As urban centers in mountainous regions continue to grow, a new study has found that more people are building on steeper slopes that are prone to slow-moving landslides. These slow-moving landslides are often overlooked in estimates of landslide risk, but they have the potential to threaten hundreds of thousands of people around the world. Slow-moving landslides can move as little as one millimeter per year to up to three meters per year, and settlements on these slopes may appear safe at first, as the slides themselves may be inconspicuous or undetected altogether. However, as the slide creeps along, it can cause damage to homes and infrastructure, and can abruptly accelerate, especially in response to changes in precipitation, which can worsen damage and, in rare cases, lead to fatalities.
The increasing pressure from urban growth drives people to settle on these slopes despite the risks, particularly after floods force people from lower-elevation areas to seek refuge higher up. According to the Intergovernmental Panel on Climate Change, nearly 1.3 billion people currently live in mountainous regions, and this number is expected to grow. Joaquin Vicente Ferrer, a natural hazards researcher at the University of Potsdam and lead author of the study, highlights the unseen risk that communities face as they establish settlements on unstable slopes where slow-moving landslides can occur. The study, published in Earth’s Future, provides the first global assessment of exposure to slow-moving landslides, a risk factor typically left out of most assessments of populations exposed to landslide danger.
Through extensive landslide mapping and inventory efforts, the researchers have compiled a database of 7,764 large slow-moving landslides with areas of at least 0.1 square kilometers in regions categorized as “mountain risk” by the IPCC. Of these landslides, 563 are inhabited by hundreds of thousands of people. The densest settlements on slow-moving landslides were found in northwestern South America and southeastern Africa, with central Asia, northeast Africa, and the Tibetan Plateau also facing significant exposure. Urban expansion in these regions was shown to correlate with an increase in exposure to slow-moving landslides, as new growth may inadvertently occur in hazardous areas, including slopes known to be susceptible to slow landslides.
Researchers have identified a connection between climatic drivers such as intense precipitation and fluctuations between dry and wet conditions with the activation of slow-moving landslides. These factors can also lead to increased flooding, which may drive people to seek higher ground, inadvertently settling on slopes at risk for slow-moving landslides. The study revealed that populations in regions experiencing increased flooding were more likely to have settlements on these dangerous slopes, with western North America and southeast Africa showing the strongest associations. The authors emphasized the need for increased landslide detection and mapping efforts in poorer regions with known landslide risks, such as the Hindu-Kush Himalayas, to improve the understanding of risks in these areas.
The study called for enhanced mapping and monitoring efforts for slow-moving landslides in regions like the East African Rift, Hindu-Kush-Himalayas, and South American Andes to better grasp the factors that drive exposure. Despite a limited number of landslide inventories in Africa and South America, the study found that communities in cities densely inhabit slow-moving landslides in these regions. Even in areas with well-documented landslide risks like northern North America and New Zealand, settlements remain on slow-moving landslides, underscoring the need to consider these risks in future planning and development. By leveraging a new global database of large slow-moving landslides, the study provides a critical estimation of exposure to these often-overlooked threats, highlighting the urgent need for improved monitoring and understanding of landslide risks worldwide.