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Role of geological structure in the occurrence of earthquake-induced landslides, the case of the 2007 Mid-Niigata Offshore Earthquake, Japan
Engineering Geology 182 (2014) 25–36
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Role of geological structure in the occurrence of earthquake-induced landslides, the case of the 2007 Mid-Niigata Offshore Earthquake, Japan Baator Has a,⁎, Tamotsu Nozaki b a b
Asia Air Survey, 1-2-2 Manpukuji, Asao District, Kawasaki City, Kanagawa, Japan Nozaki EG Consulting, 3-10-14 Yamafutatsu, Chuo-ku District, Niigata City, Niigata, Japan
a r t i c l e
i n f o
Article history: Accepted 6 September 2014 Available online 20 September 2014 Keywords: Earthquake Earthquake-induced landslide Geological structure Non-tectonic
a b s t r a c t The landslides induced by the Mid-Niigata Offshore earthquake in 2007 (MJMA6.8) occurred mostly along the shore of the Japan Sea. This study focused on the landslides that occurred near the Hijirigahana Cape, Yoneyama Town in the Niigata Prefecture. The study area is located in the western region of the central Niigata Prefecture, along the Japan Sea. The geology of the landslide area is dominated by late Miocene sandstone-rich member of alternating beds of sandstone and siltstone. These strata gently dip to the north at approximately 25° to 30° and form cataclinal and orthoclinal slopes on the north- and west-facing slopes, respectively. The central Niigata region is one of the most landslide-prone areas in Japan. The study area is located approximately 30 km from the epicenter of the earthquake with an estimated intensity of 6 lower in the JMA (Japan Meteorological Agency) scale. A group of landslides occurred in the study area. The area contains the most concentrated set of landslides from this earthquake, even though it is located far from the epicenter. Most of the landslides occurred on north-facing slopes, but two occurred on west-facing slopes. The landslides on the north-facing slopes are characterized as translational slides. The largest rock slide in the study area, Unit D (width 100 m, length 230 m), slipped along the bedding plane of the strata. Observations from slickenlines from the slip surface and outcrop in the upper portion of Unit D suggest that before this earthquake, landslides frequently occurred on the north-facing slopes and were controlled by the bedding plane. With the exception of the largest one, other landslides on north-facing slopes were mostly debris slides, indicating that sediments from older landslides exist. The second largest landslide occurred on a west-facing orthoclinal slope. A rotational rock slide, Unit A, was 80 m wide and 100 m long. During the excavation after the landslide, a high angle fault with a SW dip was identified behind the crushed zone. Based on field observation, the fault was considered to be a non-tectonic fault and created by gravitational movement. During the excavation, the crushed zone was identified on the cutting wall of the slope. These results revealed that the relatively larger rotational rock slides occurred because of the existence of the crushed zone due to older events that caused the slippage along a non-tectonic fault. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Landslides triggered by strong earthquakes are one of the most important issues regarding natural disasters for scientific communities and administrative agencies because of the severe damage caused by their tremendous numbers and large-scale dimensions. Recently, many strong earthquakes associated with a large number of landslides have demonstrated the significance of studying earthquake-induced landslides. The most remarkable earthquake in recent years, that induced a tremendous number of landslides is the Wenchuan earthquake, China (e.g., Chigira et al., 2010; Qi et al., 2010; Gorum et al., 2011; Xu et al., 2014a). Xu et al. (2014a) identified 197,481 landslides that were induced by the earthquake, established three landslide inventories ⁎ Corresponding author. Tel.: +81 44 967 6302; fax: +81 44 965 0028. E-mail address: [email protected] (B. Has).
http://dx.doi.org/10.1016/j.enggeo.2014.09.006 0013-7952/© 2014 Elsevier B.V. All rights reserved.
and revealed spatial features of landslides with topographic, lithologic and seismic parameters. Other strong earthquakes that induced a large number of landslides were also well documented, such as the Mid-Niigata (also called Chuetsu) earthquake, Japan, 2004 (JMA, 2004; Chigira and Yagi, 2006; Yamagishi and Iwahashi, 2007); the Pakistan earthquake, 2005 (Sato et al., 2007); the Iwate-Miyagi Inland earthquake, 2008, Japan (JMA, 2008; Yagi et al., 2009); the Haiti earthquake, 2010 (Xu et al., 2014b); the Yushu earthquake, 2010, China (Xu et al., 2013); and the Lushan earthquake, 2013, China (Chen et al., 2014; Xu and Xu, 2014). In addition to the Mid-Niigata and Iwate-Miyagi Inland earthquakes, a series of strong earthquakes have struck eastern Japan in recent years, such as the Mid-Niigata Offshore (also called Chuetsu-Oki) earthquake, 2007 (JMA, 2007), the northern Nagano Prefecture earthquake (JMA, 2011a) and the Fukushima Hamadori earthquake (JMA, 2011b), which occurred along with the great 3.11 Tohoku earthquake in 2011
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(http://www.jma.go.jp/jma/en/2011_Earthquake/2011_Earthquake. html). These earthquakes caused serious damage to the focal areas and their vicinities because of the associated landslides. Several studies have been published regarding the landslides induced by the earthquakes (Has et al., 2010, 2012, 2014; Collins et al., 2012). The literature, however, mostly focused on landslide inventories and discussed the spatial distribution and relationship between the causative factors and landslide occurrences. The Mid-Niigata earthquake (M6.8, October 24, 2004) and the MidNiigata Offshore earthquake (M6.8, July 16, 2007) occurred in the central Niigata Prefecture, eastern Japan (Figure 1). The epicenters of the two earthquakes only had a distance of 35 km. The Mid-Niigata earthquake, which caused a large number of landslides (Chigira and Yagi, 2006), was one of the largest earthquakes that induced numerous large landslides inland Japan since the Zenkoji earthquake in 1847 (Zenkoji Earthquake Disaster Study Group, 1994). Compared to the Mid-Niigata earthquake, the Mid-Niigata Offshore earthquake caused a few middle-scale (JLS, 2004) and small to micro-scale landslides despite the similar topography, geological conditions and magnitude. In this case, most of the landslides occurred along the coastline of the Japan Sea, and the largest ones were associated with smaller ones concentrated in and around Hijirigahana Cape, Yoneyama Town, Niigata Prefecture. The distribution and size of the earthquake-induced landslides from the Mid-Niigata Offshore earthquake have been studied (Has et al., 2010; Collins et al., 2012). However, there are no sufficient
detailed studies that identified the mechanism of the individual landslides, except Nozaki and Has (2013). Detailed geological and geomorphological surveys are necessary for individual landslides because they provide important implications to identify the mechanism of landslides and are useful for preventing second-hand hazards. In this paper, we focused on the geological structure to understand its roles in landsliding induced by this earthquake. 2. Topographical and geological settings The study area is located along the coast of the Japan Sea in the central Niigata Prefecture, eastern Japan (Figure 1). Among the small number of landslides induced by the Mid-Niigata Offshore earthquake, many landslides occurred in the study area, even though it is far from the epicenter. The Niigata Prefecture and its surrounding regions are characterized by an NE-SW topographical configuration. The geology of the central Niigata Prefecture is mostly composed of Neogene and Quaternary sedimentary rocks (GSJ, 2012). The geological structure is mainly characterized by NE-SW striking faults and active folds with NE-SW extending axes, reflecting the NW–SE compression (Taira, 2001; Otsubo, 2008) caused by the Pacific plate subducting under the Eurasian plate. The central Niigata Prefecture is also known as one of the most landslide-prone regions in Japan. A tremendous number of old landslides have been identified in this region (Shimizu et al., 2004).
