Hokkaido Eastern Iburi Earthquake Research Articles

Post-seismic topographic shifts and delayed vegetation recovery in the epicentral area of the 2018 Mw 6.6 Hokkaido Eastern Iburi earthquake

The 2018, Mw 6.6 Hokkaido Eastern Iburi earthquake in Japan triggered over 10,000 landsliding in an area spanning about 500 km2, altering the local topography and leading to the accumulation of loose deposits on hillslopes and in valleys. However, a comprehensive post-seismic landslide inventory and an assessment of topographic changes are lacking, hindering a quantitative hazard assessment. Additionally, the extent of vegetation recovery in areas affected by coseismic landslides, a key indicator of post-seismic debris flow hazard, has not been evaluated. Here, we utilize high-resolution digital elevation models and multi-temporal satellite imagery to analyze topographic changes and vegetation dynamics in the earthquake’s epicentral area (seismic intensity >5.5). We observe that the event roughened the overall gentle topography of the region and made the slopes steeper. Owing to the absence of significant rainstorms and snowmelt post 2018, only a few debris remobilizations (60) and new landslides (80) have occurred in the affected region. Moreover, we noticed a slow vegetation recovery in the post-seismic phase, suggesting that the likelihood of debris flows and gully erosion remains elevated, highlighting the need for continued monitoring and assessment.

Massively parallel Bayesian estimation with Sequential Monte Carlo sampling for simultaneous estimation of earthquake fault geometry and slip distribution

In inverse analysis, Bayesian estimation is useful for understanding the reliability of the estimation result or prediction based on it because it can estimate not only optimal parameters but also their uncertainties. The estimation of an earthquake source fault based on observed crustal deformation is a typical inverse problem in the field of earthquake research. In this study, a method for simultaneous Bayesian estimation of earthquake fault plane geometry and spatially variable slip distribution on the plane has been developed. The developed method can be applied to stochastic models with arbitrary probability distribution settings, and it enables to incorporate appropriate constraints for slip distribution in the estimation process, which can lead to enhanced robustness and stability of estimation. Since this method is computationally more expensive than conventional methods, large-scale parallel computing was introduced to cope with the increased computational cost and a supercomputer was used for the analysis. To validate the proposed method, simultaneous Bayesian estimation of fault geometry and slip distribution with slip constraints was performed using the crustal deformation observed in the 2018 Hokkaido Eastern Iburi earthquake. Hierarchical parameterization and massively parallelized Bayesian inference used in this study have broad applicability not only in earthquake research but also in other scientific and engineering fields.

Geofluid behavior prior to the 2018 Hokkaido Eastern Iburi earthquake: insights from groundwater geochemistry

A notable decrease of groundwater Na/K ratios was observed several months before the 2018 Hokkaido Eastern Iburi earthquake (M6.7) at a site approximately 20 km west of the earthquake’s epicenter. To investigate the cause of the decreased Na/K ratios, we analyzed groundwater samples (commercially bottled drinking water) to assess the contribution of deep-derived fluids using Li and Sr isotopic ratios, which are reliable indicators of deep fluid contributions. No pronounced changes in the 7Li/6Li and 87Sr/86Sr ratios were observed; thus, the pre-earthquake decrease of groundwater Na/K ratios did not result from the input of deep-derived fluids. The pre-earthquake decrease in the Na/K ratio observed in this study may instead be related to CO2 influx into the aquifer. The CO2 may have leaked from carbon dioxide capture and storage (CCS), because CCS was in operation near the epicenter of the 2018 Hokkaido Eastern Iburi earthquake. Decreases of the Na/K ratios and increases of the CO2 concentrations in groundwater have been reported before other large earthquakes; thus, CO2 influx into groundwater may be a common phenomenon preceding earthquakes.

