Iburi Earthquake Research Articles

The Mobility of Landslides in Pumice: Insights from a Flume Experiment

Risk of landslide hazards strongly depends on how far landslide sediment travels, known as landslide mobility. Previous studies mentioned enhanced mobility of earthquake-induced landslides in volcanic deposits compared to those from other geologic/soil settings. A flume apparatus constructed at a 1:300 scale was used to examine the mobility of landslides with pumice. Four pumice samples were collected from landslides induced by the 2018 Eastern Iburi earthquake, Hokkaido, Japan. Laboratory tests confirmed the unique low specific gravity of the pumice (1.29–1.33), indicating numerous voids within pumice particles. These voids allowed pumice to absorb a substantial amount of water (95–143%), about 9–15 times higher than other coarse-grained soils. Our flume experiments using various saturation levels (0–1) confirmed the influence of this inner-particle water absorption on pumice mobility. Because a low value of specific gravity indicates a low strength of soil, grain crushing may occur on the pumice layer, causing water from the internal voids to discharge and fluidize the transported landslide mass. Our findings indicate that such earthquake-induced landslides can be as mobile as those induced by rainfall, depending on the initial water content of the pumice layers. These conditions might be associated with water accumulation from previous rainfall events and the water-holding capability on pumice layers.

Was the 2018 Hokkaido Eastern Iburi Earthquake in Japan Related to Carbon Capture and Storage (CCS)-CO2 Injection? Insights From Geomechanical Analysis

In a recent paper [“Groundwater anomaly related to CCS-CO2 injection and the 2018 Hokkaido Eastern Iburi earthquake in Japan” by Sano et al. (Front. Earth Sci., 2020, 8)], the authors claimed that CO2-enriched fluid may have initially migrated through permeable channels, blocking the fluid flow from the source region, increasing pore pressure in the focal region and triggering a natural earthquake where the brittle crust was already critically stressed. The proposed model is very interesting, but the authors have not shown any quantitative evaluation supporting their conclusion. Here, through geomechanics model analysis, even under extreme conditions, which overestimate the impact of the injection, the impact of the CO2 injection on the Iburi earthquake fault, whether the deep section or shallow part of the fault, is much lower than that caused by Earth tides. In addition, no convincing mechanism exists that would allow fluid channels to heal within a short period of time and block the natural fluid flow along the fault. Therefore, the occurrence of earthquakes was not related to CO2 injection. Geological storage of CO2 is expected to become an effective option for global warming countermeasures, and the assessment of its environmental impact must be carefully conducted.

Rheological properties of halloysite soil slurry: a case study of weathered tephra involved in a shallow landslide triggered by the 2018 Eastern Iburi earthquake in Hokkaido, Japan

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

Hydraulic properties of and pressure-head dynamics in thick pyroclastic-fall deposits in Atsuma, Northern Japan: implications for the role of water in shallow landslides induced by the 2018 Hokkaido Eastern Iburi Earthquake

Hydraulic properties of and pressure-head dynamics in thick pyroclastic-fall deposits in Atsuma, Northern Japan: implications for the role of water in shallow landslides induced by the 2018 Hokkaido Eastern Iburi Earthquake

北海道胆振東部地震発生後の集合住宅居住者の都市公園水道施設利用に関する一考察

This study aimed at clarifying the actual situation in which water facilities in urban parks were used to secure water for citizens' daily lives during a large-scale power outage caused by the Hokkaido Eastern Iburi Earthquake. As a result of the research, the following points were clarified: 1. Citizens' use of water facilities was confirmed in 305 urban parks during a power outage, of which block parks accounted for 84.3%, 2. An interview survey with the manager of the housing complex revealed the situation in which the citizens used the urban park water supply facility in the event of a power outage, 3. As a result of spatial analysis by GIS, it became clear that there are 15 "blank areas" where there are no urban parks where water facilities can be used. In these areas, multiple apartments with a high possibility of water outage in the event of a power outage were locally concentrated.

Generation and Subsequent Transport of Landslide-driven Large Woody Debris Induced by the 2018 Hokkaido Eastern Iburi Earthquake

The earthquake that occurred on 6 September 2018, in the eastern part of the Iburi region of Hokkaido, Japan (the Hokkaido Eastern Iburi Earthquake) caused thousands of shallow landslides in mountain areas. In areas where many landslides occurred, the trees on the slope became large woody debris (LWD) and were supplied to the catchment. Understanding the properties of LWD during the earthquake and its subsequent movement after the earthquake are important to manage the produced LWD and implement disaster prevention measures. This study evaluates the risk of future LWD disasters based on the sequence of LWD generation, its spatial distribution, and LWD relocation linked to temporal fluctuations in rainfall events. The study site is the upper Habiu River catchment (0.37 km2), where multiple shallow earthquake-related landslides occurred. Orthophotos and elevation data acquired before and after the earthquake were used to detect the properties of LWD. To evaluate the risk of an LWD disaster, we examined the correspondence between the hydraulic quantities, including the precipitation for 2 years after the earthquake and the water depth. It was estimated that approximately 7,000 LWD pieces (9,119 m3 km−2) were produced during the earthquake. Orthophoto interpretations indicate that over 80% of the LWD produced at the time of the landslide moved from the slope to the channel accompanied by the landslide debris; some of that then flowed down, accumulated, and formed logjams. In the river channel approximately two years after the earthquake, the destruction of logjams and the clear and drastic movement of LWD could not be confirmed. In this catchment, the uneven LWD distribution and the formation of logjams were fixed almost immediately after the landslide at the time of the earthquake; these characteristics are important when considering future actions. The water depth evaluation based on the difference in the excess return period indicate that the degree of risk differs depending on the deposition location in the channel. This suggests that not all LWD in the catchment are dangerous and that a risk assessment focusing on the LWD location can be effective. This study also makes it possible to determine high priority areas for LWD treatment.

