This study investigated the spatial distribution of heavy rainfall that enhanced the occurrence of potential landslide hazards throughout Japan during the period 2001-2008, using the Soil Water Index (SWI). We calculated SWI using a tank model, with Radar/Raingauge-Analyzed Precipitation for the period 1991-2008 as input data provided by the Japan Meteorological Agency. The SWI can represent and elucidate the conceptual soil water contents during a rainfall-event associated with the shallow landslide initiation. Comparing the SWI of the present rainfall event with the upper level record of SWI during the past decade, we can evaluate the occurrence of potential landslide hazards at a location during rainfall. For this research, by comparing the SWI for the past decade, we defined the top 3 heavy rainfall events that raised the SWI, enhancing the occurrence of potential landslide hazards. We then calculated the frequency of such rainfall events. The results showed regional differences of heavy rainfall frequency that raised the SWI during the last 8 years. The high-frequency regions were eastern Hokkaido, central Tohoku District, the Oga Peninsula, the Izu Peninsula, Shikoku Island, southeastern Kyushu Island, Amami-Oshima Island, and others. In these high-frequency regions, the maximum hourly rainfall event that raised the SWI became larger. On the other hand, the low-frequency regions were western Hokkaido, eastern Kanto District, central Kinki District, Setouchi District of Hiroshima Prefecture, northwestern Kyushu Island, and others. In these low-frequency regions, the maximum hourly rainfall event that raised the frequency became progressively smaller. Therefore, we conclude that these regional differences are brought about by interannual variation of the maximum hourly precipitation, which enhanced the potential landslides to occur. Moreover, the heavy rainfall frequency increased in eastern Hokkaido, the Shimokita Peninsula, the Oga Peninsula, and central Tohoku District. In contrast, heavy rainfall decreased at parts of the Kii Peninsula, and part of northeastern Kyushu Island. The results suggest that the frequency of heavy rainfall events has changed during the last 8 years in some regions.
