Relationship between the initiation of a shallow landslide and rainfall intensity—duration thresholds in Japan

The empirical rainfall intensity and duration (I–D) threshold for the initiation of shallow landslide is newly defined for Japan where heavy rainfalls frequently occur during the East Asian summer monsoon season. The rainfall causes sediment-related disasters annually. This paper presents an examination of 1174 rainfall-induced shallow landslides that occurred during 2006–2008. Their I–D conditions were analyzed objectively from rainfall data (Radar-Raingauge Analyzed Precipitation) to derive the I–D threshold using the quantile-regression method: I = 2.18 D− 0.26, where I is measured in millimeters per hour and D in hours, as measured from the beginning of rainfall to the landslide occurrence. Rainfall events are separated by the absence of rainfall for 24 h. We then established a rescaled I–D threshold by dividing the rainfall intensity by the mean annual precipitation (MAP), as IMAP = 0.0007 D− 0.21, where IMAP is the rescaled average per-hour rainfall intensity. These thresholds were defined by the second percentile regression line for D of 3–537 h.The new thresholds are considerably lower than those previously reported for the world, humid subtropical regions, the Asian monsoon region, and Japan. The result suggests that Japan is highly prone to rainfall-induced shallow landslides because of its high-relief topography, geologic conditions, human interference, and rainfall characteristics during the East Asian summer monsoon season.