Fig. 1. Location of the study area. The shaded area indicates the Niigata Prefecture; rectangles indicate the projected area of source faults. (A) Mid-Niigata Offshore earthquake; (B) MidNiigata earthquake. Kashiwazaki St.: location of the K-net seismic station.
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The Mid-Niigata Offshore earthquake occurred 3 years after the Mid-Niigata earthquake, approximately 35 km northwest of the epicenter of the latter. Both earthquakes occurred on reverse faults. Based on aftershock observation and seismic reflections, the source fault of the Mid-Niigata earthquake was dipped west (Hikima and Koketsu, 2005; Sato et al., 2005). Various studies have discussed that the main source of the Mid-Niigata Offshore earthquake was a southeast dipping reverse fault (Mori, 2008; Shinohara et al., 2008; Miyake et al., 2010), even though there are arguments regarding the dip of the source fault (e.g., Aoi et al., 2008). This result was supported by seismic surveys (No et al., 2009; Nakahigashi et al., 2012), so it is reasonable to consider that the source fault of the Mid-Niigata Offshore earthquake dipping to the southeast (Figure 1). The study area is approximately 30 km southwest of the epicenter of the Mid-Niigata Offshore earthquake, on the hanging wall of the source fault. A late Miocene sandstone-rich member with alternating beds of sandstone and siltstone, part of which is beautifully outcropped along the local road passing through the cape (Figure 2), is conformably overlain by volcanic rocks and disconformably underlain by a siltstone member intercalated with thin beds of sandstone and a thick pumice tuff on its upper horizon (Figure 3). The upper and the lower members of sedimentary rocks were named the Hijirigahana Formation by the Yoneyama Research Group (Yoneyama Research Group, 1973). The disconformity between both members, however, was identified during our
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investigation of this disaster. These strata gently dip (25–30°) toward the sea, making cataclinal and orthoclinal slopes on the north and west side of the cape, respectively. This cape is located just on the axis of a broad open anticline, where the axis plunges to north. That is why the bedding plane in the study area trends EW and dips N despite NS trending anticlinal axis. Although vertical joints slightly tilted to the SW tend to develop NW–SE and less other directions, there was no noteworthy tectonic fault, except for some bedding-plane slips found on new and old sliding surfaces. The study area is composed of north- and west-facing slopes. The elevation of the ridge dividing between them ranges from 100 to 150 m above sea level (Figure 4). On the north-facing slopes, a wide range of old landslide topography can be identified. Near the upper region of Unit D, an old scarp can be clearly recognized in photographs taken before the earthquake (Figure 2). On the west-facing slopes, however, no landslide topography can be recognized. 3. Features of the earthquake-induced landslides A group of landslides triggered by the earthquake occurred on the north- and west-facing slopes in and around the Hijirigahana Cape (Figure 4). The landslides were classified into two types (Table 1). Most of the landslides occurred on the north-facing slopes were translational slides. Two rotational landslides occurred on the west-facing
Fig. 2. Photographs of part of the study area before and after landsliding (after Nozaki and Has, 2013). (a) Before landsliding, taken on Aug. 10, 2005 by Atsushi Ueki, Kashiwazaki City Education Center; (b) after landsliding, taken on Aug. 4, 2007. The dotted line indicates the trace of fault observed at the box, in Figure 19).
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Fig. 3. Geological map of the landslides and their vicinities (after Nozaki and Has, 2013). The letters “A” to “I” are the landslide units as in Figure 4.
slopes. The north-facing slope is considered cataclinal, and the westfacing slope is orthoclinal (Nozaki and Has, 2013). Old landslides can be recognized on the north-facing slopes by interpreting pre-earthquake aerial photographs (Figure 5). A sharp northeast-southwest directed scarp can be recognized on the aerial and field photos.
3.1. Landslides on the north-facing slope Unit D, a translational rock slide that occurred on the north-facing slope, was the largest among those induced by the earthquake (see Figure 4). This slide unit was obviously one of the pre-existed ones as previously described, and reactivated, extending mainly to the upper
Fig. 4. Distribution of earthquake-induced landslides in the study area. The shaded areas indicate old landslides.
