The analysis of the earthquake in central Italy six years after the tragedy
It was exactly 3.36 on 24 August when a magnitude 6 quake occurred, with epicenter near Accumoli and hypocenter about 4 kilometers deep. 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 the president of Ingv Carlo Doglioni said, "the two abundant minutes mentioned for the Amatrice shock are due to the reverberation, also because an earthquake of this kind lasting two minutes would have caused total devastation, then there is a psychological component because everything in those moments can seem like an eternity ". And it was only the first: shortly after, in fact, about an hour later, a second quake arrived, this time with a magnitude of 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 over a thousand tremors.
An earthquake occurred in a region already known to be at very high seismic risk, very active, rich in faults, but so powerful as to deform the conformation of the affected areas. The earthquake, in fact, originated from an "anomalous" fault, 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 African tectonic plate meets the 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 in correspondence with Accumoli. "This is an approximately 25 km long fracture plane that plunges south-west (towards Rieti) with an inclination of 50 degrees", explained expert Stefano Salvi of Eng. “This plane corresponds to a fault already partly known from surface geological studies”.
As the Ingv experts say today, "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 shift on the surface for both the two main events of 24 August and 30 October (a quake of magnitude 6.5, between Norcia, Preci and Castelsantangelo sul Nera), while the movement in depth has been 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 deform and how seismic sequences are generated and evolve ”, concludes Claudio Chiarabba, director 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 understood why an earthquake occurs here and now ”.
