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GEOMATICS, NATURAL HAZARDS AND RISK 2021, VOL. 12, NO. 1, 1–28 https://doi.org/10.1080/19475705.2020.1856201 Investigation and mechanism analysis of disasters under Hokkaido Eastern Iburi earthquake a a b c Hanxu Zhou , Ailan Che , Lanmin Wang and Lin Wang School of Naval Architecture, Ocean and Civil Engineering, Shanghai Jiao Tong University, Shanghai, China; Key Laboratory of Loess Earthquake Engineering, China Earthquake Administration, Lanzhou, Gansu Province, China; Chuo Kaihatsu Corp, Tokyo, Japan ABSTRACT ARTICLE HISTORY Received 18 May 2020 An Mw6.6 earthquake occurred at 3a.m. on September 6, 2018, in Accepted 20 November 2020 the eastern part of Iburi, Hokkaido. This earthquake killed 41 peo- ple, caused a series of service interruptions in Sapporo city. Soil KEYWORDS liquefaction and approximately 6000 landslides were triggered. A Hokkaido Eastern Iburi field investigation was conducted on liquefaction and typical earthquake; liquefaction; landslides during September 18 to 24, 2018. The liquefaction dis- regional landslides; aster exhibited soil flow, subgrade collapse, uneven settlement, continuous heavy rainfall; earthquake subsidence, and deformation of buildings. After inves- shallow translational earth tigation of typical landslides, it was found that owing to the con- slide; earth flow tinuous heavy rainfall and typhoon, the surface soil had a high- water content. A layered loam of approximately 2 m below ground surface developed into a weak surface. Based on the ana- lysis of sliding morphology, the landslides were classified into translational earth slides and earth flow. To clarify the mechanism of regional landslides, the stability and permanent displacement of slopes considering effect of continuous heavy rainfall and seis- mic motion was analyzed. Limit equilibrium analysis was applied based on the pseudo-static method. Then Newmark displacement calculation was conducted based on the seismic acceleration record. The distribution range of analysis results showed agree- ment with actual landslides disasters. The results verified the con- tribution of continuous rainfall and strong motion to the failure of regional slopes. 1. Introduction Strong earthquakes have occurred frequently around the world. In the last 10 years, there have been five strong earthquakes that have caused serious damage, including the 2004 Indian Ocean earthquake, 2008 Chinese Wenchuan earthquake, 2010 Chile earthquake, 2011 Great East Japan earthquake, and 2015 Nepal earthquake. In 2004, the Indian Ocean earthquake was recorded at a magnitude of 9.1 with an epicenter off the west coast of northern Sumatra, and it induced a series of tsunamis up to CONTACT L. WANG email@example.com 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (http:// creativecommons.org/licenses/by-nc/4.0/), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 2 H. ZHOU ET AL. Figure 1. Hokkaido Eastern Iburi earthquake seismic intensity distribution map. Source: Geospatial Information Authority of Japan. 30 m high (Stein and Okal 2007). In 2008, the Wenchuan earthquake with a magni- tude of 7.9 occurred in the Longmenshan tectonic zone and caused nearly 60,000 landslides (Chigira et al. 2010; Gorum et al. 2011). On February 27, 2010, the Chile earthquake occurred offshore from the Maule region with a magnitude of 8.8, induc- ing a tsunami that caused severe damage along the coasts (Lorito et al. 2011). In 2011, the earthquake off the Pacific coast of Tohoku triggered strong tsunami and caused nuclear leak at the Fukushima Daiichi Nuclear Power Station (Krausmann and Cruz 2013; Wartman et al. 2013). On April 25, 2015, a large (M7.8) earthquake occurred in central Nepal and triggered thousands of landslides in the steep topog- raphy of that country (U.S. Geological Survey 2015; Zhao 2016). The disasters caused by these earthquakes including tsunami, ground motion, soil liquefaction, and land- slides have claimed many lives. Therefore, it has become an essential research topic to reduce the threat of earthquakes to the safety of life and property. The Hokkaido Eastern Iburi earthquake occurred at 3 a.m. on September 6, 2018. The earthquake’s epicenter was located at 42.72 N and 142.00 E with a depth of 37 km. A moment magnitude of 6.6 was registered, and the maximum intensity of 7 was registered in Atsuma (Figure 1). A peak acceleration of 1796 Gal (composite value of three directions) was registered at the observation station of Oiwake. The crustal deformation in Atsuma and Hidaka was 5.1 cm and 6.9 cm to the southeast, respectively. The motion form was reverse fault motion on a high-angle fault plane in the north-south direction. The top of the fault was about 15 km deep, and the bottom of the fault was approximately 31 km deep. The seismic moment was 8.59 10 Nm (the above data is from Geospatial Information Authority of Japan). The peak seismic acceleration wave recorded at the Oiwake observation station by K-net of National Research Institute for Earth Science and Disaster Resilience, and the acceleration response spectrum are shown in Figure 2. The predominant period of acceleration wave was approximately 0.5 s. The tremendous impact of the earth- quake on Hokkaido resulted in a 3-day blackout in Sapporo. In this earthquake, 41 GEOMATICS, NATURAL HAZARDS AND RISK 3 Figure 2. Recorded data of Oiwake observation station from K-net. Source: National Research Institute for Earth Science and Disaster Resilience. lives were lost and approximately 15,000 buildings were damaged (partially or fully). In particular, this huge earthquake triggered nearly 6000 landslides, which claimed 36 lives. The distribution of landslides was concentrated in an area of 20 km 20 km in Atsuma, and the location of most landslides was identified through vertical orthopho- tographs (Figure 3) by Geospatial Information Authority of Japan. In addition, the liquefaction phenomena occurred in Sapporo city and Kitahiroshima city, causing a large range of ground deformation. The causes of