Slow-moving landslides are a growing but ignored threat to mountain communities

As urban centers in mountainous regions expand, people are increasingly building on steep slopes that are prone to slow-moving landslides, a new study finds. Though slow-moving landslides are often excluded from landslide risk estimates, the researchers conclude that they could threaten hundreds of thousands of people worldwide.

Slow-moving landslides move only about one millimeter per year, up to a maximum of three meters. Slopes with slow-moving landslides may appear safe to live on, but the landslides themselves may not be noticeable or may not be detected at all.

Slow moving landslides can cause damage to homes and other infrastructure. Slow moving landslides can also suddenly accelerate in response to changes in precipitation, exacerbating damage and, in rare cases, causing deaths.

The same slopes may become inhabited again in the coming years due to pressures from urban growth, especially floods that displace people from low-lying areas. According to the IPCC, about 1.3 billion people live in mountainous areas, and this number is growing.

“As people migrate to higher ground and build settlements on unstable slopes, burgeoning populations become exposed to unknown degrees of slow-moving landslides as the ground shifts beneath their homes,” said Joaquin Vicente Ferrer, a natural hazards researcher at the University of Potsdam and lead author of the study.

The study, which presents the first global assessment of exposure to slow-moving landslides, a group that is excluded from most assessments of people at risk from landslides, is published in the journal Earth’s Future.

Discovery of slow-moving landslides

Through their landslide mapping and inventory efforts, the authors created a new database of 7,764 large, slow-moving landslides of at least 0.1 square kilometers (about 25 acres) in areas classified as “mountain risk” by the Intergovernmental Panel on Climate Change. The authors used this database to explore regional and global landslide drivers through statistical models.

Of the recorded landslides, 563 (about 7 percent) are inhabited by hundreds of thousands of people. The highest concentrations of settlements in slow-moving landslides are in northwestern South America and southeastern Africa. The largest number of settlements affected by slow-moving landslides are in Central Asia, northeast Africa, and the Tibetan Plateau. Western Central Asia, especially the Alai Mountains of Kyrgyzstan, also had many inhabited slow-moving landslides.

The study looked at only permanent settlements and did not include nomadic or refugee settlements.

In all regions studied, the expansion of urban centers was associated with increased risk of slow-moving landslides. As cities expand in size, they may be forced to build new growth in risky areas, including on slopes known for slow-moving landslides. But poorer people may have few other options, the authors note.

Floods and future climate

The link between climatic factors and the increased activity of slow-moving landslides is unclear, but in general, scientists believe that more intense rainfall and a dry to wet climate change could cause the acceleration of slow-moving landslides. These factors could increase flooding, which may lead people to seek higher ground.

The study found that people facing increased flooding tend to live in areas with slow-moving landslides, and the strength of this relationship varied by region, with western North America and southeastern Africa showing the strongest association.

The study also highlighted a lack of information in poor regions with known landslide risk, such as the Hindu Kush Himalayas, and called for improved landslide detection and mapping to improve understanding of the hazard in those areas.

“We highlight the need for improved mapping and monitoring of slow-moving landslides in the East African Rift Valley, the Hindu Kush Himalayas and the South American Andes to better understand what triggers them,” said Ferrer. “Landslide inventories in Africa and South America are limited, but urban communities have been found to be clustered in the locations of slow-moving landslides.”

Even in well-mapped areas of landslides, such as northern North America (Canada and Alaska) and New Zealand, settlements lie on slow-moving landslide areas that were not included in the study but are important to consider, the authors say.

“Our study provides insights from a new global database of large, slowly moving landslides and provides the first global estimate of slow-moving landslide hazard,” Feller said. “Our method quantifies the underlying uncertainties amid variability in monitoring levels and available knowledge about landslides.”