Geologic factors contributing to landslide generation in a pyroclastic area: August 1998 Nishigo Village, Japan

Vertical contrasts in permeability, particularly where permeable surface materials overlie impermeable materials that prohibit the downward infiltration of groundwater, concentrate the groundwater and become an important focus of landslides that are triggered by intense rainfall. Just such a hydrogeological structure is present within the pyroclastics in Nishigo Village in Fukushima Prefecture, Japan, where intense rainfall of 1200 mm in 6 days generated more than 1000 landslides in August 1998. Three types of landslides occurred. The first type occurred along the edges of small plateaus, where horizontal beds of permeable ash, scoria, and pumice overlie impermeable mudflow deposits consisting of tuffaceous fines and andesite blocks, and massive, weakly consolidated ignimbrites. The rainfall on the plateaus infiltrated downward first, then laterally within the permeable beds, finally gushing out at the plateau edges and triggering landslides. The second type of landslide occurred where weathered tuff of the same ignimbrite was present with a slip surface at the base of the heavily weathered zone. Within this heavily weathered zone, the tuff exfoliated into thin weak plates running parallel to the slope surface. The third type of landslide involved failure of colluvium or ash that filled hollows. This type occurred as a result of subsurface erosion caused by the groundwater infiltrating the superficial beds above the impermeable tuff.