geological hazards after strong earthquakes are complex. Studies, including field investigations, laboratory tests, and numerical simulations, had been carried out by researchers on the characteristics and mechanisms of earthquake- induced disasters. Since the Wenchuan earthquake, many researchers had conducted field investigations on earthquake-induced landslides and studied the characteristics of disasters to demonstrate the mechanism of slope failure (Xu et al. 2009; Yin et al. 2009; Gorum et al. 2011). Sassa et al. performed an investigation on the landslides induced by the 2004 Mid-Niigata earthquake and examined the triggering and move- ment mechanisms through real earthquake wave loading and cyclic loading tests (Sassa et al. 2005). The shaking table test, which is an intuitive and accurate method for analyzing the dynamic response of a slope under a seismic action, had been widely used by researchers (Lin and Wang 2006; Wang and Lin 2011; Varghese and Madhavi 2014). Numerical simulation had also been extensively adopted to analyze the failure mode and dynamic response of disasters as the capabilities of computers increased (Liyanathirana and Poulos 2002; Zhou et al. 2013; Xing et al., 2016). In recent years, the application of GIS in disaster assessment has attracted much atten- tion. At present, relevant researchers mainly use statistical probability or physical deterministic models to analyze regional disaster susceptibility. Some of the statistical probability models, frequency ratio methods, logistic regression analysis methods, and artificial neural network models have been applied on landslide susceptibility map- ping (Akgun et al. 2008; Dahal et al. 2008; Raja et al. 2018). However, landslide sus- ceptibility map based on statistical analysis lacks physical and mechanical significance. Salciarini et al. (2006) and Baum et al. (2005) applied the TRIGRS model to analyze the stability of regional slopes under the effect of rainfall. Romeo (2000), 4 H. ZHOU ET AL. Figure 3. Investigation area and regional landslides inventory map. Source: Geospatial Information Authority of Japan. Refice and Capolongo (2002), and Carro et al. (2003) assessed landslide disasters induced by earthquakes using the Newmark slope stability model. Moreover, the limit equilibrium method with GIS had also been used to obtain the spatial distribution of unstable slopes (Jibson et al. 2000; Shou and Wang 2003; Zhou et al. 2003). On September 18, 2018, a field investigation was conducted at Hokkaido. The sur- vey areas included Sapporo city, Kitahiroshima city, and Atsuma town, as shown in Figure 3. We made a detailed investigation of the liquefaction of foundation soil and landslide disasters triggered by the Hokkaido Eastern Iburi earthquake. The character- istics of the disasters were summarized and analyzed to study the failure mechanism of regional landslides. In this paper, we review the historical earthquake, topography, and geology information of the seismic-affected area. During the investigation, drone flying, field photography, and sampling test were performed to comprehend the fail- ure mode of regional landslides. Then, the influencing factors were proposed and the failure mechanism was clarified by stability analysis and permanent displacement ana- lysis of regional slopes. 2. Historical earthquakes and geological information in seismic- affected areas Large areas were affected by the Hokkaido Eastern Iburi earthquake. An area from 42 to 44 N latitude and from 140 to 144 E longitude was selected as the study area. 2.1. Historical earthquakes The Hokkaido area had suffered many strong earthquakes in its history. Figure 4 shows the location of the earthquakes with magnitude > 5.0 in the Iburi area from January 2000 to September 2018. The size of circle represents the magnitude of his- torical earthquakes. It illustrates that earthquakes with magnitude < 5.6 are relatively GEOMATICS, NATURAL HAZARDS AND RISK 5 Figure 4. Historical earthquakes distribution. Source: National Research Institute for Earth Science and Disaster Resilience. frequent in the Hokkaido area and their locations are concentrated in the southeast- ern part of Hidaka mountains. In 2007, an earthquake of magnitude 5.6 occurred in the same location as the epicenter of the Eastern Iburi earthquake. The peak ground acceleration recorded by the 2007 earthquake was 56 Gal, and it did not cause any geological disasters. The earthquake with the highest magnitude that occurred in the study area was the 2003 Tokachi-Oki earthquake. This great earthquake measured 8.3 on the moment magnitude scale, and it occurred nearly 100 km off the coast of Hokkaido and caused tsunami, landslides, and soil liquefaction. This earthquake was recorded as the most intense earthquake since modern record-keeping started, although it just caused some slight disasters. Some coastal facilities were damaged by the tsunami, and a few landslides were triggered by the tremor. In addition, soil liquefaction occurred in Satozuka and Utsukushigaoka blocks of Kiyota ward in Sapporo, Urakawa town in Urakawa district, Mukawa town in Yufutsu district, and Tomakomai port in Tomakomai city. In general, previous earthquakes did not cause serious disasters to Hokkaido. Compared with the 2018 East Iburi earthquake, the 2003 Tokachi-Oki earthquake was an earthquake occurred below sea with a depth of 42 km, while the epicenter of the former was located on land with a depth of 37 km. The magnitude of the 2018 East Iburi earthquake is Mw 6.6, which is lower than that of the 2003 Tokachi-Oki earthquake, although it caused more serious disasters. Moreover, in the Satozuka and Utsukushigaoka blocks of Kiyota ward where soil liquefaction was induced by the 2003 Tokachi-Oki earthquake, the liquefaction disaster appeared again during the 2018 Eastern Iburi earthquake. The recurrence of a similar liquefaction phenomenon in the same region indicates that the soil has a certain liquefaction risk. 2.2. Topography From a digital elevation model (DEM) photographed by NASA and Ministry