Osamu Shimizu

Relationship of tephra stratigraphy and hydraulic conductivity with slide depth in rainfall-induced shallow landslides in Aso Volcano, Japan

Intense rainfall on July 12, 2012, triggered numerous shallow landslides on steep grassy hillslopes of Aso Volcano, Kyushu, Japan. The hillslopes are mantled by several meters thickness of fallout tephra accumulation from Holocene eruptions. The landslides occurred about 1 m deep in surficial tephra deposits. Stratigraphic surveys of three landslides showed that the tephra deposits beneath the ground consist of two layers, an upper blackish and a lower yellow-brown layer, and that the upper layer represents the accumulation of tephra during the last 1000 years. The surveys also demonstrated that the slip surfaces were formed near the boundary of the two layers, resulting in the sliding of the upper layer. We measured the saturated hydraulic conductivities of both the layers. The hydraulic conductivities of the lower layer are 1 to 2 orders of magnitude lower than those of the upper layer, suggesting that the lower layer acts as an aquiclude. Therefore, pore water pressure locally increases near the boundary between the two layers and failure occurs. We also examined the soil hardness, which has a high correlation with soil shear strength parameters, of the tephra layers at the three landslides. The soil hardness of the lower layer is greater than that of the upper layer in two of the landslides, suggesting that the lower layer collapses less readily than the upper layer. Comparison with previous landslides in the study area demonstrates that this type of rainfall-induced landslide event has occurred in the past and will recur in the future.

Using Dendrochronological Analysis In A Study On History of Landslide Movement

In general, landslide mass can be divided into small blocks depending upon their activities. The objective of this study is to clarify the historical movement of these blocks using dendrochronological analysis. Firstly, we investigated microtopography and inclination of trees (Abies sachalinensis) planted on gentle slope of the landslide. Secondly, we examined compression woods of tree rings formed in the inclined stems. In terms of compression wood, date of initiation, change in forming direction, and increase in areal extent are important factors to analyze historical movement of landslide.The landslide investigated is formed by two blocks which indicate different directions and histories of mass movement, although tree ring analysis clarified extensive movements from 1972 to 1973 and from 1986 to 1987 in both blocks. Ring samples in which compression woods are formed in different directions were found at the central area of each block, whereas samples showing extensive compression wood were found near the crack within each block. These results suggest frequent and complex movements in a small scale within a block.