Assessing Forest Carbon Stocks Transported by Landslides Associated with the 2018 Hokkaido Eastern Iburi Earthquake

Assessing Forest Carbon Stocks Transported by Landslides Associated with the 2018 Hokkaido Eastern Iburi Earthquake

Particle crushing and critical state of volcanic pumice – 2018 Hokkaido Eastern Iburi Earthquake

Volcanic pumice, with special characteristics such as crushable particles and high water retention, is distributed throughout Japan and serves as the source layer for slope hazards characterised by post-failure gentle slope flows and long-distance flows. The aim of this study is to determine the relationship between the crushing characteristics and the mechanical properties of porous pumice, which often contributes to such disasters. As the porous pumice material, Ta-d pumice, which caused numerous slope disasters during the 2018 Hokkaido Eastern Iburi Earthquake in Japan, was collected and subjected to a series of triaxial compression tests. The grain size distribution of the pumice before all the tests was adjusted to be uniform, and the amount of crushing was quantified by measuring the grain size distribution after the tests. The results suggest that the critical state and isotropic consolidation of porous pumice can be systematically expressed in a three-dimensional space with the axes of the void ratio, mean effective stress, and degree of particle crushing. Furthermore, a gentle slope disaster with an inclination of less than 21°, that had occurred at the site from which the Ta-d pumice was collected, was discussed in terms of its flow potential, showing that the flow distance can be adequately explained.

Geotechnical characteristics and seismic stability evaluation of pumice-fall deposits soil on collapse slope by the 2018 Hokkaido eastern Iburi earthquake

The September 6, 2018, earthquake in the eastern part of Hokkaido, Japan, caused extensive slope failures in Atsuma-town, Hokkaido, Japan. In this study, the authors performed in situ investigations, including trenching and portable dynamic cone penetration tests, on weathered fallen pumice sediments, which are one of the causes of the slope failures. In addition, we performed direct box shear tests on undisturbed samples collected from an undisturbed area under various shear conditions to characterize mechanical properties of the soil. The parameters obtained from the mechanical tests were used to evaluate slope stability under normal and seismic conditions with an infinite-length slope model. The results showed that the slopes where seismic failures occurred had a fragile layer from the surface to a depth of approximately 1.5 m, which generally corresponded to the depth of failure. Weathered pumice deposits with extremely high-water content existed at the boundary between the weak layer and the basement layer, and their shear strength was velocity dependent. It has been shown that an infinite-length slope stability analysis can be performed by using mechanical parameters for which velocity dependence of horizontal acceleration and shear strength due to seismic motion are accounted for.

Controlling role of ground motion time histories on the north‒south difference in coseismic landslide distribution during the 2018 Hokkaido Eastern Iburi earthquake

Controlling role of ground motion time histories on the north‒south difference in coseismic landslide distribution during the 2018 Hokkaido Eastern Iburi earthquake

Electrical Tests and Current Distribution of 500-m-Long DC Superconducting Power Cable

The third cooling test of 500m-long superconducting DC power transmission (SCDC) line constructed in Ishikari city, Hokkaido, was carried out in 2021. The system consists of underground superconducting cable with its electric capacities, 5kA and 100MVA. After completing two cooling tests in 2015, the system remained in a shutdown state. Therefore, this test was the first operation test in six years. The targets of the 3rd cooling test were to confirm the presence/absence of the damage due the Hokkaido Eastern Iburi Earthquake (Mj.6.7) that occurred in and acquire basic data for improving the efficiency of SCDC. Regarding electrical tests, the various tests, such as the capacitance/dielectric loss tangent temperature characteristic measurement, the insulation resistance measurement, the DC withstand voltage test, the rated current loading test were conducted to confirm the soundness of cables and the stability of current loading. One of the characteristic tests conducted this time is the current diversion test. Despite the six-year outage and the massive earthquake that occurred during that time, all tests confirmed that there was no deterioration in the performance of the cable system. In the current diversion test, it was confirmed that the current was evenly distributed to each conductor at the cable joint.

