What Makes Italy Shake?

The Emilia-Romagna region of northern Italy was hit by a strong 6.1 magnitude earthquake on May 20, 2012, killing  24 people. The epicenter was near San Felice sul Panaro, 36km (22 mi) north of Bologna. Nine days later, a magnitude 5.8 aftershock killed an additional 17. The epicenter was in nearby Medolla, at a depth of 10km (6 mi). More than 400 were injured in the two shocks and property damage was extensive throughout the region.

Most of us are more familiar with major earthquakes occurring along the Pacific Rim, in places like Indonesia, Japan, Alaska, and Chile. And in the mountains of Turkey, Iran, Pakistan, and India, where the Indian and African plates are pushing into and under the Eurasian plate. But the Italian Peninsula also has a long history of geologic change that periodically brings destructive earthquakes and dangerous volcanic eruptions.

The northern Italian quakes in late May were caused by a small tectonic plate called the Adriatic Plate, which subducted, or drove under, the Eurasian Plate. The Adriatic Plate was once a part of the much larger African Plate, but split off millions of years ago. The Apennine Mountains surrounding the Po Valley of Northern Italy are growing higher under the compression and folding caused by the Adriatic Plate pushing against the continental Eurasian Plate.

While the mountains around it are growing, the floor of the Po Valley is subsiding. Venice, which is gradually sinking into the sea, is part of the same geologic complex. Most geologists who have studied the problem believe the subsidence has been caused by the people of the valley withdrawing underground water at a faster rate than rain and snow can replace it. Also, the earth under the Po Valley of northern Italy is honeycombed with small fault lines. It was one of these small faults that slipped under the pressure of the Adriatic Plate, allowing that plate to thrust under the Eurasian Plate and trigger the May earthquakes, which were felt as far away as Switzerland.