Earth’s crust under stress

THE quake in Norcia was in a shallow fault in the Apennines, the mountain chain backbone of Italy’s “boot”.

The tremor happened at a depth of 6.2 miles on a fault that runs from the north west to the south east. And the epicentre was in the mountains just 6.2 miles to the south east of Norcia.

It was caused by the stretching of the Earth’s crust as tectonic plates beneath moved apart. Since the late Miocene era, a basin has been opening under the Mediterranean Sea where the Eurasian and African plates meet. The African plate is driven under the Eurasian – but the bend in the African plate can move backwards in a process called “rollback”.

Richard Walters of Durham University said: “The Apennine belt is being stretched at a rate of 3mm per year. This causes stress to build up in the crust, which is released in quakes like this one.”

Yesterday’s was the most destructive tremor in Italy since one in 2009 that killed 300 and left 55,000 homeless.

Fabio Tortorici of Italy’s Geological Institute said: “Italy can expect a 6.3 quake every 15 years on average.”