of Economy, Trade and Industry (METI) of Japan, we obtained the topographic map of 6 H. ZHOU ET AL. Figure 5. Topographic map. Source: Geospatial Information Authority of Japan. the Hokkaido area (Figure 5). The red area in the figure represents the landslide inventory map. The topographical types of the study area mainly included mountain, plain, basin, and lowlands. In the land part of study area, the terrain in the southwest is the western Sapporo mountains. Moreover, the Ishikari plain is located to the east of the western Sapporo mountains with an altitude under 30 m. The Ishikari plain is where Sapporo city and Kitahiroshima city are situated, where soil liquefaction occurred during the 2018 Eastern Ibuli earthquake. The altitude of the liquefaction area was in the range of 0–80 m. The eastern part of the Ishikari plain is the Yubari mountains and the Kamikawa basin, while the northern part is the Mashike mountains. Moreover, the Hidaka mountains are located in the southeastern part of Yubari mountains. The Ishikari-Tomakomai lowland is distributed between the Hidaka mountains, Yubari mountains, and the coastline. The regional landslides were concentrated on the Ishikari- Tomakomai lowland. The elevation of this lowland ranges from 0 to 200 m. According to the previous study (Kamp et al., 2008), the cause of landslides can be related to slope angle, elevation, and slope direction. Therefore, the correlation between slope topographic parameters and landslides was analyzed. By dividing the slope angle, elevation, and slope direction into several intervals and counting the proportion of land- slides in each interval, we could determine the relationship between landslides distribu- tion and topographic parameters (Figure 6). It can be seen that about one third of the landslides occurred on slopes with slope angle ranging from 20 to 25 , which is consist- ent with the statistical results of a previous study indicating that the landslides were con- centrated on slopes of about 20 (Ayalew et al. 2004). Moreover, the elevation of most landslides ranges from 50 m to 100 m rather than high elevation, which is against to nor- mal principle. The reason is that distance from epicenter played a more important role than elevation. Ishikari-Tomakomai lowland is the nearest hilly area to epicenter. In add- ition, the proportion of landslides in each slope direction interval was close to each other. This fact indicates that the slopes did not slid in one direction. Generally, rock collapse is related to slope direction. Because the fractures developed in rocks are directional. The same direction of seismic wave propagation and fracture will promote the probability of GEOMATICS, NATURAL HAZARDS AND RISK 7 Figure 6. Statistical of topography factor classification and spatial distribution of landslides. Source: Shanghai Jiao Tong Univerisity/ China Earthquake Administration/ Chuo Kaihatsu Corp. collapse. But the landslides under Hokkaido eastern Iburi earthquake were all soil land- slides having no such correlation. So, there was no correlation between slope direction and landslide occurrence. 2.3. Geology Figure 7 shows a geological map of the study area from The National Institute of Advanced Industrial Science and Technology. A fault zone in the Ishikari lowlands developed along the Totsuke hills in the north-south direction near the epicenter. The north-south-oriented Totsuke hills are thought to have been formed by folding (Ayalew et al. 2011). Taking the Totsuke hills as the boundary, it can be learned that the basement in the western side of Totsuke hills is covered with Quaternary pumice- fall deposit originated from the volcanoes west of Sapporo city (Ayalew et al. 2011), and it spans from approximately 2.588 million years ago to the present. The Ishikari plain is located on the Quaternary pumice-fall deposit. Therefore, the geological morphology of the liquefaction area is Quaternary pumice-fall deposit. The basement 8 H. ZHOU ET AL. Figure 7. Geological map of study area. Source: The National Institute of Advanced Industrial Science and Technology. in the eastern region of Tosuke hills located in the epicenter and landslide disaster area is Neogene sedimentary rock, which can be dated to 23.03–2.588 million years ago. Most shallow landslides were distributed on this type of geology. There were also vol- canic ash deposits on the surface of the eastern region of Tosuke hills, although they are not identified in the geological map because of their small thickness. In the latter part of the investigation, we made a more detailed analysis of the surface soil. 3. Field investigation of earthquake disasters The forms of disaster induced by the 2018 Eastern Iburi earthquake included soil liquefaction and regional landslides. Hence, field investigation was essential to study the failure mechanism of regional landslides. On September 18, a research team was set up to conduct field investigation in the affected area. Then, on September 22, we investigated the liquefaction area including the Satozuka and Utsukashigaoka blocks in Sapporo city and the Okae block in Kitahiroshima city. The next two days, September 23 and 24, we investigated five landslides with typical characteristics including two landslides in Tomisato village (a water tower and a pumpkin field), two landslides in Yoshino village (north and south), and one landslide in Sakurazaka village. Field photography, drone shooting, sampling test, and other means were adopted to study the characteristics of disasters. 