Satellite image of the Italian peninsula, a seismically-hyperactive land.
The recent, catastrophic quake in Central Italy is the lastest of a series of seismic jolts that have rattled the country since 1908. This alarming seismic pattern can be understood in terms of the dynamics of the Italian peninsula, undoubtedly one of the most geologically-active regions on Earth.
Italy is home to two montain belts, four major active volcanoes, a supervolcano, a volcanic arc (the Aeolian archipelago, home to Stromboli Volcano); furthermore, its suface is littered with faults, that attest to this land’s dangerous past (and risky present). Most seismic hazard is concentrated in the central-southern regions of the country, with the notable exceptions of southern Apulia and the whole of Sardinia, virtually non-seismic areas.
How did Italy become so prone to seismic hazard? It’s a long story, that started back in Jurassic times (about 150 million years ago), when the North Atlantic began opening up and, at the same time, Africa and Europe started slowly moving away from each other. As a result of the latter process, a new ocean gradually replaced the void between the once-united plates (see section at the botton of the next figure).
Thousands of meters of sediments were laid down on the bottom of this ancient ocean, as well as along the coastlines of the European and African Plate. As a matter of fact, the southern shores of this ocean were part of a sub-continent linked with the African plate, and named “Adria“.
Many millions of years later, in a period known as the Cretaceous, Europe and Adria began moving towards each other. Their eventual, gigantic collision created the Alps, within which are preserved relics of the ancient ocean that once had separated them.
Tens of millions of years later, at the dawn of the Oligocene, about 30 million years ago, when parts of the Alpine belt had already been eroded away, there was another crucial “turning point”: A milestone geological event which played a paramount role in creating the mountain belt that is known as the “backbone” of the Italian peninsula, the Appennines.
When Sardinia and Corsica, that formerly had been tightly linked to the European plate, started rotating counterclock-wise towards the East, this resulted in the opening of the Balearic Sea and the gradual build-up of the Apennines.
This realtively young mountain belt is nowadays subject to a state of extension, so that the land in the Regions of Abruzzo, Latium, Umbria, is being literally streched by huge geologic forces that have been forming “low-lying areas”, limited by active, seismogenic faults. It is the activity of such extensional faults that makes the central and southern Italian peninsula so much prone to be struck by sudden, catastrophic jolts.
Not all of the Apennines are being shaped by extensional forces: there are other sectors of the chain, both north and south of the Central Apennines, which are dominated by a compressional state of tectonic stress, acting on huge portions of rock that are pushed to the limit of resistance, until stress is released along faults in the form of earthquakes, such as the twin-jolts that struck the Po Plain area in May 2012.
The evolution of the Apennines is such that tremors such as the ones that destroyed the city of L’Aquila in 2009 and the town of Amatrice in August 2016 are nothing like “the norm” in geological terms.
It is time for Italian politicians and decision-makers to start thinking about earthquakes as the expression of “business as usual” geological activity in a shaky land like Italy. They can and must do all they can to improve our resistance to earthquake, natural events that are capable to wreak havoc and cause widespread loss of life, only if we allow them to.
Federico Pasquaré Mariotto (Insubria University, Varese, Italy)
Corrado Venturini (Bologna University, Bologna, Italy)