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POZZUOLI – The seismic events of the past week have rekindled the “focus of attention” on the Flegrean caldera, reopening the debate on the possible future scenarios that the city could face. After the bradysismic crisis of 82-84, which caused the evacuation of the city center, there was a long downward phase that lasted until 2005, after which the phlegony ground began to rise again. From 2005 to today, the Rione Terra, the point of maximum deformation, has experienced an increase of about 73 cm, 39 cm of which since January 2016. The “climb” speed was generally low on average, with acceleration phases such as that of 2012, which determined the transition to the yellow alert level and the operational attention phase, in which it still persists. In the last two years, the average lifting speed has been 0.7-0.6 cm / month, going in September 2020 to the current value of 1 cm / month. At the same time, seismic activity intensified with the 248 earthquakes in October, 225 in November and 159 in the first 20 days of December, of which 96 in the week 14-20.
THE INTERVIEW WITH DE NATALE – We spoke with Dr. Giuseppe De Natale, previously in charge of the Vesuvian Observatory, to try to make a picture of the situation.
WHAT HAPPENS? – In recent years, and particularly in recent months, there has been an increase in seismicity. What’s going on? “It is an absolutely physiological phenomenon, seismicity will always increase as lift increases and more and more rapidly. That is to say, the “pressure” produced in the subsoil that determines the uplift of the soil also produces the breakage of rocks and therefore earthquakes; as “pressure” increases, both uplifts and earthquakes increase».
WHAT SCENARIOS? – The soil is slowly returning to a level close to that of the crisis of the 1980s. If this trend continues, how long will it take to reach the same “quotas” as at the end of 1984 and what scenarios would we face? “In a couple of years, but we take into account that the elevation is not constant, so the rate can increase or decrease, thus varying over time. The most likely scenario is that seismicity, in this case, reaches 1984 levels (much higher than the current ones, both in earthquake frequency and magnitude). Furthermore, our calculations, reported in a 2017 article (1), which are based on the physical parameters of phlegreal rocks but without taking into account the effect of high temperatures, tell us that the system, if the ground level exceeds that of 1985 (about 4.5 meters more than in 1950), it would be very close to a critical state, therefore close to an eruption. However, the experience of the Monte Nuovo eruption, which according to posthumous reconstructions would have been preceded by at least 17 meters of elevation, would suggest that the effect of the very high temperature, even at shallow depth, makes the rocks much more ‘ductile. ‘(that is, capable of plastically deforming without breaking) can greatly raise the threshold of critical state, that is, of a possible eruption. To make accurate accounts, it would be necessary to know in detail the thermal and physical state of the phlegreal rocks up to at least 3 km deep; which is feasible, with dedicated but expensive drilling that would require a different mental and political attitude from our country ”.
PREMONITORY SIGNS – What are the “warning” signs that should alert us? «The problem in Campi Flegrei is that for 70 years, although with ups and downs in various periods, the so-called ‘alert’ signals: which are precisely the uplift of the ground, seismicity and geochemical anomalies. In the vicinity of an eruption, we expect a further increase in these anomalies, but it is not known how much; in the Rabaul caldera (New Guinea), in 1994, the eruption occurred after a rise of about ten cm and a very modest seismicity, after 10 years of inactivity as there had been increases in meters, as in the Flegrea zone and in the same periods, as well as earthquakes of more than 5 degrees.
WHAT TO DO? – Are there any actions that administrators should have and / or should have taken? «The mitigation of volcanic risk in areas of extreme population density, such as Campi Flegrei and Vesuvius, is extremely complex: ours is an almost unique case in the world due to its proportions. The only strategy with certain results is to decrease the residential density in the red zones; These spaces have a great value, and must be protected and promoted for all those activities: tourist, cultural, advanced tertiary, etc., compatible with the territory. But they cannot be considered “bedrooms”, as they are today; People must mainly reside outside these areas, and then return, with fast connections, to carry out work, cultural or leisure activities. In our most recent work (2), invited by the journal of the most important European association of our disciplines, we explained the scientific basis for an effective risk mitigation strategy. However, to implement these strategies, which would also solve many of the endemic problems of the South, it would take seriousness and a vision of the future that is difficult to imagine in our country: we are trying, however, and we will always try ”.
THE HOPE” – If we want to be optimistic, what can we expect for the uprising to cease and the downswing phase to resume? “I think the improvement will stop before it reaches the 1984 level (unless there are substantial changes to the system); Obviously, mine is a belief based on current scientific data, yes, but on a system that can evolve, totally changing its behavior from one moment to another “.
(1) https://www.nature.com/articles/ncomms15312
(2) https://nhess.copernicus.org/articles/20/2037/2020/nhess-20-2037-2020.html
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