ABSTRACT
The 2011 Tohoku Earthquake (Mw 9.0) triggered catastrophic landslides in the Shirakawa area, Fukushima, where slopes composed of volcanic deposits failed. In Hanokidaira, a large-scale collapse occurred on sliding surfaces located underneath the paleosol. Similar land-slides have been observed in areas with volcanic deposits, often associated with halloysite-rich layers. This study investigates the Nasu-Shirakawa pyroclastic deposits in Nishigo village, near Shirakawa. A white volcanic ash clayey layer, presumed to be halloysite-rich, was identified and tested. The clayey soils exhibited high water content (119%) and remained moist despite minimal rainfall. Unconfined compression tests showed up to 86% strength reduction and a sensitivity ratio of 6.15 in externally affected specimens. These findings suggest that weak, highly plastic volcanic ash clayey layers are vulnerable to seismic loading and may contribute to catastrophic landslides due to their fragility.