B. Has, T. Nozaki / Engineering Geology 182 (2014) 25–36 Table 1 Type and dimension of earthquake-induced landslides in the study area (landslide classification is based on Varnes, 1978). Unit
Type
Length(m)
Width(m)
A B C D E F G H I
Rotational Rock Slide Rotational Rock Slide? Rotational Debris Slide Translational Rock Slide Translational Debris Slide Translational Debris Slide Translational Debris Slide Translational Debris Slide Translational Debris Slide
100 25 115 230 130 60 35 60 60
80 70 100 100 40 30 35 60 40
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Additionally, the volume accumulated at the foot of the slope was less than 10,000 m3 (Nozaki and Has, 2013), indicating that the thickness of the slide was limited (Figure 8). The upper end of the sliding surface reached beyond the ridge along a bedding plane, hence no head scarp remained (Figure 7a). On the eastern side of the slide, however, a “detached scarp” rose more than 10 m, and the sequence of alternating beds of sandstone and siltstone was easily observed (Figure 7a). On the western side of the landslide, a 5 to 8 m high side scarp was formed (Figure 6), and part of it toppled as rock-panels separated by high-angle joints trending NW (see Figures 3 and 20). Almost the entire sliding mass reached the beach. The sparsely distributed debris on the sliding surface was washed away by rainfall within a few months, and microstepped bedding planes have been widely exposed. On the higher portion of the sliding surface, a large number of slickenlines, which were directed approximately to the dip of the strata, were extensively carved on the bed of siltstone (Figure 7d). Slickenlines found on the sliding surface at the foot of the western scarp, however, were approximately parallel to the strike of the strata. It means that these ones had evidently occurred as a flexural slip during the past folding: tectonic movement, and functioned as a potential sliding surface. The sliding, however, occurred not only along one bedding plane but also along the other ones connected in a stepwise manner making a micro-staircase. This means that the sliding surface shifted to the lower sequence of the strata and to the downslope. The whole sliding surface, therefore, looked a slightly steeper than the dip of the strata. With the exception of Unit D and the rotational debris slide of Unit C, all other smaller ones on the north-facing slopes are translational debris slides. This fact indicates that old landslides had occurred along the north-facing slopes in the past and that the debris remained on the slope (Figure 9). 3.2. Landslides on the west-facing slope
Fig. 5. Aerial photograph of the study area taken in 1975 by GSI, Japan. A clear scarp continued to the sea along the right side of Unit D.
slope. Only thin strata, however, must have been ripped off of the central to lower region of the unit because part of the road that ran across the central part of the unit remained intact (Figures 6 and 7c).
Units A and B are rotational rock slides that occurred on the westfacing slope with a total volume of approximately 100,000 m3. The thickness to the sliding surface of Unit A was approximately 10 m. On the western side of the cape, the slope looks toward southwest, and the strata dip north, forming orthoclinal structure (Figure 3). Except for the upper region of the slope, where the rock consists of alternating
Fig. 6. Whole view of Unit D from the toe area. The dashed lines indicate the location of the lost road. The rectangles indicate the location of the photographs in Figure 7.
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Fig. 7. Observations of Unit D. (a) Alternating beds outcropped on the eastern side scarp, (b) sliding surface, (c) concrete block of road remaining after the landsliding and (d) sliding surface with slickenlines.
beds of sandstone and siltstone, the basement rock consists of siltstone member sparsely intercalated with thin layers of loosely indurated sandstone and a thick soft pumice tuff in its upper region. An approximately 5 m high head scarp was created below the crown of Unit A along the ridge associated with parallel cracks. The vertical crack, which was 0.3 to 0.7 m high at the head area of Unit B, obliquely ran across the road. There was no clear boundary between Units A and
B. The tip of the sliding surface of both units, including the toe of the debris of Unit A did not reach the foot of the slope, and the JR tunnel under the landslides had no damage. Although a slightly gentler slope was created at the head of Unit A, the gradient of this orthoclinal slope was more than 40° as a whole. On the outcrop of alternating beds at the northern end of the ridge that meets with the road, a few consistent vertical joints directed NW were observed (see Figure 2). New open spaces
Fig. 8. Geological cross section (III–III′ on Figure 3) of Unit D (after Nozaki and Has, 2013).
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Fig. 9. Earth slides of Unit E, view from the toe to the head.
associated with small debris were observed along these joints and bedding planes, and the sedimentary rocks were loosened by tremor of the earthquake. Some of the joints, however, had been already loosened by pre-events as evidenced by sealing cracks with asphalt on the road, and grass roots invaded some of those open cracks. After the landsliding, cut and slope protection works were executed as a countermeasure on Unit A (Figure 10). 4. Discussion: role of geological structure in landsliding 4.1. Concentrated landslides in the study area Compared to the Mid-Niigata earthquake, the Mid-Niigata Offshore earthquake induced only a few middle-scale landslides but was
associated with many other smaller ones. The earthquake-induced landslides are hazardous, however, because of their concentrated occurrence, especially in the study area. The seismic source area of the latter earthquake was located under the sea, while the former one was located inland (see Figure 1). The study area, however, was located on the hanging wall of the source fault, and the distance from its southern verge was less than 10 km. In addition, the PGA at this site was estimated to be greater than 500 gal and the intensity was 6 lower in the JMA scale (Hasi et al., 2011). The particle motion of the seismic station Kashiwazaki (NIG018, K-Net, from NIED) showed that horizontal motion dominated along the NW–SE and N-S directions (Figure 11), and it stimulated landslides on the same facing slopes because of directivity effects of ground motion (Somerville et al., 1997). The same phenomenon was also suggested in a few previous literatures (e.g., Huang, 2013; Xu et al., 2013).
Fig. 10. Cutting works on Unit A.
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Fig. 13. Stereonet projection of Unit D. The solid line indicates the bedding plane. The black dashed line indicates the plane of slope, and the dashed gray lines indicate the plane of joints on the western side scarp. The solid arrow shows the direction of landslide.
Fig. 11. Particle motion of acceleration at the Kashiwazaki seismic station.
4.2. Role of geological structure 4.2.1. Bedding plane as a sliding surface For the landslides that occurred on the north-facing slopes, especially for Unit D, the bedding plane played the main role for the landsliding. Fig. 2 shows that cataclinal slope consists of beautiful alternating beds of sandstone and siltstone that are easily sheared off along bedding planes. Especially under strong ground motion by earthquakes, bedding-planeslip must easily occur, as observed in the outcrop (Figure 12). In the cases of the Neogene sedimentary rocks, strong earthquakes cause displacement along bedding planes on dip slip (e.g., Abe et al., 2006; Nozaki, 2008), indicating that the bedding plane is a weak surface. This suggests that the bedding plane of layered formations can control both the landslide mechanism and the activity (Cruden and Varnes, 1996). The bedding-plane slips observed in places must have fostered the development of gravitational sliding as a potential sliding surface. These slips must have occurred with flexural slip on the axial part of the anticline because the slickenlines and the dip of the bedding plane crossed each other at nearly right angles, but each of those slips was not so long extended. The sliding surface of Unit D drew a very gentle arc, creating micro-steps. This configuration made it slightly steeper in its upper region. It, therefore, may have been one of the reasons why the sliding mass did not disintegrated and reached the foot of the slope. Additionally, it may have helped the large displacement.
Fig. 12. Bedding plane slip appeared on the outcrop (see the location in Figure 2) after the earthquake.