Combining portable cone penetration test and electrical resistivity tomography to assess residual risks after shallow landslides: a case at the Hokkaido Eastern Iburi earthquake in 2018 in Japan

Landscapes disturbed by shallow landslides present several residual risks. For example, residual soils on sliding surfaces may lead to the re-occurrence of shallow landslides, and landslide deposits on valley floors may introduce multiple hazards associated with river blocking or landslide damming. Despite such risks, few studies have investigated the residual risks following shallow landslides, and thus, no methodology has been established for assessing such risks. In this study, we combined portable cone penetration test (PCPT) and electrical resistivity tomography (ERT) to investigate the post-event risks associated with a landslide scar resulting from the 2018 Hokkaido Eastern Iburi earthquake (HEIE) in Japan. The multi-point PCPT identified the interface between a weak volcanic soil layer and underlying weathered sedimentary rocks. This interface exhibited an abrupt spatial change in resistivity, ranging from 20 to 100 Ωm. The thickness of the residual soil layer was 0.3–3.2 m and that of the landslide deposit exceeded 2.0 m. An infinite-slope stability analysis was performed to predict the re-occurrence of shallow landslides due to a lower intensity earthquake than that of HEIE in this region. Considering the area affected by the HEIE and the physical properties of soil layers, our findings imply a widespread risk of re-occurrence of shallow landslides over an area of 20 km × 20 km. Although the areal coverage of the combined ERT/PCPT method needs to be extended, it is effective for assessing the residual risks associated with shallow landslides.

Emergency Medical Team Response during the Hokkaido Eastern Iburi Earthquake 2018: J-SPEED Data Analysis.

In the last ten years, Japan has experienced several large-scale earthquakes with devastating social and health impacts. Earthquakes directly and indirectly cause a variety of health problems. Further investigation is required to increase preparedness and preventive efforts. In response to the Hokkaido Eastern Iburi Earthquake on September 6, 2018, 32 Emergency Medical Teams (EMTs) employed the Japanese version of Surveillance in Post-Extreme Emergencies and Disasters (J-SPEED) as a national standard daily reporting template, gathering data on the number and type of health problems treated. The purpose of the study is to conduct a descriptive epidemiology study using the J-SPEED data to better understand the health problems during the earthquake disaster. Reported items in J-SPEED (Ver 1.0) form were analyzed by age, gender, and time to better understand the health issues that have arisen from the earthquake. Most consultations (721; 97.6%) occurred between Day 1 and Day 13 of the 32-day EMT response. During the response period, disaster stress-related symptoms were the most common health event (15.2%), followed by wounds (14.5%) and skin diseases (7.0%). The most often reported health event during the response period was stress-associated illnesses related to disasters, followed by wounds and skin conditions. The health consequences of natural disasters depend on diverse local environment and population. As a result, this initial study was hard to generalize; however, it is expected that data accumulated using the J-SPEED system in the future will strengthen and extend the conclusions.

How a Household Survived a Natural Hazard-Triggered Blackout with Photovoltaic and Battery Energy System: A Report of 2018 Hokkaido Eastern Iburi Earthquake in Japan

Energy-generating and storage systems, such as photovoltaic (PV) panels and energy storage batteries in homes, are becoming increasingly popular in the context of decarbonization. The systems are also expected to increase household resilience to natural hazard-triggered blackouts. However, how these systems contribute to the use of electrical appliances in households in actual cases is not sufficiently known. Therefore, this report aims to describe the activities that a household with an energy-generating and storage system could undertake during a natural hazard-triggered blackout. We focused on the blackout triggered by the 2018 Hokkaido Eastern Iburi Earthquake (approximately 2.95 million households lost power) and conducted an interview with a household living in a detached all-electric house with a PV and battery system. The results showed that the household lived without inconveniences during the blackout due to the power supply from the installed system, despite the weather. They charged cell phones and used a television, refrigerator, microwave oven, cooking heater, and bath. Moreover, the household’s electricity was also supplied to other households. These results clarified the actual benefit of enhancing household and community resilience of the systems. The results will aid household decision-making for the installation and governmental consideration of subsidies.

