For people living along Italy’s picturesque Apennine mountain spine, these are worrying times. Since 24 August there have been three significant earthquakes in the region, the most recent of which was the magnitude 6.6 Norcia quake, on Sunday 30 October. Add to this the thousands of smaller aftershocks, and right now the Apennines feel very unstable. So why is the backbone of Italy being pummelled repeatedly?
Italy sits on the boundary between the African and Eurasian tectonic plates, with the African plate diving down beneath the Eurasian plate, just to the east of Italy. The Apennine mountain chain is made up from a network of small faults (around 10 to 20km long) which stretch open and help accommodate the movement of the big plates colliding. These small mountain faults move apart at an average rate of around 3mm per year, but the movement occurs in fits and starts, as the recent quakes demonstrate.
The first in the current cluster of quakes – a magnitude 6.2 which killed nearly 300 people – broke two neighbouring faults known as Laga and Vettore. Stress redistribution in the fault network then led to the subsequent two quakes. “This process can continue indefinitely, with one big quake weakening a sister fault in a domino process that can cover hundreds of kilometres, in principle,” Gianluca Valensise, a seismologist at Italy’s National Institute for Geophysics and Vulcanology, told The Sydney Morning Herald.
Similar quake clusters have occurred in the past, including a sequence of five major quakes that shook the Calabria region during two months of 1783, and the more recent trio of quakes near Assisi in 1997.
