A recent study reveals that as cities in mountainous areas expand, more individuals are moving to construct homes on steeper slopes that are susceptible to slow-moving landslides. These types of landslides are often overlooked in landslide risk assessments, but researchers believe they could endanger the lives of hundreds of thousands worldwide.
Slow-moving landslides can advance at a pace of merely one millimeter to three meters annually. Although slopes with these landslides might appear safe for building, the movement of the soil may be subtle or even entirely unnoticed.
As the landslide slowly progresses, it can cause damage to homes and infrastructure. There’s the potential for these slow movements to speed up suddenly, often triggered by rainfall changes. Such abrupt shifts might amplify the extent of damage and, in rare situations, result in fatalities.
These same slopes could be re-settled years down the line due to urbanization pressures, particularly when floods displace residents from lower elevations. According to the IPCC, nearly 1.3 billion people inhabit mountainous regions, and this number continues to rise.
“As individuals migrate to higher altitudes to form communities on unstable slopes, a swiftly increasing population is left facing an unknown level of risk from slow landslides, which causes the ground to shift beneath their homes,” stated Joaquin Vicente Ferrer, a natural hazards researcher at the University of Potsdam and the primary author of the study.
The research presents the first comprehensive global evaluation of the risk posed by slow-moving landslides, which are typically omitted from many assessments concerning landslide hazards. This study was published in Earth’s Future, a journal dedicated to interdisciplinary research about Earth’s past, present, and future.
Identifying Slow-Moving Landslides
Through mapping and inventory initiatives, the authors developed a new dataset comprising 7,764 sizeable slow-moving landslides, each covering at least 0.1 square kilometers (approximately 25 acres), located in areas designated as “mountain risk” by the Intergovernmental Panel on Climate Change. They utilized this database to analyze regional and global factors contributing to exposure via statistical models.
Of the recorded landslides, 563—about 7%—are home to hundreds of thousands of residents. The most densely populated areas with slow-moving landslides are found in northwestern South America and southeastern Africa. Central Asia, northeast Africa, and the Tibetan Plateau feature the largest populations at risk from these landslides. Additionally, regions in west-central Asia, particularly the Alai Range of Kyrgyzstan, have a significant number of populated slow-moving landslides.
This study focused solely on permanent settlements, so it did not include nomadic or refugee communities.
In every region examined, the growth of urban centers correlated with increased vulnerability to slow-moving landslides. As urban areas expand, new developments often occur in hazardous locations, including slopes already known for slow-moving landslides. However, the authors noted that economically disadvantaged populations may have limited options.
Flooding and Future Climates
The relationships between climatic factors and the activation of slow-moving landslides are still not fully understood, but scientists generally believe that intense rainfall and shifts between dry and wet conditions can trigger the acceleration of slow-moving landslides. Such conditions may also elevate flooding risks, pushing people to seek refuge on higher ground.
The study indicated that communities facing rising flood risks showed a tendency to establish more settlements on slow-moving landslides. The strength of this link varied by region, with western North America and southeastern Africa exhibiting the most robust associations.
The research underscored a significant lack of data in poorer areas that are aware of landslide hazards, such as the Hindu-Kush Himalayas, and called for enhanced landslide detection and mapping efforts to better comprehend risks in these regions.
“We emphasize the necessity to enhance mapping and monitoring activities for slow-moving landslides in areas such as the East African Rift, Hindu-Kush-Himalayas, and South American Andes to gain deeper insights into exposure drivers,” Ferrer remarked. “Even with limited landslide databases from Africa and South America, we found that communities in cities densely inhabit slow-moving landslides in those regions.”
Even in regions where landslide mapping is advanced, like northern North America (including Canada and Alaska) and New Zealand, there are communities located on slow-moving landslides. Although these were not part of the study’s dataset, the authors highlighted their importance.
“Our study provides insights from a newly established global database of large slow-moving landslides, offering the initial global evaluation of exposure to these landslides,” Ferrer concluded. “Our methodologies allow us to quantify uncertainties amidst varying degrees of landslide monitoring and knowledge availability.”