3.1. Soil liquefaction Figure 8 shows the soil liquefaction in investigation area, containing three blocks: Satozuka and Utsukashigaoka blocks in Sapporo city and Okae block in Kitahiroshima city. In the Satozuka and Utsukashigaoka blocks, soil liquefaction had occurred earlier during the 2003 Tokachi-Oki earthquake. GEOMATICS, NATURAL HAZARDS AND RISK 9 Figure 8. Soil liquefaction investigation area. Source: Shanghai Jiao Tong Univerisity/ China Earthquake Administration/ Chuo Kaihatsu Corp. 1. Liquefaction disaster Obvious phenomena including soil flow (Figure 9a), subgrade collapse, uneven settle- ment (Figure 9b), earthquake subsidence (Figure 9c), and deformation of buildings (Figure 9d) could be observed on the streets. The main characteristic of liquefaction is the liquefaction flow of soil in the foundation. The liquefied foundation soil flowed out of the cracks, and soil loss in the foundation led to subgrade collapse, uneven settlement, earthquake subsidence, and deformation of buildings. In Satozuka block, the liquefied soil flowed out of the foundation about 200 m long along a slope of 1.5% to 3%, which caused a maximum settlement of 3 m (Figure 9b). Finally, the liquefied soil accumulated on the pavement (Figure 9a). The liquefied soil exhibited strong fluidity. 2. Re-liquefied area In the Satozuka and Utsukashigaoka blocks, soil liquefaction had occurred during the 2003 Tokachi-Oki earthquake. Therefore, these two blocks were the re-liquefied areas. In the 2018 Eastern Iburi earthquake, water and sand were spewed out from the cracks in the asphalt pavement and accumulated on the concave area of road surface. There were traces of running water on the blasting sand. Some affected streets were exactly the same disaster area where soil liquefaction occurred in 2003. Figure 10 shows a comparison of images taken in 2003 and 2018 of the same location. The comparison result shows that thedisasterseverity in 2018 was lighterthan that in 2003. Thevolume of spewedsand and water in 2018 was smaller than that in 2003. The liquefied area in 2003 had not been repaired and strengthened. Therefore, the liquefied soil reconsolidated in the natural state after 2003. From consolidation, liquefaction, to reconsolidation under self-weight, the density and shear wave velocity of the soils decreased first and then increased. The density and shear wave velocity of the soils after reconsolidation were even larger than the initial values before the liquefaction (Meng et al. 2017). After the 2003 Tokachi-Oki earthquake and reconsolidation, the density and shear wave velocity of the soils 10 H. ZHOU ET AL. Figure 9. Photos of soil liquefaction disaster.Source: Shanghai Jiao Tong Univerisity/ China Earthquake Administration/ Chuo Kaihatsu Corp. Figure 10. Comparison of liquefied sites between 2003 and 2018 in Utsukushigaoka block. Source: Shanghai Jiao Tong Univerisity/ China Earthquake Administration/ Chuo Kaihatsu Corp. GEOMATICS, NATURAL HAZARDS AND RISK 11 Figure 11. Landslide disaster investigation areas. Source: Shanghai Jiao Tong Univerisity/ China Earthquake Administration/ Chuo Kaihatsu Corp. increased. Therefore, the severity of the liquefaction disaster under the 2018 earthquake was not so serious as that under the 2003 earthquake. 3.2. Regional landslides The total number of landslides reached 6000 and 36 people died from being buried by landslides. The landslides damaged farmland and buried large areas of housing and roads. In the areas most affected by the landslides, five landslides with typical characteristics were investigated including two landslides in Tomisato village (a water tower and a pumpkin field), two landslides in Yoshino village (north and south), and one landslide in Sakurazaka village (Figure 11). 1. Disaster investigation Figure 12a shows photos of the landslide in Yoshino village (south). The sliding body carried a number of trees and buried the houses at the foot of the slope, causing the death of 36 residents. The red circle is where the dead residents lived (Figure 12a). The wreckage of destroyed houses was mixed with the deposits. The elevation of the slope in Yoshino village was about 60 m, and the original slope degree was 27 . The photos show that the categories of the landslides belong to translational earth slide type and earth flow type. The shape of earth flow landslide looks like an “hourglass.” There is a contraction section between the source area and the depositional area (U.S. Geological Survey 1996). The width of translational earth slide was 300 m. The sur- face soil was mainly pumice and was approximately 3 m thick. The deposit soil con- tained a high-water content close to saturation. On the other hand, Figure 12b shows photos of the landslide in Yoshino village (north). There were no casualties, but structures and roads were buried. The sliding state of this 12 H. ZHOU ET AL. Figure 12. Photos of landslide disaster.Source: Shanghai Jiao Tong Univerisity/ China Earthquake Administration/ Chuo Kaihatsu Corp. GEOMATICS, NATURAL HAZARDS AND RISK 13 Table 1. Characteristics of investigated landslides. Slope Sliding Surface soil Position of landslide Elevation (m) degree ( ) distance (m) thickness (m) Failure mode Yoshino(south) 60 27 70–120 3 Translational earth slide Yoshino(north) 60 26 70–120 3 Translational earth slide Tomisato (pumpkin field) 50 21 90–130 2 Earth flow Tomisato (water tower) 60 25 70–100 2 Earth flow Sakurazaka 50 23 60–80 2–3 Earth flow landslide was similar to that of the southern landslide in Yoshino village, although the width of this landslide was longer reaching700 m (Figure 12b). It was found that trees remained upright on the deposits (Figure 12b). Therefore, the sliding body slid in its entirety. Figure 12c shows photos of the landslide in Tomisato village (pumpkin field). The sliding body spread into the pumpkin field at the foot of the slope. The elevation of this slope was about 50 m, and the original slope was 21 . The surface soil was approximately 2 m thick. Water was gushing out of the sliding bed. The deposits at the bottom of the slope had a high-water content. The landslide was classified as earth flow type. The sliding body exhibited strong fluidity. The sliding distance of the soil was about 130 m. The sliding bed was close to flat, and the shrub on the surface of the slope slid along with surface soil and remained intact. Figure 12d shows the landslide in Tomisato village (water tower). This landslide was a collection of many small slides. The sliding mode was similar to that of the landslide in the pumpkin field of Tomisato village. The elevation of this slope was about 50 m. There was a water factory at the bottom of the slope. The water tower and building were buried by the deposits, although they did not collapse. It can be inferred that the velocity of sliding body and the impact force on the water tower were not significant. The surface soil slid down because of instability, but without carrying abundant energy. However, ground motion had an impact on the other structures. Cracks were found on a bridge near the landslide in Tomisato village (water tower) (Figure 12d). 