Along the ridgeline between the cataclinal and orthoclinal slopes, vertical joints developed trending NW–SE (Figure 13), controlling the west-side scarp of Unit D. Large blocks consisting of alternating beds in the debris at the toe of Unit D must have been derived from the pre-existing east-side high scarp that receded during this earthquake. The huge blocks of sandstone and siltstone observed at the toe of Unit D were considered to have broken out from the remaining old scarp. Almost the entire slope, including Units C, D, E, F and G, were located in a larger old landslide unit (Figure 4), and it is reasonably inferred that translational slides were repeated on this slope before the earthquake. Comparing the photographs in Fig. 2, we found that extents of Unit D were almost the same as the previous sliding. Although thick layers of small parts incipiently moved down at the head area, only a 2 to 3 m thick veneer of alternating beds on the upper half of the slope must have slid down to the beach area as a new rock slide, including some old debris, because the volume of the debris was less than 10,000 m3 (Nozaki and Has, 2013). The same type of rapid translational rock slides as in Unit D, represented by the Yokowatashi landslide (Nagata and Nozaki, 2007), were triggered in many places by the Mid-Niigata Prefecture earthquake. Lithological and structural complexity is one of the main factors that control landslide movement. In moderate mountainous areas in the Niigata Prefecture, where is geo-structurally active areas such as the MidNiigata region mainly composed of Neogene sedimentary rocks, geological structure has always influenced landslides. Many landslides in this region tend to occur along the axes of anticlines (Iwamatsu et al., 1975), possibly because of the weakened rock strength at the hinge of folds. A large amount of dip-slip sliding induced by the Mid-Niigata earthquake indicates that earthquake-induced landslides tend to be strongly influenced by the geological structure. Above all, the geological structure played a dominant role in the north-facing rock slide affected by strong ground motion. 4.2.2. Non-tectonic fault and crashed zone in west-facing slope The mechanism for landslides on the west-facing orthoclinal slope of Units A and B is different from that of Unit D. Excavations for countermeasures in Unit A and its vicinity extensively exposed the interior structure of the slope, which helped clarify the mechanism of landsliding. Fig. 14 is a sketch map of the cut-slope observed during the excavation of Unit A. We conducted field reconnaissance and observed of the cores drilled just after the earthquake in Unit A and drew this map and geological section (Figure 15). Although landslide debris moved by the earthquake had been completely excavated, a high angle normal fault (Figure 16) dipping to the west appeared at the head of the cut slope. The hanging wall of this fault was accompanied by heavy crashed zones “Cr” as shown in Fig. 16. A thick layer of pumiceous tuff was displaced more than 5 m, and a “tuff tail” was traced along the fault
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Fig. 14. Sketch map of cut-slope during the excavation of Unit A (after Nozaki and Has, 2013).
plane (see “b” in Figure 16). The displacement along the fault was less in the upper region (“a” in Figure 16) but became larger in the lower region. Thick crushed zones along the bedding-planes and/or beddingplane-slip faults also appeared in the middle and lower region of the slope (Figure 17). At the foot of the cut slope, part of their lower boundary was accompanied by a wedge (Figures 17 and 18a) along slip-planes with intercalated clay seams (Figure 18b). This crushed zone occasionally looked very loose (Figure 18c), part of which may have been caused by this event. We also observed siltstone blocks confined to the tuff layer (Figure 18d). These phenomena indicate that the structure of the hanging wall had been undoubtedly disturbed before this event, and part of the pre-existing slip-surface and/or slip-planes played a strong role in the occurrence of Units A and B by this earthquake. Another sharp fault also appeared on the cutting wall during the excavation along the ridge between Units A and D (Figure 19). The photograph shows a typical normal dip separation fault, and the right side is the hanging wall. The fault looked intact, but a thin crushed zone existed along the fault plane. Additionally, the hanging wall and foot wall of the fault looked intact, and no crushed zone was recognized. Therefore, a
tectonic fault was suggested. The upper central bushy region in Fig. 2 was the cut slope in Fig. 19, and the southern extension of this fault was considered to be connected to the normal fault in Unit A. However, on the northern extension of the outcrop (Figure 2), no such fault was confirmed despite the wide and extensive outcrop. This fault, and therefore the fault in the area of Unit A (Figure 16), was considered to be a non-tectonic fault and part of an old sliding surface (Nozaki and Has, 2013). It reasonably extends to west toward the natural outcrop shown in Fig. 6, suggesting that the whole structure was non-tectonic and was created by the old gravitational movement. Above all, in Unit A, the orthoclinal landslide occurred on a wedge of a pre-existing weak zone between the sharp normal fault and the bedding-slip-fault (Figure 20). This result indicated that the geological structure caused by older sliding played a dominant role in landsliding. 5. Conclusions Induced by the Mid-Niigata Offshore earthquake, a group of landslides occurred in the Hijirigahana Cape, mainly on the north-facing
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Fig. 15. Geological section of Unit A, I–I′ on Figure 14 (after Nozaki and Has, 2013).
Fig. 16. A sharp, nearly vertical normal fault appeared on the cutting wall during the excavation in Unit A. The black arrows indicate fault, and the white arrow indicates the movement of the hanging wall of the fault; Cr: crushed zone; Tf: tuff; S:scale, 4 m.
slope. Many translational landslides represented by Unit D that occurred on the north-facing slope slipped along the bedding planes, creating micro-steps. Most of them were reactivations of old landslides, except for the head region of Unit D, at which the rock mass thickly receded toward the ridge. Almost all slid masses were less than 2 to 3 m
thick because of the cataclinal structure and reactivation of thin old debris left on the gently tilted dip-slopes. The mechanism of landslides on the orthoclinal slope was rather complicated and interesting. The geological structure underneath the Unit A landslide on the steep slope showed interesting features. A
Fig. 17. View of the foot of the cutting wall of Unit A. Cr: crushed zone; Rd: road; S:scale, 3 m.
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Fig. 18. Features inside Unit A. Sis: siltstone; Cr: crushed zone; Tf: tuff. (a) sharp bedding plane as the boundary between the crushed zone and siltstone bed; (b) boundary between the crushed zone and siltstone bed; (c) loose siltstone blocks confined to the crushed zone; (d) siltstone block included in tuff strata.
non-tectonic normal fault, which looked like a typical tectonic fault, bedding-slip faults, which were flexural slips caused by folding and/or old primary landsliding, and broad zones of crushed rock, which were considered to be induced by older events, were found under Unit A.
