Destructive landslides were triggered by the 6.7 M_w Eastern Iburi earthquake that struck southern Hokkaido, Japan, on 6 September 2018. Heavy rainfall on 4 September in addition to intermittent rainfall around the Iburi Tobu area saturated and weakened the slope-forming materials (mostly altered volcanoclastic soils), making them susceptible to failure because of the earthquake’s strong ground motion. Most of the shallow landslides exhibited long runouts along gentle hill slopes, with characteristic halloysite-bearing slip surface at the base of the volcanic soils. This study investigated the mineralogical and physico-chemical properties of the slip surface material with the aim of understanding weakening and post-failure behaviors during the landslides. Halloysite in the slip surface had irregular-to-hollow-spherical morphology with higher mesopore volumes than tubular halloysite, which is related to a high capacity for water retention after rainfall. To reproduce possible chemical changes in the slip surface during rainfall, the sample was immersed in varying amounts of rainwater; solution pH increased and ionic strength decreased with increasing water content. These findings, alongside electrophoretic analysis, suggest that rainwater infiltration could have increased the absolute zeta potential value of the slip surface material. It is suggested that rainfall before the earthquake enhanced the colloidal stability of halloysite particles within the slip surface, owing to an increase in electrostatic repulsion. This decreased the material’s cohesive strength, which might have led to destabilization of the slope during ground shaking generated by the earthquake, and subsequent high-mobility flow after failure.

6.7 M _w引发破坏性山体滑坡2018 年 9 月 6 日袭击日本北海道南部的伊布里东部地震。 9 月 4 日的强降雨以及伊布里东武地区周围的间歇性降雨使斜坡形成材料（主要是变质的火山碎屑土壤）饱和并削弱了坡度，使其易于破坏因为地震的强烈地面运动。大多数浅层滑坡沿着平缓的山坡表现出较长的滑坡，在火山土壤底部具有特征性的含埃洛石的滑移面。本研究调查了滑面材料的矿物学和物理化学性质，目的是了解滑坡过程中的弱化和破坏后行为。滑移面埃洛石具有不规则到中空的球形形态，比管状埃洛石具有更高的介孔体积，这与降雨后的高保水能力有关。为了重现降雨期间滑面可能发生的化学变化，将样品浸入不同量的雨水中；随着含水量的增加，溶液pH值增加，离子强度降低。这些发现连同电泳分析表明，雨水渗透可能增加了滑动表面材料的绝对 zeta 电位值。这表明，由于静电斥力的增加，地震前的降雨增强了滑面内埃洛石颗粒的胶体稳定性。这降低了材料的内聚强度，这可能导致在地震产生的地面震动期间斜坡不稳定，以及随后在破坏后的高流动性流动。