A slow-moving landslide in the Italian town of Motta Montecorvino. The top of the landslide is marked in blue. An arrow indicates the direction of the slide. (Credit: Francesco Notarangelo)

Disaster didn’t strike overnight in the town Motta Montecorvino, Italy. Rather, a slow-moving landslide is tearing the hilltop community apart a few painful centimeters a year. Leaning telephone poles and ominous cracks in walls tell the tale of a town sliding away.

Motta Montecorvino is not the only community slipping off a hill. Researchers have found 133 slow-moving landslides in the Eastern Apennine Mountains of southern Italy. The scientists are mapping the size, shape and area of these unusual and little-studied natural hazards.

Heather Hoey presented results from the first 37 sites Thursday afternoon in a poster session at the American Geophysical Union’s Fall Meeting. Hoey, a master’s student in hydrogeology at Tulane University in New Orleans, helped classify the evolution of the slipping land.

“We wanted to figure out if we can differentiate the slides by morphology,” Hoey said. She grouped slides into boxy, elongated and teardrop based on their shape. The teardrop slides are the oldest, Hoey said. They gain their distinctive shape after most of the foot – or lower edge of the slide – is obscured either by erosion or by human activities like farming.

The Italian villages in question are built on limestone that is covered by clay. The molasses-like slumps started in the clay of the hillsides. They range in area from 2.5 to 373 square kilometers (1.2 to 144.0 square miles). Initially, one of the researchers found the landslides by scrutinizing aerial photographs from the 1940s. Governmental surveys of the area pinpointed each slide and the team supplemented their data with on-ground measurements and photos.

They found that the slides have a lower slope and curvature than their surroundings, and that slide area increases with decreasing slope. So far, they haven’t uncovered any patterns associated with the slide age but Hoey said further analysis might reveal trends.

It is clear, however, that slow-moving landslides are long-lived.

“Our Italian counterparts definitely think that some of these slides are hundreds of years old,” Hoey said.

Mapping the slides is valuable, since they’re hard to spot with an untrained eye.

“People would tell me there was a landslide,” said Hoey. “But, it honestly took me a week in the field to see the landslides.”

And the landslides pose a significant problem, she said. The towns have called in engineers to stop the landslides, with little success. Hoey explained that the concrete walls built to hold back creeping masses of earth are put too close to the top of the landslides. The pressure of the wall bears down on the earth below and can speed the slide on its way.

The team will continue analyzing the data set and look for more relationships between the movement and size of each mass. The next step is to calculate depth and volume.

These clues will help scientists understand the evolution of slow-moving landslides, knowledge that might help communities in the path of the wave of soil.

-Marissa Fessenden is a science communication graduate student at UC Santa Cruz