2. Characteristics of the landslides The elevation, slope degree, sliding distance, and surface soil thickness of the investigated landslides are summarized in Table 1. Notice that the basic characteristics of these land- slides are identical. The sliding distance of the landslides is mainly around 100 m, and the thickness of surface soil is commonly shallow. The elevation of the landslides is mainly around 50 m, and the slope degree of these landslides mainly ranges from 20 to 30 , which accords with the statistical results in Section 2.2. Based on the collation of field investigation data and analysis of inventory map of landslides, the characteristics of the regional landslides can be summarized into eight aspects: 1) The number and scale of the landslides were huge. About 6000 landslides occurred in the range of 20 km. 2) The landslides were mostly classified as earth flow type and translational earth slide type. 3) The original slope of most landslides was between 20 and 30 . 4) The thickness of surface soil ranged from 2 m to 5 m, and most landslides were shallow. Most of the sliding beds were along the loam layer. 5) Sliding 14 H. ZHOU ET AL. Figure 13. DEM, profiles and sketch of the landslide in Yoshino village (south). Source: Shanghai Jiao Tong Univerisity/ China Earthquake Administration/ Chuo Kaihatsu Corp. body had a high-water content. During the investigation, it was found that the deposits were close to saturation and that groundwater gushed from some landslide surfaces. 6) The landslides had no directionality. The landslides in different locations slid in all direc- tions. 7) The sliding body kept fairly intact. The vegetation on the surface of the soil GEOMATICS, NATURAL HAZARDS AND RISK 15 Figure 14. DEM, profiles and sketch of the landslide in Tomisato village (pumpkin field). Source: Shanghai Jiao Tong Univerisity/ China Earthquake Administration/ Chuo Kaihatsu Corp. remained intact and upright during the sliding process. 8) The sliding distance of the landslide body ranged from 70 m to 250 m. The sliding body exhibited strong fluidity. 16 H. ZHOU ET AL. 3. Failure mode Most landslides belong to the category of translational earth slide and earth flow. The failure modes of these two types of landslides were analyzed based on the characteris- tics of landslides. a. Translational earth slide Drone was adopted to build DEM of the landslides and to protect investigators from the impact of aftershocks. Figure 13a shows the DEM of the landslide in Yoshino vil- lage (south), and Figure 13b illustrates the sketch and profiles extracted from DEM. The profile started at the top of the slope and ended at the edge of deposits. The pro- files of no. 3–6 were from the slid slope, while those of no. 1–2 were from the unslid slope near the slid slope, which can be regarded as a comparison with the slid slope. The red dotted line in the figure is the outline of the unslid slope, which can be used to simulate the original topography before landslides. From the profiles, it can be seen that the sliding bed is a planar surface. This explains that the landslides belong to the translational earth slide type. The trees on the surface of the slope slid down along with the soil and rock to produce a thicker deposit. The thickness of surface soil is small; however, the distance between the red dotted line and the sliding bed is larger than the thickness of surface soil, as this distance is counted in the height cal- culation of the trees on the slope. The height of the trees can reach 10–20 m. The landslide was actually shallow. Based on the analysis and characteristics summary of the disasters, the following conclusion could be made about the failure mode of the translational earth slide: Before the earthquake, typhoon Jebi landed on the western part of Hokkaido and heavy rainfall resulted in the increase of water content in the surface soil. Therefore, the sliding body had a high-water content. Then, a planar surface grew into the weak surface of the slope. Under the effect of earthquake motion, the stability of the sur- face soil was broken and the surface soil above the weak surface slid down along the weak surface, which is the reason why the vegetation on the surface of the soil remained intact during the sliding process. b. Earth flow Figure 14a shows the DEM of the landslide in Tomisato village (pumpkin field), and Figure 14b shows the sketch and profiles extracted from DEM. The sliding body was mainly pumice and was approximately 2 m thick. The vegetation on the slope was dominated by shrubs without tall trees. Therefore, there was no thick deposit at the bottom of the slope. The height of the slope in Tomisato village was about 50 m, and the original slope degree was 21 . The length of the southward flow of sliding body was 90–130 m, and the sliding body exhibited strong fluidity. The sliding body of a gentle slope flowed at such a long distance. Combining all these results, it was found that the sliding body had a high-water content and it might have liquefied under the effect of water, resulting in a long flow distance. The failure mode of earth flow is shear and disaggregate action of sliding body under the impact of motion. When earthquake occurs, seismic force acts on the soil particles. If the seismic motion is strong enough to break the original bonding strength and GEOMATICS, NATURAL HAZARDS AND RISK 17 Figure 15. Soil layers in Sakurazaka village. Source: Shanghai Jiao Tong Univerisity/ China Earthquake Administration/ Chuo Kaihatsu Corp. structural state of particles, the soil particles would separate from each other and slide along the weak surface. In the meanwhile, the surface soil might have liquefied under the effect of rainfall. Thus, the sliding body slid at a long distance and exhibited strong fluidity. 