This complicated geological structure was one of the main factors of this hazardous landslide of Units A and B. The rotational and translational earthquake-induced landslides in the Hijirigahana Cape strongly suggest that the geological structure
Fig. 19. A sharp fault appeared during the excavation in Unit A (see location in Figure 2, after Nozaki and Has, 2013).
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Fig. 20. Stereonet projection of Unit A. The black solid line indicates the normal fault that appeared at the head (see Figure 16). The gray solid line indicates the bedding plane. The black dashed line indicates the plane of slope, and the gray dashed line indicates the joint plane. The solid arrow shows the direction of landsliding.
played an important role in landslide occurrence under strong ground motion. This results implicated geological structure should be one of important factors for considering countermeasures to landslides in tectonically active mountainous regions like Japan.
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Fracture surface markings on landslide scarps and movement of the landslide induced by the Mid Niigata Prefecture earthquake in 2004. In: The Japan Landslide Society (Ed.), Earthquake-induced Landslide Disasters in middle Mountains. “Study Report on the 2004 Mid-Niigata Earthquake PartI”. Geomorphology & Geology, pp. 160–167. Nakahigashi, K., Shinohara, M., Kurashimo, E., Yamada, T., Kato, A., Takanami, T., Uehira, K., Ito, Y., Iidaka, T., Igarashi, T., Sato, H., Hino, R., Obana, K., Kaneda, Y., Hirata, N., Iwasaki, T., Kanazawa, T., 2012. Seismic structure of the source region of the 2007 Chuetsu-oki earthquake revealed by offshore-onshore seismic survey: Asperity zone of intraplate earthquake delimited by crustal inhomogeneity. Tectonophysics 562–563, 34–47. No, T., Takahashi, N., Kodaira, S., Obana, K., Kaneda, Y., 2009. Characteristics of deformation structure around the 2007 Niigata-ken Chuetsu-oki earthquake detected by multi-channel seismic reflection imaging. 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Sato, H., Kato, N., Echigo, T., Ogino, S., Tateishi, M., Toda, S., Kato, H., Koshiya, S., Toyoshima, T., Ito, T., Imaizumi, T., Abe, S., Suzuki, N., Misawa, E., Oda, S., Kagohara, K., Koike, T., Akai, S., Noda, K., 2005. High-resolution seismic reflection profiling across the surface rupture associated with the 2004 Mid-Niigata Prefecture earthquake, central Japan: data acquisition and processing. Bull. Earthq. Res. Inst., Univ. Tokyo 80, 1–9 (in Japanese with English abstract). Sato, H.P., Hasegawa, H., Fujiwara, S., Tobita, M., Koarai, M., Une, H., Iwahashi, J., 2007. Interpretation of landslide distribution triggered by the 2005 Northern Pakistan earthquake using SPOT 5 imagery. Landslides 4, 113–122. Shimizu, F., Oyagai, N., Miyagi, T. and Inoguchi, T. 2004. Landslide topography map, vol. 17 (Nagaoka & Takada), 1/50000 and 1/25000. Technical Note of National Institute of Disaster Prevention, No.244. Shinohara, M., Kanazawa, T., Yamada, T., Nakahigashi, K., Sakai, S., Hino, R., Murai, Y., Yamazaki, A., Obana, K., Ito, Y., Iwakiri, K., Miura, R., Machida, Y., Mochizuki, K., Uehira, K., Tahara, M., Kuwano, A., Amamiya, S., Kodaira, S., Takanami, T., Kaneda, Y., Iwasaki, T., 2008. Precise aftershock distribution of the 2007 Chuetsu-oki earthquake obtained by using an ocean bottom seismometer network. Earth Planets Space 60, 1121–1126. Somerville, P.G., Smith, N.F., Graves, R., Abrahamson, N.A., 1997. Modification of empirical strong ground motion attenuation relations to include the amplitude and duration effects of rupture directivity. Seismol. Res. Lett. 68 (1), 199–222. Taira, A., 2001. Tectonic evolution of the Japanese island arc system. Ann. Rev. Earth Planet. Sci. 29, 109–134. Varnes, D.J., 1978. Slope movement types and processes. In: Schuster, R.L., Krizek, R.J. (Eds.), Landslides, analysis and control, special report 176: Transportation research board. National Academy of Sciences, Washington, DC, pp. 11–33. Xu, C., Xu, X.W., 2014. The spatial distribution pattern of landslides triggered by the 20 April 2013. Lushan earthquake of China and its implication to identification of the seismogenic fault. Chin. Sci. Bull. 59 (13), 1416–1424. Xu, C., Xu, X.W., Yu, G.H., 2013. Landslides triggered by slipping-fault-generated earthquake on a plateau: an example of the 14 April 2010, Ms 7.1, Yushu, China earthquake. Landslides 10, 421–431. Xu, C., Xu, X.W., Yao, X., Dai, F.C., 2014a. Three (nearly) complete inventories of landslides triggered by the May 12, 2008 Wenchuan Mw 7.9 earthquake of China and their spatial distribution statistical analysis. Landslides 11, 441–461. Xu, C., Shyu, J.B.H., Xu, X.W., 2014b. Landslides triggered by the 12 January 2010 Port-auPrince, Haiti, Mw = 7.0 earthquake: visual interpretation, inventory compiling, and spatial distribution statistical analysis. Nat. Hazards Earth Syst. Sci. 14 (7), 1789–1818. 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Engineering Geology journal homepage: www.elsevier.com/locate/enggeo
Role of geological structure in the occurrence of earthquake-induced landslides, the case of the 2007 Mid-Niigata Offshore Earthquake, Japan Baator Has a,⁎, Tamotsu Nozaki b a b
Asia Air Survey, 1-2-2 Manpukuji, Asao District, Kawasaki City, Kanagawa, Japan Nozaki EG Consulting, 3-10-14 Yamafutatsu, Chuo-ku District, Niigata City, Niigata, Japan
a r t i c l e
i n f o
Article history: Accepted 6 September 2014 Available online 20 September 2014 Keywords: Earthquake Earthquake-induced landslide Geological structure Non-tectonic
a b s t r a c t The landslides induced by the Mid-Niigata Offshore earthquake in 2007 (MJMA6.8) occurred mostly along the shore of the Japan Sea. This study focused on the landslides that occurred near the Hijirigahana Cape, Yoneyama Town in the Niigata Prefecture. The study area is located in the western region of the central Niigata Prefecture, along the Japan Sea. The geology of the landslide area is dominated by late Miocene sandstone-rich member of alternating beds of sandstone and siltstone. These strata gently dip to the north at approximately 25° to 30° and form cataclinal and orthoclinal slopes on the north- and west-facing slopes, respectively. The central Niigata region is one of the most landslide-prone areas in Japan. The study area is located approximately 30 km from the epicenter of the earthquake with an estimated intensity of 6 lower in the JMA (Japan Meteorological Agency) scale. A group of landslides occurred in the study area. The area contains the most concentrated set of landslides