Half a million inhabitants under direct threat from a super volcano: the risk of an eruption on the Phlegraean Fields, near Naples, has never been greater, warns an Anglo study -Italian published on Friday.
Less known than Vesuvius, which wiped Pompeii off the map almost two millennia ago, the volcano of the Phlegraean Fields, which last erupted in 1538, exposed hundreds of thousands of inhabitants to a deluge of lava, ash and rocks.
“It is an extremely dangerous volcano”, explains to AFP Stefano Carlino, co-author of the study of the London university UCL and the Italian Institute of geophysics and vulcanology (INGV), published in the Communications Earth & Environment.
“We are not saying that an eruption will occur, we are saying that the conditions for an eruption are more favorable” today, specifies Christopher Kilburn of UCL, responsible for the works .
The energy of the volcano is such that its eruption 30,000 years ago would have contributed to the extinction of Neanderthals, according to some hypotheses.
A resurgence of activity in the early 1980s led to the evacuation of 40,000 residents, but the volcano has not been talked about since then.
And yet: the tens of thousands of small earthquakes from the 1950s weakened the caldera – a flat-bottomed volcanic depression -, “parts of which were tested almost to the breaking point”, indicates the study.
These tremors, even more numerous since 2019, have upset the underground strata and the municipality of Pozzuoli on which the volcano is located has risen by four meters over the decades.
The researchers point out that the effects activity of the volcano are “cumulative”: it is therefore not necessary for the intensity of this activity to increase significantly to increase the probability of an eruption.
“A possible eruption could be preceded relatively weak signals, such as a modest level of ground heave, and a smaller number of earthquakes,” they note.
They cite the example of the Rabaul caldera in Papua New Guinea, which erupted in 1994 when the tremors that preceded it were far fewer than during the eruption ten years earlier.
The probability of a mega eruption is however “very low”, tempers Stefano Carlino. “What's more likely are small eruptions.”
Moreover, even in the event of a rupture of the crust, “the magma must rise to the right place”, underlines Christopher Kilburn.
Scientists, who are only interested in volcanoes in the phase of waking up after a long period of sleep, use an innovative method to auscultate this flat volcano, almost invisible to the naked eye, and which wells under the apparently peaceful coast of the Neapolitan basin.
In the field, they measure both earthquakes and ground movements, its deformation, to draw up a behavior model of the volcano. In the laboratory, they observe the fracturing of the rock.
Then they go back in time to compare them to other episodes, other eruptions of other similar volcanoes, when more conventional approaches are satisfied with statistical series.
If “we cannot say with certainty what will happen, what matters is to be prepared for all eventualities”, recalls Stefano Carlino .
Half a million inhabitants live in a high-risk perimeter, 800,000 others in a lower-risk zone.
In the event of an alert, the authorities' plan local authorities provide for the evacuation of the population by public transport. The alert level – green, yellow, orange, red – is reviewed each month.
“Currently in Pozzuoli, the alert level is yellow,” a spokeswoman for the municipality, Giordana Mobilio.
“We have a constant channel of communication with the inhabitants of the municipality that we inform about tremors” of a magnitude greater than 1.5, she underlines.< /p>