Household resilience realized by photovoltaic and battery energy system in natural hazard-triggered blackouts: Evidence from Japan

Photovoltaic (PV) panels, increasingly popular due to decarbonization, are expected to increase household resilience during natural hazard-triggered blackouts. Their effectiveness is also expected to improve with the addition of storage batteries. However, PV ownership does not necessarily lead to electricity use in blackouts; even when used, the actual use of electrical appliances is unclear. We examined households that owned PV panels and experienced a natural hazard-triggered blackout to determine what barriers and facilitating factors existed for using PV electricity, which electrical appliances could be used with PV, and whether the benefits of PV changed with storage batteries. A questionnaire survey targeted households in Japan that experienced blackouts caused by the 2018 Hokkaido Eastern Iburi earthquake and the 2019 Typhoon Faxai, respectively, and their responses (n = 282 and 259, respectively) were analyzed. Descriptive statistics showed that while it was sometimes necessary to switch PV systems to stand-alone operation mode during blackouts, most respondents who did not use PV systems did not know how to operate them, which was a barrier to their use. The main facilitating factors were preparation like advance explanations from the sales staff and the regular manual check for use in case of blackouts. Bayesian item response theory models demonstrated that PV electricity enabled households to use a variety of electrical appliances, including those related to food and communication (i.e., refrigerators and cell phone chargers), whose needs increased during the blackouts. The probability of each appliance being used increased in households with batteries. The above results were commonly confirmed regardless of hazard type. These findings will improve the disaster resilience of households equipped with PV panels and batteries, as well as spread the installation of such equipment by highlighting their benefits.

Examination of one-dimensional S-wave velocity structure using strong-motion data for high-seismic-intensity area during the 2018 Hokkaido Eastern Iburi earthquake

The Yufutsu Plain, a sedimentary basin surrounded by mountains, is located in the southern part of the Ishikari–Yufutsu Lowlands, Hokkaido, Japan. The Hidaka arc–arc collision zone, located in the eastern part of the Yufutsu Plain, forms the Hidaka Mountain range in central Hokkaido, with the Ishikari–Teichi–Toen Fault Zone of the Ishikari–Yufutsu Lowlands on the west side, which forms part of a major geological boundary that extends in the north–south direction. The 2018 Hokkaido Eastern Iburi earthquake (Mw 6.6) occurred at 03:08 JST on September 6, 2018, in this arc–arc collision zone. The K-NET HKD126 station in Mukawa Town, which is close to the severely damaged basin margin area, recorded strong-motion data with strong power for a predominant frequency of 0.5–1.0 Hz during the main shock. The peak ground acceleration was 661 cm/s2 in the east–west direction. The site amplification characteristics of the shallow S-wave velocity structure, which was estimated from microtremor array observations and surface wave explorations, were one of the causes of this strong ground motion. It is essential to accurately estimate the depth of the seismic bedrock and basin margin to evaluate the long period of large earthquakes. In this study, we used strong-motion data recorded at HKD126 and three temporary strong-motion stations near the basin margin area to tune the deep S-wave velocity structure. First, we performed microtremor array observations and surface wave explorations to estimate the S-wave velocity structure to a depth of 1 km beneath the station at the hill site because a detailed shallow structure is not available for this site. Then, with a combination of the estimated S-wave velocity structure and the existing structure, we tuned the deep structures based on an autocorrelation function analysis using strong-motion data. The validity of the estimated structures from the shallowest depth to the seismic bedrock was verified based on the differences between the observed arrival time difference and theoretical travel time difference for the S-wave initial motion. We estimated the seismic bedrock of the four stations to be at a depth of 7–10 km. In addition, an autocorrelation function analysis suggests topological bedrock undulations.Graphical

Changes in sediment erosion rate in the Azuma River basin after the Hokkaido eastern Iburi earthquake in 2018

The Hokkaido Eastern Iburi earthquake in 2018 induced the largest landslide over the Azuma River basin in Hokkaido, the north island of Japan. This study estimated the sediment erosion rate over the Azuma River basin using the universal soil loss equation (USLE) for the pre- and post-earthquake conditions. The results showed that the potential sediment erosion rate from the entire basin increased by 8–29 times more than the pre-earthquake condition. The primary cause of the increase in sediment erosion rate was deforestation due to the landslide, which generated 13% of the bare land area in the entire basin. In addition, the influence of interannual variability of rainfall on sediment erosion rate was calculated from observed rainfall data. As a result, the minimum annual sediment erosion rate after the earthquake exceeded the maximum before the earthquake, indicating that the post-earthquake sediment erosion rate is substantially higher than the pre-earthquake rate. These findings suggest that in order to assess the sediment erosion risk over the Azuma River basin, it is necessary to predict and monitor the vegetation recovery, considering the interannual variability of rainfall.