from this earthquake, even though it is located far from the epicenter. Most of the landslides occurred on north-facing slopes, but two occurred on west-facing slopes. The landslides on the north-facing slopes are characterized as translational slides. The largest rock slide in the study area, Unit D (width 100 m, length 230 m), slipped along the bedding plane of the strata. Observations from slickenlines from the slip surface and outcrop in the upper portion of Unit D suggest that before this earthquake, landslides frequently occurred on the north-facing slopes and were controlled by the bedding plane. With the exception of the largest one, other landslides on north-facing slopes were mostly debris slides, indicating that sediments from older landslides exist. The second largest landslide occurred on a west-facing orthoclinal slope. A rotational rock slide, Unit A, was 80 m wide and 100 m long. During the excavation after the landslide, a high angle fault with a SW dip was identified behind the crushed zone. Based on field observation, the fault was considered to be a non-tectonic fault and created by gravitational movement. During the excavation, the crushed zone was identified on the cutting wall of the slope. These results revealed that the relatively larger rotational rock slides occurred because of the existence of the crushed zone due to older events that caused the slippage along a non-tectonic fault. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Landslides triggered by strong earthquakes are one of the most important issues regarding natural disasters for scientific communities and administrative agencies because of the severe damage caused by their tremendous numbers and large-scale dimensions. Recently, many strong earthquakes associated with a large number of landslides have demonstrated the significance of studying earthquake-induced landslides. The most remarkable earthquake in recent years, that induced a tremendous number of landslides is the Wenchuan earthquake, China (e.g., Chigira et al., 2010; Qi et al., 2010; Gorum et al., 2011; Xu et al., 2014a). Xu et al. (2014a) identified 197,481 landslides that were induced by the earthquake, established three landslide inventories ⁎ Corresponding author. Tel.: +81 44 967 6302; fax: +81 44 965 0028. E-mail address: [email protected] (B. Has).
http://dx.doi.org/10.1016/j.enggeo.2014.09.006 0013-7952/© 2014 Elsevier B.V. All rights reserved.
and revealed spatial features of landslides with topographic, lithologic and seismic parameters. Other strong earthquakes that induced a large number of landslides were also well documented, such as the Mid-Niigata (also called Chuetsu) earthquake, Japan, 2004 (JMA, 2004; Chigira and Yagi, 2006; Yamagishi and Iwahashi, 2007); the Pakistan earthquake, 2005 (Sato et al., 2007); the Iwate-Miyagi Inland earthquake, 2008, Japan (JMA, 2008; Yagi et al., 2009); the Haiti earthquake, 2010 (Xu et al., 2014b); the Yushu earthquake, 2010, China (Xu et al., 2013); and the Lushan earthquake, 2013, China (Chen et al., 2014; Xu and Xu, 2014). In addition to the Mid-Niigata and Iwate-Miyagi Inland earthquakes, a series of strong earthquakes have struck eastern Japan in recent years, such as the Mid-Niigata Offshore (also called Chuetsu-Oki) earthquake, 2007 (JMA, 2007), the northern Nagano Prefecture earthquake (JMA, 2011a) and the Fukushima Hamadori earthquake (JMA, 2011b), which occurred along with the great 3.11 Tohoku earthquake in 2011
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(http://www.jma.go.jp/jma/en/2011_Earthquake/2011_Earthquake. html). These earthquakes caused serious damage to the focal areas and their vicinities because of the associated landslides. Several studies have been published regarding the landslides induced by the earthquakes (Has et al., 2010, 2012, 2014; Collins et al., 2012). The literature, however, mostly focused on landslide inventories and discussed the spatial distribution and relationship between the causative factors and landslide occurrences. The Mid-Niigata earthquake (M6.8, October 24, 2004) and the MidNiigata Offshore earthquake (M6.8, July 16, 2007) occurred in the central Niigata Prefecture, eastern Japan (Figure 1). The epicenters of the two earthquakes only had a distance of 35 km. The Mid-Niigata earthquake, which caused a large number of landslides (Chigira and Yagi, 2006), was one of the largest earthquakes that induced numerous large landslides inland Japan since the Zenkoji earthquake in 1847 (Zenkoji Earthquake Disaster Study Group, 1994). Compared to the Mid-Niigata earthquake, the Mid-Niigata Offshore earthquake caused a few middle-scale (JLS, 2004) and small to micro-scale landslides despite the similar topography, geological conditions and magnitude. In this case, most of the landslides occurred along the coastline of the Japan Sea, and the largest ones were associated with smaller ones concentrated in and around Hijirigahana Cape, Yoneyama Town, Niigata Prefecture. The distribution and size of the earthquake-induced landslides from the Mid-Niigata Offshore earthquake have been studied (Has et al., 2010; Collins et al., 2012). However, there are no sufficient
detailed studies that identified the mechanism of the individual landslides, except Nozaki and Has (2013). Detailed geological and geomorphological surveys are necessary for individual landslides because they provide important implications to identify the mechanism of landslides and are useful for preventing second-hand hazards. In this paper, we focused on the geological structure to understand its roles in landsliding induced by this earthquake. 2. Topographical and geological settings The study area is located along the coast of the Japan Sea in the central Niigata Prefecture, eastern Japan (Figure 1). Among the small number of landslides induced by the Mid-Niigata Offshore earthquake, many landslides occurred in the study area, even though it is far from the epicenter. The Niigata Prefecture and its surrounding regions are characterized by an NE-SW topographical configuration. The geology of the central Niigata Prefecture is mostly composed of Neogene and Quaternary sedimentary rocks (GSJ, 2012). The geological structure is mainly characterized by NE-SW striking faults and active folds with NE-SW extending axes, reflecting the NW–SE compression (Taira, 2001; Otsubo, 2008) caused by the Pacific plate subducting under the Eurasian plate. The central Niigata Prefecture is also known as one of the most landslide-prone regions in Japan. A tremendous number of old landslides have been identified in this region (Shimizu et al., 2004).
