Historical and future trends in population exposed to landslides in Japan

Population in landslide-prone areas (Pop_landslide) in 2015 is quantified for all municipalities in Japan using population data with a high spatial resolution (250-m grid cells) and a national investigation of landslide-prone areas. Municipalities are classified into 11 classes using the ratio of Pop_landslide to the total population (population exposure ratio). Although Pop_landslide does not significantly differ in eight of 10 classes with non-zero Pop_landslide, classes with smaller population exposure ratios have a larger number of municipalities. Therefore, classes with small population exposure ratios contribute more to the total Pop_landslide in Japan than classes with large population exposure ratios. The trends in Pop_landslide from 1920 to 2050 for each municipality are also examined, combining Pop_total from 1920 to 2050 and the population exposure ratio in 2015. The Pop_landslide for Japan as a whole reached its maximum in 1995 and subsequently decreased. Although each municipality shows similar trends to Japan as a whole, municipalities with larger population exposure ratios have an earlier peak and larger fluctuations than those with smaller relative exposure ratios. Furthermore, during the period of 1950–2019, in Japan as a whole, the hazard (number of landslides) and vulnerability trends correspond well to the risk (number of landslide fatalities) trend, while the exposure trend does not. The effects of exposure on risk may be less significant than those of hazards and vulnerability at the national scale.