Physical, Mechanical and Mineralogy Properties of Altered Halloysite‐Bearing Tephra Layers Causing Pervasive Regional Landslides Induced by 2018 Hokkaido Eastern Iburi Earthquake

Tephra mantled terrain in the vicinity of active volcanic regions has been affected by catastrophic, regional distributed landslide clusters. Our study presents physical, mechanical, and mineralogy characterization of weathered Ta‐d pumice (tephra from Tarumae volcano) from Asahi and Uryu landslides triggered by the 2018 Hokkaido Eastern Iburi Earthquake. Laboratory examination included grain‐size distribution analysis, Atterberg limits, 1D consolidation tests, and static triaxial compression tests conducted on undisturbed and reconstituted samples. X‐ray diffraction (XRD) examination and scanning electron microscopy (SEM) analysis was performed to identify the clay mineral proportion and micromorphology of clay minerals. Laboratory results indicated that yellowish‐brown Ta‐d pumices had low dry density and high natural water content, liquid limit values of 89.34% and 61.76%, plasticity index values of 24.13% and 17.52%, and permeability value of about cm/s, as well as peak effective cohesion of 11.2 kPa and 5.8 kPa and peak effective internal friction of 20.8° and 23.1°. XRD analysis shows that halloysite is the dominant clay mineral present in all samples and SEM observations revealed that it is dominantly present in “crinkly halloysite” habits, which is associated with the spherical halloysite that can facilitate saturation of the slip layer and a critical reduction in shear resistance.