Fig. 1. Location of the study area. The shaded area indicates the Niigata Prefecture; rectangles indicate the projected area of source faults. (A) Mid-Niigata Offshore earthquake; (B) MidNiigata earthquake. Kashiwazaki St.: location of the K-net seismic station.
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The Mid-Niigata Offshore earthquake occurred 3 years after the Mid-Niigata earthquake, approximately 35 km northwest of the epicenter of the latter. Both earthquakes occurred on reverse faults. Based on aftershock observation and seismic reflections, the source fault of the Mid-Niigata earthquake was dipped west (Hikima and Koketsu, 2005; Sato et al., 2005). Various studies have discussed that the main source of the Mid-Niigata Offshore earthquake was a southeast dipping reverse fault (Mori, 2008; Shinohara et al., 2008; Miyake et al., 2010), even though there are arguments regarding the dip of the source fault (e.g., Aoi et al., 2008). This result was supported by seismic surveys (No et al., 2009; Nakahigashi et al., 2012), so it is reasonable to consider that the source fault of the Mid-Niigata Offshore earthquake dipping to the southeast (Figure 1). The study area is approximately 30 km southwest of the epicenter of the Mid-Niigata Offshore earthquake, on the hanging wall of the source fault. A late Miocene sandstone-rich member with alternating beds of sandstone and siltstone, part of which is beautifully outcropped along the local road passing through the cape (Figure 2), is conformably overlain by volcanic rocks and disconformably underlain by a siltstone member intercalated with thin beds of sandstone and a thick pumice tuff on its upper horizon (Figure 3). The upper and the lower members of sedimentary rocks were named the Hijirigahana Formation by the Yoneyama Research Group (Yoneyama Research Group, 1973). The disconformity between both members, however, was identified during our
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investigation of this disaster. These strata gently dip (25–30°) toward the sea, making cataclinal and orthoclinal slopes on the north and west side of the cape, respectively. This cape is located just on the axis of a broad open anticline, where the axis plunges to north. That is why the bedding plane in the study area trends EW and dips N despite NS trending anticlinal axis. Although vertical joints slightly tilted to the SW tend to develop NW–SE and less other directions, there was no noteworthy tectonic fault, except for some bedding-plane slips found on new and old sliding surfaces. The study area is composed of north- and west-facing slopes. The elevation of the ridge dividing between them ranges from 100 to 150 m above sea level (Figure 4). On the north-facing slopes, a wide range of old landslide topography can be identified. Near the upper region of Unit D, an old scarp can be clearly recognized in photographs taken before the earthquake (Figure 2). On the west-facing slopes, however, no landslide topography can be recognized. 3. Features of the earthquake-induced landslides A group of landslides triggered by the earthquake occurred on the north- and west-facing slopes in and around the Hijirigahana Cape (Figure 4). The landslides were classified into two types (Table 1). Most of the landslides occurred on the north-facing slopes were translational slides. Two rotational landslides occurred on the west-facing
Fig. 2. Photographs of part of the study area before and after landsliding (after Nozaki and Has, 2013). (a) Before landsliding, taken on Aug. 10, 2005 by Atsushi Ueki, Kashiwazaki City Education Center; (b) after landsliding, taken on Aug. 4, 2007. The dotted line indicates the trace of fault observed at the box, in Figure 19).
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Fig. 3. Geological map of the landslides and their vicinities (after Nozaki and Has, 2013). The letters “A” to “I” are the landslide units as in Figure 4.
slopes. The north-facing slope is considered cataclinal, and the westfacing slope is orthoclinal (Nozaki and Has, 2013). Old landslides can be recognized on the north-facing slopes by interpreting pre-earthquake aerial photographs (Figure 5). A sharp northeast-southwest directed scarp can be recognized on the aerial and field photos.
3.1. Landslides on the north-facing slope Unit D, a translational rock slide that occurred on the north-facing slope, was the largest among those induced by the earthquake (see Figure 4). This slide unit was obviously one of the pre-existed ones as previously described, and reactivated, extending mainly to the upper
Fig. 4. Distribution of earthquake-induced landslides in the study area. The shaded areas indicate old landslides.
B. Has, T. Nozaki / Engineering Geology 182 (2014) 25–36 Table 1 Type and dimension of earthquake-induced landslides in the study area (landslide classification is based on Varnes, 1978). Unit
Type
Length(m)
Width(m)
A B C D E F G H I
Rotational Rock Slide Rotational Rock Slide? Rotational Debris Slide Translational Rock Slide Translational Debris Slide Translational Debris Slide Translational Debris Slide Translational Debris Slide Translational Debris Slide
100 25 115 230 130 60 35 60 60
80 70 100 100 40 30 35 60 40
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Additionally, the volume accumulated at the foot of the slope was less than 10,000 m3 (Nozaki and Has, 2013), indicating that the thickness of the slide was limited (Figure 8). The upper end of the sliding surface reached beyond the ridge along a bedding plane, hence no head scarp remained (Figure 7a). On the eastern side of the slide, however, a “detached scarp” rose more than 10 m, and the sequence of alternating beds of sandstone and siltstone was easily observed (Figure 7a). On the western side of the landslide, a 5 to 8 m high side scarp was formed (Figure 6), and part of it toppled as rock-panels separated by high-angle joints trending NW (see Figures 3 and 20). Almost the entire sliding mass reached the beach. The sparsely distributed debris on the sliding surface was washed away by rainfall within a few months, and microstepped bedding planes have been widely exposed. On the higher portion of the sliding surface, a large number of slickenlines, which were directed approximately to the dip of the strata, were extensively carved on the bed of siltstone (Figure 7d). Slickenlines found on the sliding surface at the foot of the western scarp, however, were approximately parallel to the strike of the strata. It means that these ones had evidently occurred as a flexural slip during the past folding: tectonic movement, and functioned as a potential sliding surface. The sliding, however, occurred not only along one bedding plane but also along the other ones connected in a stepwise manner making a micro-staircase. This means that the sliding surface shifted to the lower sequence of the strata and to the downslope. The whole sliding surface, therefore, looked a slightly steeper than the dip of the strata. With the exception of Unit D and the rotational debris slide of Unit C, all other smaller ones on the north-facing slopes are translational debris slides. This fact indicates that old landslides had occurred along the north-facing slopes in the past and that the debris remained on the slope (Figure 9). 3.2. Landslides on the west-facing slope
Fig. 5. Aerial photograph of the study area taken in 1975 by GSI, Japan. A clear scarp continued to the sea along the right side of Unit D.
slope. Only thin strata, however, must have been ripped off of the central to lower region of the unit because part of the road that ran across the central part of the unit remained intact (Figures 6 and 7c).