Changes in morphological characteristics of drainage basins following coseismic landslides by the 2018 Hokkaido Eastern Iburi Earthquake

Changes in morphological characteristics of drainage basins following coseismic landslides by the 2018 Hokkaido Eastern Iburi Earthquake

Estimation of the Sedimentary Basin Structure of the Southeastern Part of the Ishikari Lowland, Hokkaido, Inferred from Receiver Function

There is a very thick (>10 km) sedimentary basin in the Ishikari lowland, the central Hokkaido. This thick sedimentary basin affects strong ground motions observed from earthquakes, e.g., the 2018 Hokkaido Eastern Iburi Earthquake occurred in the southeastern part of the Ishikari lowland. Such a thick sedimentary basin structure is difficult to explore by using microtremor array or seismic reflection. This study investigated the sedimentary basin structures at 16 strong motion stations in the southeastern part of the Ishikari lowland by using the borehole-to-surface spectral ratio and the receiver function method based on the observed strong-motion records. First, at the KiK-net stations, the surface-to-borehole spectral ratio was obtained from observed strong-motion records. The S-wave velocity structure models between the surface and the borehole seismograph were estimated by fitting theoretical spectral ratios to the observed one. Next, the velocity structure models between the surface and the seismic bedrock were derived from the receiver function calculated from the observed records. We used the downhill simplex method to obtain the velocity structure model from the receiver function. The inverse analysis with three different initial models resulted in variations in the estimated velocity structure models. Synthetic velocity waveforms of the aftershock of the 2018 Eastern Iburi earthquake were calculated based on the estimated velocity structure models. The synthetic velocities (0.1-1 Hz) agree with the S waves of the observed records. The synthetic waveforms obtained from three models at each site were very similar. We compare the resulted velocity models with the microtremor array surveys carried out by previous studies. The fundamental mode phase velocities calculated from the velocity structure models obtained from the receiver functions correspond to the observed phase velocity from the microtremor array survey in higher (>0.4-0.8 Hz) frequencies. However, in lower frequencies (<0.4 Hz), the observed phase velocities at HKD184, IKRH03, and IBUH03 agree with the higher-mode phase velocities than the fundamental ones. This agreement suggests that the higher-mode Rayleigh waves dominate the observed phase velocities in the lower frequencies. The velocity structure models estimated from the receiver function analysis indicated that the low Vs (<1.1 km/s) layer deposited thickly in the western part of the Ishikari lowland and the middle Vs (1.1<Vs<2.7 km/s) layers were thick in the eastern part of the lowland. The depth of the estimated seismic bedrock was 12 km at HDKH04, located in the southeastern part of the lowland, which was the deepest between the study sites.

Characteristics of physical properties of the sliding and its surrounding layers in landslides caused by the 2018 Hokkaido Eastern Iburi Earthquake

A 6.6-Mw earthquake struck the Iburi region of Hokkaido, Japan, in 2018, triggering massive landslides. Most of these landslides were shallow and occurred mostly in the Atsuma and Abira towns. Ta-c and Ta-d tephra layers have been found in the Towa landslide at Atsuma from the Tarumai volcano, while Ta-d, En-a, and Spfa-1 tephra layers have been found in the Mizuho landslide at Abira from the Tarumai and Eniwa volcanos, as well as the Shikotsu caldera. Field observations from previous studies revealed that the sliding layers were located in the Ta-d and En-a layers at the Towa and Mizuho landslides, respectively. Unlike previous research on earthquake-induced landslides, which were investigated using mechanical properties, this study investigates the characteristics of physical properties, saturated permeability properties, and content of clay minerals on sliding and surrounding tephra layers. Results from this study reveal that the physical properties of sliding layers from two landslides demonstrated the same characteristics: non-plastic soil with a low density of soil particles, void ratio, and dry density; these characteristics could influence earthquake-induced landslides. It also reveals a relationship between the plasticity chart and the age of tephra materials, including the relationship between the weathering process and density of soil particles and the dissimilarity in characteristics of saturated permeability properties in tephra materials.

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.

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.