Contrasting Rainfall Thresholds for Landslide Initiation: A Multi-Temporal Analysis of the 2014 and 2018 Disasters in Hiroshima, Japan

Rainfall-induced landslides are a major geohazard in Japan, yet the contrasting triggering mechanisms of short-duration convective storms and prolonged frontal rainfall remain poorly quantified. This study examines rainfall conditions associated with the August 2014 Hiroshima and July 2018 Western Japan landslide disasters using intensity–duration (I–D) threshold analysis, percentile-based thresholds, Monte Carlo Receiver Operating Characteristic (ROC) analysis, and evaluation of topographic and geological controls. The 2014 event produced approximately 600 landslides with 376 mm of rainfall over 40 h, whereas the 2018 event produced 1960 landslides after > 1000 mm of rainfall over 11.6 days. Distinct I–D thresholds were established for the events: the 2014 threshold (I = 95.795D−0.574, R2 = 0.94) indicates intensity-controlled landslide initiation, whereas the 2018 threshold (I = 61.76D−0.42, R2 = 0.97) reflects duration-controlled slope saturation. ROC analysis identified 2 h cumulative rainfall as the most effective rainfall indicator (AUC = 0.61 and 0.69 for the 2014 and 2018 events). Statistical analyses showed that lithology was a stronger influence on rainfall-triggering conditions than slope angle, with weathered granitic terrains requiring rainfall thresholds for landslide initiation. These results underscore the need to integrate rainfall characteristics with geological conditions in the development of landslide early-warning systems for humid mountainous regions.