Two Types of Rainfall Conditions Associated with Shallow Landslide Initiation in Japan as Revealed by Normalized Soil Water Index

Two types of rainfall conditions for shallow landslide initiation are objectively identified in Japan. We examined rainfall intensity-duration conditions of 1,174 shallow landslides that occurred during 2006-2008 using the Normalized Soil Water Index (NSWI), which represents conceptual soil water contents. We classified rainfall in Japan into two types using principal component analysis: short-duration-high-intensity (SH) type, and long-duration low-intensity (LL) type. Based on this classiication, we proposed a new method herein for predicting shallow landslides resulting from the rapid increase of NSWI with short duration in the SH type, and from the gentle rise of NSWI followed by heavy rainfall with long duration in the LL type. We verified this method by analyzing the rainfall-induced shallow-landslide and debris disasters in 2009, for which we succeeded in predicting those of the SH and LL types, respectively, at an earlier stage of rainfall events. These results are important for shallow landslide disaster warnings in Japan.