4. Mechanism of the regional landslides For the regional landslides during Eastern Iburi earthquake, the main influencing fac- tors were the geological structure, heavy rainfall, and strong motion. Considering the effect of heavy rainfall and strong motion, based on the analysis results of failure mode, regional slope susceptibility evaluation using GIS is carried out to clarify the mechanism of regional landslides. 4.1. Influencing factors 1. Geological structure The landslide in Sakurazaka village was also investigated. This landslide was located in the woods, and it caused no damage to people and facilities. However, there was an intact and clear scarp in the source area of this landslide. According to the investi- gation results, most landslides were shallow, several meters deep seated (Yamagishi and Yamazaki 2018). Therefore, the occurrence of landslides should be related to the properties of surface soil. Thus, in the landslide in Sakurazaka village, the geological structure of the surface soil was investigated. According to previous geological data (Nakagawa et al. 2018) and results of the field investigation, the soil layers in Sakurazaka village could be identified. It can be seen from the vertical scarp of the landslide in Figure 15 that the soil had an obvious 18 H. ZHOU ET AL. Table 2. Soil physical properties (Sakurazaka village). Soil specific Water Liquid Plastic Plasticity Point Location gravity q (g/cm ) content (%) limit (%) limit (%) index 1 Tarumae d 2.527 150.2 191.1 88.8 102.3 pumice-fall deposit 2 Black sand 2.509 279.1 No plasticity No plasticity No plasticity 3 Tarumae d loam 2.672 30.5 47.6 22.8 24.8 stratified structure. Moreover, the main composition of the surface soil was pumice deposit from the Tarumae volcano, mixed with black sand and loam. The sliding bed corresponded to the Tarumae-d loam, and this explains the fact that the sliding body slid down along the loam layer. The loam layer is the weak surface of unstable slope. Soil samples were taken during the investigation. Three samples were taken from the following locations: Tarumae-d pumice-fall deposit layer, black sand layer, and Tarumae-d loam layer, respectively (Figure 15). Laboratory tests were conducted, and the physical properties of soil samples are shown in Table 2. The water content of the Tarumae d deposit and black sand was high, while that of the loam was relatively low. The following could be a reasonable inference: Because of the persistent heavy rainfall before earthquake, rainwater led to the increase in water content and the loam layer being an impermeable layer turned into the weak surface of unstable slope. 2. Continuous heavy rainfall On September 4th, 2018, typhoon Jebi hit the western part of Hokkaido, initiating strong rainfall in the Iburi region. Figure 16a shows the information about accumu- lated precipitation in Atsuma town from June to September of each year from 2012 to 2018. Compared with the accumulated precipitation in the same period of previous years, that in 2018 in Atsuma town is relatively higher. The average and cumulative daily pre- cipitation in 1 month before earthquake in Atsuma town are shown in Figure 16b. Beginning in August, the rain began continuously. The cumulative rainfall of 1 month before earthquake reached 200 mm. On September 5th, typhoon Jebi affected Atsuma town and produced a lot of rainfall. The daily precipitation on September 5th was 12.5 mm. According to the data of Japan Meteorological Agency, it will take about one day for rainwater to penetrate underground in Atsuma town, so the earthquake occurred at the time when groundwater level was relatively higher. Based on rainfall data and observation from field investigation, it can be found that surface soil was nearly satu- rated. Water table is considered to be on the ground surface. Heavy rainfall triggered regional landslides from three aspects: (1) rainwater reduced the effective stress between soil particles, which decreased the sliding resistance; (2) rainwater increased the liquefac- tion risk of soil; and (3) with the increase of soil water content above the loam layer, the soil sliding force of the gravity component along slope direction increased. 3. Strong motion In addition to heavy rainfall, strong ground motion is another major cause of regional landslides. The additional force caused by the reciprocating motion destroyed the equilibrium condition of slope and led to the occurrence of landslides. As the instant maximum of seismic records, the peak ground acceleration (PGA) can GEOMATICS, NATURAL HAZARDS AND RISK 19 Figure 16. Precipitation information of Atsuma town. Source: Japan Meteorological Agency. represent the whole action of ground motion to a certain extent (Wang et al. 2010). In addition, PGA is a quantity that can be quickly acquired after earthquake to reflect the magnitude of ground motion. Therefore, using PGA to analyze earthquake- induced landslides has physical significance and practical application value. PGA data recorded by KiK-net can be obtained from the website. The seismo- graphs of KiK-net could record the acceleration data in three directions, i.e., E-W, N- S, and U-P, at every station. Through kriging interpolation of PGA point data, the planar distribution of PGA in the study area can be obtained. Figure 17 shows the distribution of horizontal and vertical PGA of the 2018 Hokkaido Eastern Iburi earth- quake. The points represent the location of the observation stations in study area. The color represents the value of PGA, where red represents big value and blue rep- resents small value. The horizontal PGA is derived from the product square of E-W PGA and N-S PGA as a scalar in horizontal direction. 