Changes in the factors contributing to the reduction of landslide fatalities between 1945 and 2019 in Japan

Landslides are natural hazards that cause severe damage and human losses. Japan has succeeded in reducing the number of landslide fatalities and is one of the few countries with long-term databases of landslide fatalities. In this study, we identified the factors that contributed to the decrease in fatalities associated with rainfall-triggered landslides in Japan between 1945 and 2019. We examined trends in landslide fatalities and six factors for Periods I, II, III, IV, and V—each period spans 15 years of the study period—and for Periods I–II, II–III, III–IV, and IV–V. We examined the trends in the number of landslides (N_L) and in the ratio between the number of fatalities (N_F) and the number of landslides (N_F/N_L), and considered fatalities as the product of the number of landslides and the probability of fatalities. The number of fatalities decreased continuously between Periods I and IV; the rate of the decrease declined over time. During Period I–II, N_F/N_L decreased, whereas N_L remained unchanged. Decreases in the average number of household members, changes in building structure, and increases in the number of people evacuated may have contributed to the decrease in N_F/N_L. During Periods II–III and III–IV, N_L also decreased. During Period II–III, the area of mature forests increased slowly. During Period III–IV, the implementation of structural measures (i.e., hard measures) was aggressively pursued. The factors that contributed to the decrease in landslide fatalities changed with time, suggesting that measures for reducing landslide fatalities changed according to the degree of maturity of the nation. Furthermore, we identified increases in rainfall and N_L in Period V, which might indicate a future increase in landslide fatalities.