Characteristics of the occurrence of shallow landslides depend on the process of whole-mountain denudation in granitic areas of western Japan.

Abstract:

The degree of mountain denudation depends on the occurrence of shallow landslides, and on granite slopes the characteristics of the soil layer structure and the occurrence characteristics of shallow landslides vary with the degree of mountain denudation [1]. However, it is not well known how the degree of whole-mountain denudation affects the occurrence characteristics of shallow landslides. Therefore, this study estimated the degree of whole-mountain denudation and then examined the characteristics of shallow landslides.

First, we focused on four granitic regions in which shallow landslides had occurred recently. The slopes were categorized into gentle slopes on the summit (Gen-S), upper dissected slopes (Up-S), and lower dissected slopes (Low-S) in ascending order of denudation. The top convex line of Up-S represented the upper convex break of the slope (Up-C), which was the dissection front between the Pliocene and the Pleistocene. The top convex line of Low-S represented the lower convex break of the slope (Low-C), which was the dissection front for the postglacial period [1]. Second, we calculated the rate of Low-S, which represented the degree of whole-mountain denudation in the postglacial period. Third, we determined the occurrence frequency of shallow landslides from the landslide area as a ratio of the number of square kilometers [%].

Greater numbers of shallow landslides occurred in Up-S in some areas where the degree of whole-mountain denudation was as low as 4% and Low-C was located downstream. On the other hand, greater numbers of shallow landslides occurred in Low-S in some areas where the degree of whole-mountain denudation was as high as 35% to 44% and Low-C was located upstream. In conclusion, areas with lesser degrees of whole-mountain denudation occur readily with shallow landslides in Low-S, whereas those with higher degrees of denudation occur with shallow landslides in Up-S.

[1] Matsuzawa, M., Kinoshita, A., Takahara, T., Ishizuka, T.: The effects of the degree of mountain denudation on soil layer structure and shallow landslide properties in granite areas in Japan, Japanese Geomorphological Union, 36-1, 23-48, 2015.(in Japanese)