Hydrated halloysite: the pesky stuff responsible for a cascade of landslides triggered by the 2018 Iburi earthquake, Japan

Coseismic landslides generated by strong earthquakes can substantially increase the devastation imposed on society and humanity, especially when such seismic events occur in unconsolidated pyroclastic fall deposits that are prone to destabilization. The catastrophic 2018 Hokkaido Eastern Iburi Earthquake triggered thousands of shallow liquefied landslides in pyroclastic fall deposits, 1 day after the passage of Typhoon Jebi. The landslides were highly mobile and had long runouts. We here report novel findings pertaining to distinctive properties of the widely distributed, weathered Plinian Ta-d tephra deposit from Tarumae volcano, and their impact on the spatial clustering of the Iburi landslides. Distribution of the landslides is positively correlated with the dispersal of Ta-d. Liquefaction occurred in the weathered Ta-d pumice, despite the absence of unconfined groundwater. The Ta-d pumice also has lower soil strength than other local pumice units. The volumetric soil moisture content of weathered Ta-d pumice is very high (>90%) and exceeds other soil layers, regardless of precipitation variation. The presence of hydrated halloysite was confirmed by X-ray diffraction. The halloysite enhances the shaking-triggered liquefaction, because it maintains weathered Ta-d pumice in a highly saturated and exceedingly loose state, even in the absence of unconfined groundwater. This is inconsistent with the traditional concept of liquefaction that generally occurs in sandy soils below the groundwater level.