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The 6.7 Mw Eastern Iburi earthquake of 6 September 2018 triggered destructive landslides in southern Hokkaido, Japan, many of which were characterized by the flow-like downslope movement of volcanic soils formed from weathered tephra containing halloysite. This study aims to elucidate the mechanism of landslide generation through rheological characterization of halloysite soil slurries. The examined slurries were prepared from either oven-dried or moist soil. Both slurries showed a power-law correlation between measured yield stress and moisture content. However, at a given water content, slurry made from dried soil showed stresses that are one-to-two orders of magnitude lower than those of slurry made from moist soil. Compared with the measurements for the slurry of dried soil, those for the slurry of moist soil are closer to prior numerical modeling of a specific landslide, indicating that the soils involved were moist. The yield stress also varied with slurry pH, generally increasing with decreasing pH, which is in part explained by the DLVO force model based on the electrical double layer and van der Waals forces between the colloidal particles. The pH dependence is more prominent in the slurry of moist soil, and thus the mechanical state of the slope appears to vary significantly with rainfall-induced changes in subsurface chemistry. Dynamic viscoelasticity measurement indicated that both initially solid-like slurries can become fluid under an applied oscillatory strain of 0.5 to 10 Hz: the susceptibility to fluidization depends greatly on water content and frequency. The ground motion during the earthquake easily fluidized the slurry, indicating this was a factor contributing to the observed flow-like landslides. Graphical Abstract

The risk management of rainfall-induced landslides requires reliable rainfall thresholds to issue early warning alerts. The practical application of these thresholds often leads to misclassifications, either false negative or false positive, which induce costs for the society. Since missed-alarm (false negative) and false-alarm (false positive) cost may be significantly different, it is necessary to find an optimal threshold that accounts for and minimises such costs, tuning the false-alarm and missed-alarm rates. In this paper, we propose a new methodology to develop cost-sensitive rainfall thresholds, and we also analyse several factors that produce uncertainty, such as the accuracy of rainfall intensity values at landslide location, the time of occurrence, the minimum rainfall amount to define the non-triggering event, and the variability of cost scenarios. Starting from a detailed mapping of landslides that occurred during five large-scale rainfall events in the Italian Central Alps, we first developed rainfall threshold curves with a ROC-based approach by using both rain gauge and bias-adjusted weather radar data. Then, based on a reference cost scenario in which we quantified several cost items for both missed alarms and false alarms, we developed cost-sensitive rainfall threshold curves by using cost-curve approach (Drummond and Holte 2000). Finally, we studied the sensitivity of cost items. The study confirms how important is the information regarding rainfall intensity at the landslide site for the development of rainfall thresholds. Although the use of bias-corrected radar strongly improves these values, a large uncertainty related to the exact time of landslide occurrence still remains, negatively affecting the analysis. Accounting for the different missed-alarm and false-alarm misclassification costs is important because different combinations of these costs make an increase or decrease of the rainfall thresholds convenient. In our reference cost scenario, the most convenient threshold is lower than ROC-based thresholds because it seeks to minimise the number of missed alarms, whereas the missed-alarm costs are almost seven times greater than false-alarm costs. However, for different cost scenarios, threshold may vary significantly, as much as half an order of magnitude.

Destructive landslides were triggered by the 6.7 Mw 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. Irregular-to-hollow-spherical halloysite on the slip surface of a shallow landslide triggered by the 6.7 Mw Eastern Iburi earthquake. Rainfall before the earthquake enhanced electrostatic repulsion of halloysite particles (owing to solution pH increase and ionic strength decrease), which might have led to destabilization of the slope.

Destructive landslides were triggered by the 6.7 Mw Eastern Iburi earthquake that struck southern Hokkaido, Japan, on 6 September 2018. In this study, we carried out 1-D inversion analysis of one of the shallow landslides near the epicenter using a Bing debris-flow model. At this site, the slope failure comprised cover soil with an initial down-slope length of ~ 80 m and a thickness of ~ 7 m on a slope with

La Spezia Province (880 km2; Liguria, northwestern Italy) is frequently hit by intense rainfalls, which often cause shallow landslides and damage to population and environment. In this regard, the Provincial Administration of La Spezia and the Earth Sciences Department, University of Pisa, promoted a study to define the rainfall thresholds for shallow landslides occurrence. In fact, on 25 October 2011 a very intense rainfall hit two parts of the provincial territory (Cinque Terre-Riviera area and Vara Valley) causing at least 3500 shallow landslides. This event was analyzed together with other 134 shallow landslide events occurred from 2008 to 2014. The rainfall conditions of these events were determined using an algorithm implemented by the CNR-IRPI of Perugia. The rainfall thresholds at different exceedance probability levels of landslide were defined using two statistical techniques: least-squares linear fit (LSF) and Quantile Regression (QR). The results highlight that the LSF thresholds seems to be the best performing from a statistical point of view and, consequently, the “best” for the study area.

The Eastern Ligurian Riviera, including the famous Cinque Terre, is frequently hit by heavy rainfall, which often induces shallow landslides and floods, causing damage and sometimes death. In this context, the assessment of the rainfall thresholds for shallow landslides initiation is very important in order to improve forecasting and to arrange efficient warning systems. With this purpose, a detailed analysis of the main rainstorms was carried out. The hourly rainfall recorded from 1967 to 2006 was analysed and compared with the occurrence of shallow landslides. Critical threshold curves were defined, in terms of duration and intensity, applying statistical techniques (logistic regression) in order to separate rainfall events that induced failures and events that did not. The rainfall thresholds obtained in this work were compared with the local and regional curves proposed by various authors. The results of this analysis suggest that in the study area landslide activity initiation requires higher amount of rainfall and intensity than elsewhere.