Coupled evolution of a city and landslides

Urbanisation activities such as landscape modifications are likely the dominant force controlling landslides in cities. Due to the lack of city-scale evidence, the urbanisation-landslide interaction has been commonly studied locally, focusing on individual landslides or road segments. We study the expansion of the metropolitan area of Medellín, Colombia, in 18-time steps since 1770 and explore the empirical relationship between urban expansion, landslide occurrences, and fatalities. Between the 1930s and 2023, Medellín’s population increased from about 120 thousand to 2.5 million inhabitants, while its area expanded from 9 km ² to 107 km ². Landslide occurrences have gradually increased since the 1930s with a volatile positive trend. In contrast, landslide fatalities have been fluctuating without a clear trend. Landslides have predominantly impacted the outskirts of the urban area, causing harm primarily to newly emerged neighbourhoods towards the city’s north, northeast, and west especially since 1960s. Thus, we argue that struggling settlement on Medellín’s margins has likely increased the city’s landslide hazards. Landslides kill 2 to 3.5 times more people per km ² in informal neighbourhoods (categorised as Informal II and III) than in the most hazardous formal neighbourhood type. We recommend that land-use policies assess whether planned future land transformations could alter an area’s inherent hazard status rather than relying solely on existing hazard levels for permits.