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It’s been six years now since the terrible earthquake hit central Italy. It was, in fact, the morning of 24 August 2016 when the earth shook very strongly, with a powerful shock that devastated the municipalities of Amatrice, Arquata del Tronto (the epicenter) and Accumoli. And the balance it was truly shocking: villages completely razed to the ground, hundreds of buildings destroyed, thousands of displaced and injured and as many as 300 victims. On the occasion of the anniversary, here is a short time-line to remember what happened on this tragic day six years ago.
It was exactly the 3.36 of 24 August when there was a tremor of magnitude 6, with its epicenter near Accumoli and its hypocenter at a depth of about 4 kilometers. Defined as the “Amatrice-Visso-Norcia seismic sequence”, one of the most important of the century, the quake lasted about 15-20 seconds, although to those who were in those areas it seemed much longer. As told by the president of Ingv Carlo Doglioni, “The abundant two minutes we talked about for the Amatrice shock are due to the reverberation, also because a two-minute earthquake would have caused total devastation, then there is a psychological component because everything in those moments can seem like a ‘eternity”. And it was only the first: shortly after, in fact, about an hour later, came a second shock, this time of magnitude 5.4, with its epicenter near Norcia and its hypocenter at a depth of 8.7 kilometers. Then the devastation, which involved 8 thousand square km, 140 municipalities and about 600 thousand people, while the seismic swarm continued in the following days, with earthquakes even of magnitude 4.3. The seismographs, in those days, recorded further a thousand shocks.
A earthquake occurred in a region already known to be at very high seismic risk, very active, rich in faults, but so powerful that it deform the conformation of the affected areas. The earthquake, in fact, originated from a fault “Anomalous”because it is located a few kilometers deep, located in the areas between Amatrice and Norcia and which passes under the municipality of Accumuli, right where the tectonic plate African meets Eurasian. The tremors of that day, thus, even changed the conformation of this area, causing a lowering of the soil, in the shape of a spoon, of about 20 centimeters at Accumoli. “It is about a 25 km long fracture plane that plunges south-west (towards Rieti) with an inclination of 50 degrees”, he explained the expert Stefano Salvi of Eng. “This plane corresponds to a fault already partly known from surface geological studies”.
How they tell today the Ingv experts, “In twenty-five years we have witnessed a progressive crumbling of a sector of the Apennine chain over 150 km long, from Nocera Umbra to L’Aquila. We know that the deformation of the Apennines is slow, 2-4 mm / year of extension perpendicular to the chain, which manifests itself with normal earthquakes on plunging west faults, in this central-northern sector ”. One of the peculiarities of the 2016 sequence, the experts continue, is that on the same fault plane there was a displacement on the surface for both the two main events of the August 24 he was born in October 30 (a quake of magnitude 6.5, between Norcia, Preci and Castelsantangelo sul Nera), while the movement in depth was reconstructed on different portions of the same system. “We certainly have a lot of data ahead of us yet to be analyzed in order to discover the physics of earthquakes in ever greater detail. The set of data acquired so far has allowed us to understand more and more deeply how the Apennines deforms and how seismic sequences are generated and evolved “, concludes Claudio Chiarabbadirector of the Earthquake Department of Ingv. “Studies aimed at recognizing possible triggering processes, capable of destabilizing faults close to the critical state, and the signals that can be generated or correlated are potentially promising. It always remains to be seen why a earthquake it happens here and now ”.
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