Shallow landslides occur frequently on the Aso volcanic mountains. The soil materials on the Aso volcanic mountains consist of tephra layers formed by volcanic activities. This study is aimed to specify the physical properties of soil that correlate with the sliding layer of a shallow landslide on the volcanic mountain area. Tephra layers consist of kuroboku and scoria layers and the differences between these layers were specified using the physical properties of soil methods. Results showed that the plasticity index and the fine fraction content can be used for estimating the sliding layer in the Aso volcanic area.

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Heavy rainfall frequently occurred in Kyushu and triggers the landslides every year. This study observes the landslides which occurred in Oita Prefecture, Kyushu, Japan. The landslides in this study, consisting of the same soil materials, tuff breccia and andesite materials; however, the landslide mechanisms were different. Two landslides occurred caused by heavy rainfall in the different timing of the landslide occurrence, and another landslide occurred without the heavy rainfall or an earthquake occurs. Therefore, this study aims to analyse the physical and mechanical properties of tuff breccia and andesite materials with diverse landslide mechanisms. This study performed soil stratigraphic analysis and soil hardness measurements in the field, and performed physical properties, saturated permeability, mechanical properties, and XRD tests in the laboratory. This study found that characteristics of tuff breccia and andesite in diverse mechanisms of landslides were not very different, especially on the landslides caused by heavy rainfall. Furthermore, the landslide in the andesite and tuff breccia areas could be divided into three types based on the timing of the landslide occurrence, scale of the landslide, and landslide mechanisms.

A 6.6-Mw earthquake struck the Iburi region of Hokkaido, Japan, in 2018, triggering massive landslides. Most of these landslides were shallow and occurred mostly in the Atsuma and Abira towns. Ta-c and Ta-d tephra layers have been found in the Towa landslide at Atsuma from the Tarumai volcano, while Ta-d, En-a, and Spfa-1 tephra layers have been found in the Mizuho landslide at Abira from the Tarumai and Eniwa volcanos, as well as the Shikotsu caldera. Field observations from previous studies revealed that the sliding layers were located in the Ta-d and En-a layers at the Towa and Mizuho landslides, respectively. Unlike previous research on earthquake-induced landslides, which were investigated using mechanical properties, this study investigates the characteristics of physical properties, saturated permeability properties, and content of clay minerals on sliding and surrounding tephra layers. Results from this study reveal that the physical properties of sliding layers from two landslides demonstrated the same characteristics: non-plastic soil with a low density of soil particles, void ratio, and dry density; these characteristics could influence earthquake-induced landslides. It also reveals a relationship between the plasticity chart and the age of tephra materials, including the relationship between the weathering process and density of soil particles and the dissimilarity in characteristics of saturated permeability properties in tephra materials.

Shallow landslides have frequently occurred in the Aso volcano, Kyushu island, Japan. Yet, observations of the effects of the physical properties of the soil on the landslide stratigraphy have not been explained. In this study, we conducted field observations—at two landslide sites in the Takadake mountain (Aso volcano) area—to identify the slip surfaces. We found that slip surfaces (at both sites) were located in the lower part of the N3-4 Kuroboku soil layer. This was determined by characteristics of the physical properties of the soil, including particle size distribution and plasticity index. Furthermore, we identified the relationship between plasticity index and the fine fraction of the soil to help explain the identification of slip surfaces. Results showed that Kuroboku and Scoria layers have different characteristics according to the plasticity chart (liquid limit-plasticity index relationship) as well as plasticity index-fine fraction relationship.