20 H. ZHOU ET AL. Figure 17. PGA map. Source: Source: Shanghai Jiao Tong Univerisity/ China Earthquake Administration/ Chuo Kaihatsu Corp. It can be observed from Figure 17 that PGA decayed outward from the epicenter. In a certain area near the epicenter, PGA maintained a relatively large value and then rapidly declined to a much smaller value. The maximum value of horizontal PGA was 859 Gal and that of vertical PGA was 443 Gal. However, in the 2003 Tokachi-Oki earthquake, the maximum value of horizontal PGA was 517 Gal and that of vertical PGA was 276 Gal, much lower those in the 2018 Eastern Iburi earthquake. The occur- rence of earthquake landslides is closely related to horizontal ground motion, which has a leading role in earthquake landslides. Moreover, vertical ground motion has a promoting effect on slope instability (Huang et al. 2004). According to the result of GEOMATICS, NATURAL HAZARDS AND RISK 21 Figure 18. Sketch of stability analysis using semi-infinite slope limit equilibrium analysis. Source: Source: Shanghai Jiao Tong Univerisity/ China Earthquake Administration/ Chuo Kaihatsu Corp. force analysis in the earthquake process, the horizontal component contributes immensely to the outward movement of sliding body and the reciprocating effect of the vertical motion may reduce the cohesive and friction forces of sliding body and reduce the seismic resistance of slope. Therefore, strong ground motion is an import- ant factor causing the instability of regional slopes. 4.2. Evaluation of the landslide area based on stability analysis Stability analysis is a common method to evaluate possibility of regional landslides in risk assessment. Calculation results of stability analysis contain safety factor, perman- ent displacement, etc., which can evaluate the risk of regional slopes from many aspects. In order to verify that strong motion and continuous rainfall are the causes of regional landslides, safety factor and permanent displacement calculation consider- ing the influence of PGA distribution and water are adopted. The consistence of dis- tribution between calculated results and actual landslides indicates the control effect of strong motion and rainfall in slope failures. 1. Safety factor calculation The sketch of the stability analysis (Sun et al. 2011) is shown in Figure 18. The failure surface was assumed as a plane under the sliding body. Earthquake force is equivalent to a static force that is equal to the product of sliding body weight and seismic force coefficient. In the sketch, k and k are the coefficients of the seismic force in horizontal and h v vertical directions, which are the ratio of the peak ground acceleration to the ground acceleration; h is the thickness of the sliding body; b is the slope degree; W is the weight of the sliding body; and h is the height of the water level in the slope. According to Mohr-Coulomb’ law of strength, the slope safety factor F is ðÞ ðÞ c þ 1 k c þ kc cos b 1 k c þ kcðÞ k cos b þ k sin b h tan u t sat t sat h F ¼ ðÞ 1 k c þ kc½ ðÞ 1 k sin b þ k cos b h v h t sat (1) 22 H. ZHOU ET AL. Figure 19. Sketch of two-stage integration process in Newmark model. Source: Source: Shanghai Jiao Tong Univerisity/ China Earthquake Administration/ Chuo Kaihatsu Corp. where c is the cohesion, u is the friction angle, k ¼ h =h is the water level coefficient, c is the gravity density in the natural state, and c is the saturated gravity density. t sat The limit equilibrium method obtains the solution of problem by analyzing the bal- ance of sliding body at the moment of failure without considering the reciprocating shear action of earthquake. Reduction of strength parameters probable resulting from liquefaction was ignored. 2. Permanent displacement calculation Based on the safety factor of stability analysis, the permanent displacement of regional slopes can be calculated by Newmark model then. Newmark model was pro- posed by Newmark in 1965 for dam stability analysis (Newmark 1965). It is consid- ered that failure of dam depends on the cumulative deformation under dynamic action. There are two steps to calculate permanent displacement in Newmark model: first step is to calculate the acceleration that makes the sliding body move in critical state according to safety factor, second step is to integrate two times the seismic acceleration time-history waveform which exceeds critical acceleration from acceler- ation to displacement (Figure 19). GEOMATICS, NATURAL HAZARDS AND RISK 23 Table 3. The values of input parameters. Parameter Value c, u Fixed value in Atsuma town obtained from soil sample (Sakurazaka village) test c ¼ 40kPa u ¼ 20 c , c Fixed value in Atsuma town obtained from soil sample (Sakurazaka village) test t sat 3 3 c ¼15kN/m c ¼17kN/m t sat kk ¼ 1 for the region west of 142 30 east longitude and k ¼ 0 for the other region b Derived from Digital Elevation Model (DEM) k , k Derived from PGA map h v hh ¼ 2m in all study area considering that most landslides are shallow-seated The formula for calculating critical acceleration a is: a ¼ðÞ F 1 g sin b (2) c s In Figure 19, the top red line represents critical acceleration. And permanent dis- placement is accumulated until relative velocity between sliding body and surface comes to 0. It can be expressed as Eq. (3): ðð D ¼ ðÞ aðtÞ a dtdt (3) N c where D is the permanent displacement of Newmark model; a(t) is the time history waveform of seismic acceleration. In previous research, 5 cm of permanent placement was generally used as the threshold leading to macroscopic ground cracking and fail- ure of slopes (Jibson 2011). 