Hydraulic properties of and pressure-head dynamics in thick pyroclastic-fall deposits in Atsuma, Northern Japan: implications for the role of water in shallow landslides induced by the 2018 Hokkaido Eastern Iburi Earthquake

The 2018 Hokkaido Eastern Iburi Earthquake triggered numerous shallow landslides on slopes covered with thick pyroclastic-fall deposits. The landslides occurred more frequently on concave slopes than on convex slopes, implying that water was responsible for initiating them. As a first step toward clarifying the role of water in such landslides, we monitored the pressure-head ( ψ ) dynamics of uncollapsed concave and convex slopes and measured the hydraulic properties of each layer in the laboratory. Our results show that while the lower part of the pyroclastic-fall-deposit layers and the weathered basement complex (sedimentary rock) are always at or near saturation with little relationship to rainfall on concave slopes, the weathered basement complex never becomes saturated and shows greater fluctuation in ψ on convex slopes. From these results and the hydraulic properties, it can be inferred that water plays two important roles in co-seismic landslides: the formation of a pyroclastic-fall-deposit layer that is vulnerable to seismic motions as a result of persistently promoted weathering, and the saturation of the depths around a landslide slip surface just before earthquake onset. These factors allow sliding-surface liquefaction to be induced by earthquakes and also explain why co-seismic landslides occur more frequently on concave slopes. It was also inferred that 11 mm of rainfall 25–27 h prior to earthquakes has little effect on landslide initiation, as similar rainfall amounts do not affect ψ around a slip surface.