A new landslide cannot form until the soil has recovered to the critical depth for the recurrence of a landslide through the weathering of bedrock and soil transportation from adjacent areas. In volcanic areas with tephra deposits, landslides expose tephra and not bedrock. Therefore, the immunity of landslides in volcanic areas may be different from that of landslides in non-volcanic areas. Herein, we developed landslide inventory maps (LIMs) for 6 periods during 1955–2016 using aerial photographs and digital elevation models in northern Aso Volcano. In this area, landslides were found to continuously occur due to rainfall events and a large earthquake. Using the 6 LIMs, we examined the terrain attributes (i.e., slope angle, slope aspect, and normalized distance to ridge) and the overlap of landslides. Among the terrain attributes, slope angle was a dominant factor affecting the occurrence of landslides caused by both rainfall events and an earthquake. The total landslide areal density in 2016 was 50% for a slope angle of 35%–45%. 2 atypical events (a rainfall in July 2012 and an earthquake in April 2016) caused landslides to occur on slopes that were relatively resistant to landslides by typical amounts of rainfall, resulting in high landslide density in 2016. The intensity of rainfall for an event in July 2012 was considerably higher than that for other rainfall events. The type of landslides caused by an earthquake in April 2016 was different from that of landslides caused by rainfall. The depths of some landslides caused by this earthquake were deeper than those of landslides caused by the rainfall in July 2012. The overlap ratio was <2.3% for all combinations of the 6 LIMs. The small overlap ratio in the study area suggests that the immunity of landslides continued during the 60-year period we examined. Further research clarifying the process leading to subsequent landslides would be useful to better understand the immunity of landslide and the associated landslide susceptibility in volcanic areas.
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