Pressure Head Dynamics on a Natural Slope in Eastern Iburi Struck by the 2018 Hokkaido Earthquake

The 2018 Hokkaido Eastern Iburi Earthquake triggered numerous shallow landslides on slopes covered with thick pyroclastic fall deposits. These shallow landslides tended to occur on concave rather than convex slopes and their slip surfaces were very wet, indicating that water played an important role in landslide initiation. As a first step toward clarifying the role of water in these landslides, we used tensiometers to monitor pressure head dynamics on a natural hillside covered with thick pyroclastic deposits that remained in place throughout the earthquake. We found that on concave slopes, the lower part of the pyroclastic fall deposits throughout the weathered basement complex (sedimentary rock) were always wet. Notably, the interface between the pyroclastic fall deposits and weathered basement complex, which forms a potential slip surface for earthquake-induced landslides, was always at or near saturation. On convex slopes, the weathered basement complex was never saturated and showed greater pressure head fluctuation. We infer that the pyroclastic fall deposits over the basement complex tend to weather more easily and are more susceptible to intense ground motion on concave than on convex slopes and the landslide slip surface was saturated at the timing of the earthquake on concave slopes. We conclude that these factors contributed to the larger number of shallow landslides initiated on concave slopes.