Mount Usu is erupting frequently from 18th century. Comparing the recent eruptions, the property of the erupted materials have been found different in various eruption periods. The major eruption in 1978 had been concentrated in Mount Koushu and Mount Oushu. However, the eruption of 2000 had been localized around Nisiyama and Kompirayama area. The soil samples collected from Nishiyama and Kompirayama had shown slight difference in strength and other soil physical properties. The mineralogical properties also show the possibility of different rocks in the vicinity of Nishiyama and Kompirayama. The strength of the soil and the mineralogical property of the volcanic ash in 1978 and 2000 eruptions show considerable difference in the soil properties. Due to the difference in nature of eruption and high clay content in Nishiyama, it exhibited less shear strength than the soil from Ousu in 1978. The density of the soil near crater of Nishiyama is very low with high silt content. This clarifies the possibility of mass movement in various forms after the rainfall. Recalling the triggering of huge mass movement at mount Usu after 1978 eruption, the possible mass movement there in future can not be neglected, based on the low strength, existing land deformations and potential topography for slides.
Engineering properties of the soil around Nishiyama and Kompirayama after the eruption at Mount Usu in 2000
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