Do all roads lead to Sapporo? The role of linking and bridging ties in evacuation decisions

Why do some communities evacuate long-distances in higher rates than others after disaster? This mixed-methods study uses a new dataset of long-distance evacuation rates after Hokkaido’s Eastern Iburi Earthquake in September 2018, aggregated to the city level from geolocated Facebook user movement. We found that communities with stronger linking and bridging social capital tended to see much lower evacuation rates to distant towns. We used statistical models, fieldwork, and content analysis of 12 interviews, finding that despite rumors on social media, communities with stronger linking social networks had greater trust in government and decided to stay in local evacuation shelters. This was especially the case if these communities also had stronger bridging social networks, helping them access key information, especially among vulnerable communities. Meanwhile, residents with weaker linking or bridging networks may have believed rumors of extreme water, food, and power shortages and left town for good. This study highlights the importance of trust in local officials when managing evacuation after disasters.

Initial natural vegetation recovery at landslide scars caused by the 2018 Hokkaido Eastern Iburi Earthquake

Initial natural vegetation recovery at landslide scars caused by the 2018 Hokkaido Eastern Iburi Earthquake

Actual conditions and issues for home ventilator patients in Sapporo during the large-scale power outage due to the Hokkaido Eastern Iburi Earthquake in 2018

Actual conditions and issues for home ventilator patients in Sapporo during the large-scale power outage due to the Hokkaido Eastern Iburi Earthquake in 2018

Virchowの3徴から読み解く震災と静脈血栓塞栓症―北海道胆振東部地震からの知見から―

震災に関わる静脈血栓塞栓症（venous thromboembolism: VTE）の発生は，Virchowの3徴と共に考察することでその発生の原因，震災特有の病態を理解することができる．深部静脈血栓症（deep vein thrombosis: DVT）の発生率は10～30％と報告され，震災発生後1～2週間後に発生することから，この期間に生じる震災後の避難所環境に伴う血流鬱滞や血液凝固能の亢進に対する対策が，DVT発生の予防に寄与する．本総説では，2018年9月6日に発生した北海道胆振東部地震において避難所環境に関わる段ボールベッドとトイレ環境の重要性に着目し，かつVirchowの3徴に沿いながら震災とDVT発生及び予防について論じる．本邦で生じた複数の震災から得られた知見は，VTE発生予防という点で含蓄に富み，将来発生しえる震災に向けて有効な施策に繋がるはずである．

Quantification of sediment and fallen trees generated by landslides in the 2018 Hokkaido Eastern Iburi Earthquake

Quantification of sediment and fallen trees generated by landslides in the 2018 Hokkaido Eastern Iburi Earthquake

Stratified tephra slides induced by the Hokkaido Eastern Iburi Earthquake around Apporo-1Site, Atsuma, Hokkaido : comparison with landslides occurred in the Jomon Period

Stratified tephra slides induced by the Hokkaido Eastern Iburi Earthquake around Apporo-1Site, Atsuma, Hokkaido : comparison with landslides occurred in the Jomon Period

Analysis of Changes in Land Use/Land Cover and Hydrological Processes Caused by Earthquakes in the Atsuma River Basin in Japan

The 2018 Hokkaido Eastern Iburi earthquake and its landslides threaten the safety and stability of the Atsuma River basin. This study investigates land use and land cover (LULC) change by analyzing the 2015 and 2020 LULC maps of the basin, and its impact on runoff and sediment transport in the basin by using the soil and water assessment tool (SWAT) model to accurately simulate the runoff and sediment transport process. This study finds that the earthquake and landslide transformed nearly 10% of the forest into bare land in the basin. The simulation results showed that the runoff, which was simulated based on the 2020 LULC data, was slightly higher than that based on the 2015 LULC data, and the sediment transport after the earthquake is significantly higher than before. The rate of sediment transportation after the earthquake, adjusted according to the runoff, was about 3.42 times more than before. This shows that as the forest land decreased, the bare land increased. Conversely, the runoff increased slightly, whereas the sediment transport rate increased significantly in the Atsuma River basin after the earthquake. In future, active governance activities performed by humans can reduce the amount of sediment transport in the basin.

