Rome is an ancient wonder. It feels strange, walking along a street next to the heart of the world city, but surrounded by old ruins. Here the old survives next to the new. There was extensive damage done by the Vandals, the people who overran Italy in the 5th century. They cut off the water supply, and that alone left Rome defenceless. Much of that which now survives owed its remains to the lack of water which made the hills of Rome unattractive to live. But much is in ruins. The Colosseum is among the lucky ones: some three quarters survives and only one side has collapsed. But this wasn’t done by the Vandals or any of the invaders. Earthquakes are to blame for the vandalism.
But in fact Rome isn’t an earthquake zone. Its location is well chosen, and the Earth here is, unlike the political arena, stable. The problem is, as so often in Italy, the neighbours. 60 kilometer to the East runs the Apennine mountain chain, and here there are earthquakes. It is the distant echoes of these quakes which so damaged the Colosseum. Exactly which earthquake destroyed which part is hard to tell. There were damaging events in 346 and in 847 AD, but the worst may have been in 1349.
Italy has a mountainous spine. There are three separate mountain chains, together spanning the length of the country from the Po valley in the north to the southern tip opposite Sicily. The northern Apennines run northwest-southeast, the central Apennines follow Italy’s spine and the southern Apennines bend towards the west. Geologically, this is a young mountain chain, younger than the Alps. Most of the mountain building happened during the last 10-15 million year. The youth has made them rugged: erosion has barely gotten started. It is a continuation of the Atlas Mountains of Morocco: in geology, Italy has never lost its African heritage. The central Apennines are the highest, reaching close to 3km height.
The mountains can bring relief of the summer heat, and attract more rain than the coastal plain. There are many small towns in the hills, and more populated cities in the wide river valleys.
Tectonics at work
Italy is in a precarious position. Originally a fragment of Africa, it separated from that ancient continent and became part of the first wave reaching Europe, part of the closing of the Tethys ocean. The collision pushed up the Alps. Africa is still trying to move north, but now Italy is caught in the middle. And the motions became more complex. The Italian mainland is moving northeast, towards the Adriatic. Behind, in the west, Corsica and Sardinia are European. In front, the Adriatic foreland is part of the African plate. Italy remains the battle front, between armies that in some ways have gotten lost in the land and occupy unexpected territory.
The three different Apennines are part of this complex motion. They are thrust-belt structures, but with different origins. The northern Apennines are related to the Alps, the movement towards Europe. The central Apennines come from the movement towards the Adriatic Sea, and the southern Apennines formed due to the approach of Africa. All these movements caused the rise of the Apennines along the spine of Italy. There may be a subduction zone associated along the Adriatic but this is not certain.
The push towards the Adriatic ended a few million years ago, when the sea on the west, the Tyrrhenian basin (between Italy and Corsica) began to widen. East and West are moving apart and Italy’s peninsula is being stretched. Stretching does not build mountains, in fact quite the opposite. Thus, the Apennines no longer grow and have started sinking: it is this deflation which is at the heart of the modern earthquakes.
All along the mountain chain, volcanoes formed. The northern ones are believed extinct but activity in the south is continuing, from Naples to Sicily. Here are some of the most famous volcanoes on Earth: destructive Vesuvius, playful Stromboli, iconic Volcano and towering Etna. Vesuvius is just the most recent expression of a wide weakness, perhaps caused by the extension of the Tyrrhenian. Over the past few million years, the volcanoes have been following a graben structure on the western side of the Apennines.
The Apennine fault
Along the entire Apennines runs a fault. It is not a single fault but a large number of parallel ones. They are related to the sinking of the mountains, and are mainly dip-slip events with movement perpendicular to the mountain chain. Typical slip rates are 1-2 mm per year.
The earthquakes along the Apennine fault reach up to M7, and can be highly damaging. The typical depth is 10-15 kilometer. The earthquakes often come in sequences. An example is from 1703, when the sequence began with an event at Norcia, on Jan 14. The second event was a bit further south, at Montereale. Finally, L’Aquila was hit on Feb 2. All three quakes were M6+, with the largest one last at an estimate M6.7. All happened within 40 km of each other. In 1783, a sequence of 5 quakes hit the southern Apennines, within 2 months in February and March. The quakes may happen along the same fault, or may be on neighbouring ones. Each quake affects the stress on the surrounding region.
The main ‘quake belt’ is only around 30 km wide but hundreds of kilometers long, and with many parallel faults. East of this is an a-seismic belt. In the quake belt, there is little indication of the faults at the surface: any remains are quickly removed by the intense agriculture. Finding the faults is a major research area, and there is considerable uncertainty about which quake was caused by which fault(s). Major quakes are frequent: there have been at least 60 earthquakes of M>6 over the past 2000 years, with as many as half of these reaching M6.5.
Among the most destructive events is the triple quake of September 1349. It occurred fairly close to Rome and did major damage to its ancient buildings. One of the events had an epicentre near the oft-hit town of L’Aquila. The damage of Rome was one of the defining events of the Middle Ages and it was widely reported in the various sources. This was the time when the southern outer ring of the Colosseum collapsed.
The current sequence began in April 2009, when L’Aquila (who else?) suffered a M6.2 event. The next one was a surprise: in May 2012, two earthquakes of M6 and M5.8 struck north of Bologna, in a region not considered particularly at risk. This is at the northern end of the Apennines, in the plains. The sedimentary plain turned out to be hiding the faults. 24 August 2016 an M6.2 hit Amatrice, 26 October a double event in Visso, and finally 30 October an M6.6 hit Norcia. Hopefully the sequence will end here. But the earthquakes in Italy will not stop. It is part of the fabric of the country.