magnitude; and, when the ground level became similar or higher than the maximum level reached in 1984, the seismicity would also become greater of that period (as explained later, in 1984 the entire city of Pozzuoli was evacuated, due to continuous earthquakes and above all due to the fear of an imminent eruption). I asked them, reiterating the request in April 2022 when earthquakes with higher magnitudes were already starting to occur, to notify the national and local authorities to prepare the territory and the citizens themselves.
So there were almost six years to prepare the territory: verify the resistance of buildings, inform citizens, plan the response to probable emergencies. How prepared is the territory, almost six years after the first warnings and seven months after the enactment of the law on the Campi Flegrei? Yesterday a very notable seismic swarm occurred, with the strongest earthquake ever recorded in the Campi Flegrei (magnitude 4.4, 8.10 pm), which threw the population and services into chaos: thousands of frightened people on the streets, local railways (Cumana and Metro L2) stopped due to possible damage to the tracks, hundreds of people evacuated in emergency from seriously damaged buildings injured, schools closed the next day, tents erected in an emergency on the street. Following the strongest earthquake, two other events with a magnitude greater than 3: 3.9 And 3.1.
So let’s talk about the worst case scenario, i.e. the eventuality of an eruption in the area. Recently, there has been much discussion in the media about the possibility that the magma rose from the main reservoir towards surface depths (up to 3.0-5.0 km). Let’s first say that the depth of the upper part of the main magma chamber, 7.5-8.0km, was determined in the 2000s, very precisely with the best technique based on the analysis of the reflections of the seismic waves generated by explosions (similar to the ultrasound and tomographic techniques used in medicine); a technique known as ‘active seismic reflection’ which is not by chance used to locate oil and gas deposits.
Other tomographic techniques, based on ‘direct’ seismic waves (i.e. those that pass through the rock volumes, not those reflected by the discontinuities) of local earthquakes, can be used to study the velocity anomalies of seismic waves in the crust: for example, if the seismic waves pass through magmatic zones, i.e. containing ‘liquids ‘, both the P and S waves are slowed, and the S much more than the P.
These techniques (called ‘passive seismic’ because they do not use explosions but natural springs like earthquakes), however, they are much more ‘uncertain’ (in technical jargon they have ‘low resolution’), first of all because the locations of the earthquakes are not known a priori, but must be determined by the arrival times of the seismic waves at the various stations, together with the speeds of the seismic waves in the volume crossed. Furthermore, the areas actually ‘illuminated’ by seismic rays depend precisely on the earthquake locationsand, for example, below the maximums depth nothing of them can be seen because no seismic ray passes through. In the Campi Flegrei, there are very few earthquakes with a depth greater than 3 km; therefore, below that depth the resolution is almost zero and any resulting anomalies could be an artifact of the method.
At the Campi Flegrei, however, modern so-called ‘broad band’ seismic stations have been in operation since the early 2000s, which until now have never detected the typical seismic signals, at very low frequency, generated by the magma that rises in the fractures and conduits. However, the problem of managing an emergency while searching for scientific information that is not already available is much more general, as the Covid-19 emergency has well demonstrated: because scientific research feeds on hypotheses ‘plausible’, of which the majority prove to be incorrect and only the most valid ones are selected over time and form the wealth of ‘robust and reliable’ knowledge that we call ‘Science’.
But, precisely, to achieve ‘reliable truths’ the scientific method takes time: years or decades, times not compatible with decisions to be made in an emergency. Thus, the Phlegraean population is often dazed And confused from the most disparate hypotheses, very often conflicting with each other, which appear from time to time in scientific journals and are immediately relaunched by the media as if they were ‘absolute truth’.
Another important question concerns the possibility of eruption prediction, which is generally taken for granted but isn’t. In reality, even the (few) successful evacuation experiences to protect populations from eruptive events were not carried out in the face of simple eruption ‘predictions’, but rather following the first eruptive events, predicting if anything evolution next of phenomena. Even the best known and most celebrated example, i.e. the evacuation in relation to the eruption of Pinatubo (Philippines) in 1991, was ordered a few days after the start of a phreatic eruption, and was then continued in a ‘progressive’ way, i.e. evacuating areas gradually further away from the crater area as signs of an increase in the intensity of the eruption.
At the Campi Flegrei, therefore, it seems logical plan and make possible a possible evacuation even once the eruption has begun, starting from the area considered most at risk. In reality, we can already identify the area that generates the greatest quantity of earthquakes with the highest magnitudes, as well as the highest intensity of fumarolic emissions, as the area with the highest risk of the opening of future eruptive vents. In this area, moreover, there is the highest seismic danger and the population is highly stressed by the continuous earthquakes, here stronger and closer to the buildings, therefore extremely resentful.
In the 1984, at the height of the previous episode of bradyseism (but today the ground is over 30 cm higher than then, and therefore probably also the internal pressure), the entire town of Pozzuoli (40,000 people) mainly due to the risk of an imminent eruption (which didn’t happen), but also due to the continuous seismic shocks that stressed the population and put the stability of the buildings at great risk: exactly as happens today.
A progressive evacuation technique at the Campi Flegrei would avoid practically all the conceivable difficulties a single, massive simultaneous evacuation of 500-600 thousand people (many live in the current ‘red zone’).