2018年北海道胆振東部地震時の日高幌内川岩盤地すべりの運動像

2018年北海道胆振東部地震時の日高幌内川岩盤地すべりの運動像

Initial vegetation recovery and revegetation using regional resources in landslide areas following the 2018 Hokkaido Eastern Iburi Earthquake

Initial vegetation recovery and revegetation using regional resources in landslide areas following the 2018 Hokkaido Eastern Iburi Earthquake

Damage Survey and Residual Seismic Capacity Evaluation of Reinforced Concrete School Buildings after the 2018 Hokkaido Eastern Iburi Earthquake

ABSTRACT In this study, a damage survey was conducted on the buildings of two schools located in the town of Abira, where a Japanese seismic intensity of 6 upper was recorded during the 2018 Hokkaido Eastern Iburi Earthquake. The damage classes of the reinforced concrete structural members and residual seismic capacity ratios were examined. Both schools had performed seismic evaluation and retrofitting before the earthquake, and no serious damage occurred, but flexural and shear cracks in columns and walls were observed. Analysis using drone photographs was conducted to perform crack detection and crack width evaluation and create point cloud models.

Insights into the mobility characteristics of seismic earthflows related to the Palu and Eastern Iburi earthquakes

As an important indicator for landslide hazard and risk assessments, landslide mobility has attracted considerable attention in the literature, yet few studies have investigated seismic earthflows. Therefore, the earthflows induced by the 2018 Mw 7.5 Palu earthquake (Indonesia) and the 2018 Mw 6.5 Eastern Iburi earthquake (Japan) and their mobility characteristics were analyzed. Long-term irrigation for cultivation of wet rice and heavy rainfall produced by Typhoon Jebi introduced substantial moisture into alluvial fans (Palu earthquake) and pyroclastic fall deposits (Eastern Iburi earthquake), respectively, and seismic shaking liquefied these moist deposits, which consequently traveled long distances. Four earthflows that were triggered on very gentle slopes of 1–3° by the Palu event exhibited very high mobility levels (H/L) of 0.0129, 0.01125, 0.0209 and 0.0271 (average H/L = 0.018), whereas 7058 earthflows triggered by the Eastern Iburi event exhibited an average mobility of 0.33. Different liquefied layers were located below or above the low permeability layer, and movement restrictions and complex final flow disturbances could be responsible for obvious mobility differences triggered by the two seismic events. However, the earthflow mobility induced by the Eastern Iburi earthquake was obviously higher than that of coseismic rock landslides worldwide and is described by the following relationship: H/L = 1.46–0.24 × log10 (earthflow_area). Furthermore, the earthflow area, peak ground acceleration (PGA) and slope angle all have increase influences on mobility.

Mineralogical and physico-chemical properties of halloysite-bearing slip surface material from a landslide during the 2018 Eastern Iburi earthquake, Hokkaido

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.

1-D inversion analysis of a shallow landslide triggered by the 2018 Eastern Iburi earthquake in Hokkaido, Japan

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 < 20° dip. The landslide moved southeastward with a run-out distance of ~ 100 m. Inversion analysis of the post-failure deposit geometry was conducted with the Markov Chain Monte Carlo method (MCMC) to optimize the Bingham rheological parameters of the debris. The analysis reproduced several features of the deposit geometry with a yield stress of ~ 1500 Pa and dynamic viscosity of 800–3000 Pa s. The results suggest that the shallow landslide can be approximated by the flow of a viscoplastic fluid with high-mobility debris and a maximum frontal velocity of 6–9 m/s, with a flow duration of 2–4 min.

Tomography of the 2018 Mw6.6 Hokkaido eastern Iburi earthquake area and surrounding regions, Japan

To better understand the structural heterogeneities under the Hokkaido subduction zone and its implications in the generation mechanism of the 2018 Mw 6.6 Hokkaido eastern Iburi earthquake, three-dimensional (3-D) high-resolution seismic velocities (Vp, Vs) and Poisson's ratio (σ) images were determined using 273,078 high-quality travel-times of P- and S-wave source-receiver pairs from both onshore and offshore earthquakes. Our results reveal that the 2018 eastern Iburi earthquake likely occurred at a presumed blind thrust fault with sharply varying seismic parameters. A strong low-V and high-σ body extending to 120-km depth was clearly observed northwest of the 2018 hypocenter, indicating the presence of fluids in the mantle wedge associated with the Pacific slab dehydration. Sharp contrasted structures of Vp, Vs, and σ were clearly imaged across the hypocenter of the 2018 Iburi earthquake. The sandwiched rigid zone with high-V and low-σ anomalies in the hypocenter was considered to be the subsidence of brittle crust, which is thickened and deepened due to the colliding system of the northeastern Japan and the Kuril arcs. The mechanical strength in the source rocks with high-V and low-σ was stronger than in surrounding regions where high pore-fluid pressure accumulated. Such a fluid-pressurized regime beside the hypocenter reduced the rock effective stress on the source rocks, lowered the threshold for fault failure, and consequently led to brittle failure. Fluid intrusion in the back-arc seisomogenic layer played a key role in intraplate earthquake generation in the Hokkaido subduction zone and elsewhere.