Revealing the relation between spatial patterns of rainfall return levels and landslide density

It is known that the spatial rainfall pattern can mark landslide distribution across the landscape during extreme triggering events. However, the current knowledge of rainfall controls on this distribution remains limited. Here, to reveal what rainfall characteristics control landslide spatial distribution, we explore the spatiotemporal pattern of a rainfall event that triggered over 7500 landslides (area approximate to 10(0)-10(4) m(2)) at a regional scale with an area of approximate to 400 km(2) in Japan. Using a 5 km resolution radar-driven and gauge-adjusted hourly precipitation dataset with 32 years of records, we compared rainfall return levels for various time ranges from 1 to 72 h and landslide density in each grid cell of the precipitation dataset (approximate to 25 km(2)). The results show that, even if local slope distributions within the grid cells are comparable, the number of landslides in a approximate to 25 km(2) grid cell was substantially high when rainfall return levels exceeded the 100-year return period in all examined timespans (i.e., 1-72 h). In contrast, when only specific-duration rainfall intensities (e.g., 6-48 h) exceeded the 100-year return level, the landslide density in corresponding grid cells tended to be low. Consequently, the landslide density increased with the increase in rainfall return levels of various timespans rather than a specific rainfall intensity, such as downpours for a few hours or long-term cumulative rainfall for several days. Moreover, with the increase in the landslide density, the number of relatively large landslides exceeding approximate to 400 m(2) increased. Therefore, the spatial differences in rainfall return levels potentially constrain the density of total landsliding and relatively large landslides. In this sense, whether rainfall intensities reach high return levels rarely experienced in a wide timespan ranging from a few hours to several days is one of the key determinants of the spatial distribution of landslides and the extent of related hazards.