3. Analysis results The values of the parameters are given in Table 3. In the table, the water level coeffi- cient k was 1 for the region west of 142 30 E longitude because there was a strong rainfall in western Hokkaido caused by typhoon Jebi. Moreover, the slope degree b was derived from DEM with values varying with different spatial positions. The DEM used was the ASTER_GDEM with an resolution of 30 m and was photographed before the earthquake in 2009. Safety factor threshold in study area was determined by calculating the safety factors of five investigated landslides, as shown in Table 4. Safety factor range of five landslides was 0.95–1.09. Therefore, maximum value of 1.09 can be considered as the lower limit of threshold such that slopes with safety factor less than 1.09 were determined as unstable slopes. Considering the assumptions and simplifications made in the calculation of safety factor based on the investigation results, the safety factor threshold of 1.09 from five typical investigated landslides had certain representativeness, and was adopted to evaluate the stability of regional slopes under 2018 Hokkaido Eastern Iburi earthquake. Safety factor calculation of investi- gated landslides with surface soil of natural moisture content (k ¼ 0) was also con- ducted and listed in Table 4. The values of safety factor under natural condition are all large indicating that no slopes would slide with no rainfall. It verifies the validity of strength parameters. The safety factor of stability analysis is shown in Figure 20. The red spots in Figure 20 indicate the areas with a safety factor less than 1.09. The blue spots are the 24 H. ZHOU ET AL. Table 4. Safety factor of investigated landslides. Tomisato Tomisato Yoshino Yoshino Position of landslide (water tower) (pumpkin field) (north) (south) Sakurazaka Safety factor Surface soil in 0.95 1.04 0.96 1.01 1.09 saturated state Surface soil with 1.21 1.41 1.26 1.33 1.39 natural moisture content Figure 20. Distribution of safety factor. Source: Shanghai Jiao Tong Univerisity/ China Earthquake Administration/ Chuo Kaihatsu Corp. actual landslide hazard areas. Distribution range of unstable slopes are basically con- sistent with actual landslides. But the number of unstable slopes is less than actual landslides. Based on the results of stability analysis in Figure 20, critical acceleration can be calculated by Eq. (2). The slopes with safety factor less than 1.00 had negative critical acceleration because they had failed for sure in stability analysis. Permanent displace- ment was integrated for each grid in MATLAB. The seismic acceleration wave in Figure 2a was adopted. In order to consider the attenuation effect, the acceleration waves of different places are reduced in proportion according to the PGA map in Figure 17. The permanent displacement results are shown in Figure 21. The red spots indi- cate areas with permanent displacement larger than 5 cm. And blue spots are the actual landslide hazard areas. In Figure 21 it can be seen that the distribution of slopes with permanent displacement larger than 5 cm generally coincides with that of actual landslides. It indicates strong motion and rainfall are controlling factors result- ing in regional landslides under Hokkaido eastern Iburi earthquake. Safety factor and permanent displacement evaluate the stability of regional slopes from the aspect of force and deformation respectively. Combination of safety factor and permanent displacement offers a better assessment for damage resulting from landslides. GEOMATICS, NATURAL HAZARDS AND RISK 25 Figure 21. Distribution of permanent displacement. Source: Shanghai Jiao Tong Univerisity/ China Earthquake Administration/ Chuo Kaihatsu Corp. 5. Conclusions The 2018 Hokkaido Eastern Iburi earthquake occurred on September 6, 2018. This earthequake killed 41 people and had a great impact on lifeline engineering. It also caused geological disasters such as soil liquefaction in urban areas and regional land- slides outside urban areas. 1. Serious liquefaction disasters occurred in the residential area of Sapporo city and Kitahiroshima city under 2018 Eastern Iburi earthquake. Satozuka block, Utsukashigaoka block and Okae block were investigated. Through the compari- son of the same liquefied area between 2003 Tokachi-Oki earthquake and 2018 Eastern Iburi earthquake, it was found that the severity of liquefaction disaster under 2018 earthquake is not as serious as 2003 earthquake. The density and shear velocity of soil increased after reconsolidation. 2. The regional landslides led to death of villagers, road burial, destruction of field and other disasters. Five landslides of typical characteristics were investigated including the southern and northern landslide in Yoshino village, the landslide in pumpkin field and the landslide near the water tower of Tomisato village, the landslide in Sakurazaka village. There was an impermeable loam layer about 2m below the ground surface. Because of the continuous heavy rainfall, the water content of surface soil above loam layer increased. The loam layer grew into the weak surface and under the effect of strong motion the sliding body integrally slid along the loam layer. Most landslides belong to the translational earth slide and earth flow type. 3. Referencing to the failure mode of regional landslides, permanent displacement analysis based on pseudo static method is carried out to evaluate the susceptibil- ity of regional slopes considering the influence of continuous heavy rainfall and strong motion. Using GIS to obtain the distribution of unstable slopes, it is 26 H. ZHOU ET AL. consistent with the actual landslide disaster map. It proves that continuous heavy rainfall and strong motion are the main factors causing regional landslides under the 2018 Hokkaido Eastern Iburi earthquake. Acknowledgments The authors would like to express their gratitude to Prof. Mitsu Okamura - chairman of ATC3 (Geotechnology for Natural Hazards in Asia Pacific Regions, International Society of Soil Mechanics and Geotechnical Engineering) for investigation invitation. The authors are also grateful for the helpful advice from Arai Hayato and Hosoya Takashi of Chuo Kaihatsu Corporation. 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"Geomatics, Natural Hazards and Risk" – Taylor & Francis
Published: Jan 1, 2021
Keywords: Hokkaido Eastern Iburi earthquake; liquefaction; regional landslides; continuous heavy rainfall; shallow translational earth slide; earth flow
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