Many residents of Pozzuoli, near Naples (Italy), and surrounding towns have fled their homes following a series of earthquakes which caused slight damage to buildings. Since Monday evening, “around 150 earthquakes have been recorded” in the area of the Phlegrean fields, a volcano on which half a million people reside, said the National Institute of Geophysics and Vulcanology (INGV).
A tremor with a magnitude of 4.4 was measured by the INGV, “the most violent” since 2005. The phenomenon, described by seismologists as a “seismic swarm”, is “the most powerful in the last forty years . » 20 minutes questioned volcanologist Patrick Allard, CNRS researcher emeritus at the Institute of Globe Physics in Paris, to better understand the phenomenon of these Phlegrean fields.
How do you explain the seismic activity of the Phlegrean fields recorded Monday evening, and did it surprise you?
The seismic activity actually began on Monday morning, or even a few days earlier since seismic swarms had already been observed [séismes groupés]. What surprised everyone a little was the high magnitude of quite a few of these earthquakes, since some reached 4.4 [sur l’échelle de Richter], and several were greater than 3, so these were quite energetic earthquakes which shook the population. They were felt as far away as Naples.
A population that felt it all the more because part of it lives inside the crater itself?
Yes, it is a hollow volcano, what we call a caldera, with more than 500,000 people living inside, some of whom are directly above the seismic centers – that is to say at the place where rocks fracture. This is one of the biggest civil protection challenges in Europe.
How was this crater created?
It is a large volcano which collapsed on itself several tens of thousands of years ago, today forming a large crater 12 km in diameter. In particular, there was a very large eruption 15,000 years ago which deposited thirty to forty meters of hot ash in the Neapolitan area (and which subsequently consolidated, what we call tuff). Deposits have been found across a large part of Eastern Europe.
Is it still active?
This large volcano neighboring Vesuvius is still active, it is even one of the most monitored volcanoes in the world. We know that it has a large reservoir of magma, between eight and ten kilometers deep, which breathes, that is to say it expands, releases heat, which raises the floor of the caldera. The last eruption took place in 1538, which is recent. After that it fizzled out, then from around 1950 it went in the opposite direction. There were two acute crises of ground movement, at the beginning of the 1970s, then between 1984 and 1985. After a new period of calm, there has been a return to ground uplift since 2006, but instead of suddenly , this happens more continuously, with an acceleration in the rate of deformation, which means that the volcano breathes harder.
What scenarios can we fear in the face of these ground movements since the middle of the 20th century?
The earthquakes suggest that the reservoir is releasing excess energy, without proven magma movement. This is the most optimistic scenario. A less optimistic scenario would be an injection of magma arriving at the surface and producing an eruption of the type of 1538, which was not a large eruption, but which today would generate the evacuation of the entire population.
Are these volcanic earthquakes different from tectonic earthquakes?
They have nothing to do with each other, because they are not faults that are affected by the movement of tectonic plates. In general, volcanic earthquakes do not reach the magnitude of large tectonic earthquakes, they are more localized. They can nevertheless cause damage.
Monday’s earthquakes were also quite energetic, as you pointed out?
Yes, we are in an acute phase of this evolution which has lasted since 1950. The ground has risen so much that the rocks are cracking more and more, with magnitudes reaching 4.4. This could go up to 5, which could cause balcony collapses. After the crisis of the 1970s and 1980s, the ground rose 1.70 meters each time, which forced the town of Pouzolles to build the quays of its port lower, because they were too high to unload the fish or take on passengers… Now, the question is whether the excess pressure will continue to make the ground crack, or will we reach a plateau? We do not know.