Units A and B are rotational rock slides that occurred on the westfacing slope with a total volume of approximately 100,000 m3. The thickness to the sliding surface of Unit A was approximately 10 m. On the western side of the cape, the slope looks toward southwest, and the strata dip north, forming orthoclinal structure (Figure 3). Except for the upper region of the slope, where the rock consists of alternating
Fig. 6. Whole view of Unit D from the toe area. The dashed lines indicate the location of the lost road. The rectangles indicate the location of the photographs in Figure 7.
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Fig. 7. Observations of Unit D. (a) Alternating beds outcropped on the eastern side scarp, (b) sliding surface, (c) concrete block of road remaining after the landsliding and (d) sliding surface with slickenlines.
beds of sandstone and siltstone, the basement rock consists of siltstone member sparsely intercalated with thin layers of loosely indurated sandstone and a thick soft pumice tuff in its upper region. An approximately 5 m high head scarp was created below the crown of Unit A along the ridge associated with parallel cracks. The vertical crack, which was 0.3 to 0.7 m high at the head area of Unit B, obliquely ran across the road. There was no clear boundary between Units A and
B. The tip of the sliding surface of both units, including the toe of the debris of Unit A did not reach the foot of the slope, and the JR tunnel under the landslides had no damage. Although a slightly gentler slope was created at the head of Unit A, the gradient of this orthoclinal slope was more than 40° as a whole. On the outcrop of alternating beds at the northern end of the ridge that meets with the road, a few consistent vertical joints directed NW were observed (see Figure 2). New open spaces
Fig. 8. Geological cross section (III–III′ on Figure 3) of Unit D (after Nozaki and Has, 2013).
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Fig. 9. Earth slides of Unit E, view from the toe to the head.
associated with small debris were observed along these joints and bedding planes, and the sedimentary rocks were loosened by tremor of the earthquake. Some of the joints, however, had been already loosened by pre-events as evidenced by sealing cracks with asphalt on the road, and grass roots invaded some of those open cracks. After the landsliding, cut and slope protection works were executed as a countermeasure on Unit A (Figure 10). 4. Discussion: role of geological structure in landsliding 4.1. Concentrated landslides in the study area Compared to the Mid-Niigata earthquake, the Mid-Niigata Offshore earthquake induced only a few middle-scale landslides but was
associated with many other smaller ones. The earthquake-induced landslides are hazardous, however, because of their concentrated occurrence, especially in the study area. The seismic source area of the latter earthquake was located under the sea, while the former one was located inland (see Figure 1). The study area, however, was located on the hanging wall of the source fault, and the distance from its southern verge was less than 10 km. In addition, the PGA at this site was estimated to be greater than 500 gal and the intensity was 6 lower in the JMA scale (Hasi et al., 2011). The particle motion of the seismic station Kashiwazaki (NIG018, K-Net, from NIED) showed that horizontal motion dominated along the NW–SE and N-S directions (Figure 11), and it stimulated landslides on the same facing slopes because of directivity effects of ground motion (Somerville et al., 1997). The same phenomenon was also suggested in a few previous literatures (e.g., Huang, 2013; Xu et al., 2013).
Fig. 10. Cutting works on Unit A.
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Fig. 13. Stereonet projection of Unit D. The solid line indicates the bedding plane. The black dashed line indicates the plane of slope, and the dashed gray lines indicate the plane of joints on the western side scarp. The solid arrow shows the direction of landslide.
Fig. 11. Particle motion of acceleration at the Kashiwazaki seismic station.
4.2. Role of geological structure 4.2.1. Bedding plane as a sliding surface For the landslides that occurred on the north-facing slopes, especially for Unit D, the bedding plane played the main role for the landsliding. Fig. 2 shows that cataclinal slope consists of beautiful alternating beds of sandstone and siltstone that are easily sheared off along bedding planes. Especially under strong ground motion by earthquakes, bedding-planeslip must easily occur, as observed in the outcrop (Figure 12). In the cases of the Neogene sedimentary rocks, strong earthquakes cause displacement along bedding planes on dip slip (e.g., Abe et al., 2006; Nozaki, 2008), indicating that the bedding plane is a weak surface. This suggests that the bedding plane of layered formations can control both the landslide mechanism and the activity (Cruden and Varnes, 1996). The bedding-plane slips observed in places must have fostered the development of gravitational sliding as a potential sliding surface. These slips must have occurred with flexural slip on the axial part of the anticline because the slickenlines and the dip of the bedding plane crossed each other at nearly right angles, but each of those slips was not so long extended. The sliding surface of Unit D drew a very gentle arc, creating micro-steps. This configuration made it slightly steeper in its upper region. It, therefore, may have been one of the reasons why the sliding mass did not disintegrated and reached the foot of the slope. Additionally, it may have helped the large displacement.
Fig. 12. Bedding plane slip appeared on the outcrop (see the location in Figure 2) after the earthquake.
Along the ridgeline between the cataclinal and orthoclinal slopes, vertical joints developed trending NW–SE (Figure 13), controlling the west-side scarp of Unit D. Large blocks consisting of alternating beds in the debris at the toe of Unit D must have been derived from the pre-existing east-side high scarp that receded during this earthquake. The huge blocks of sandstone and siltstone observed at the toe of Unit D were considered to have broken out from the remaining old scarp. Almost the entire slope, including Units C, D, E, F and G, were located in a larger old landslide unit (Figure 4), and it is reasonably inferred that translational slides were repeated on this slope before the earthquake. Comparing the photographs in Fig. 2, we found that extents of Unit D were almost the same as the previous sliding. Although thick layers of small parts incipiently moved down at the head area, only a 2 to 3 m thick veneer of alternating beds on the upper half of the slope must have slid down to the beach area as a new rock slide, including some old debris, because the volume of the debris was less than 10,000 m3 (Nozaki and Has, 2013). The same type of rapid translational rock slides as in Unit D, represented by the Yokowatashi landslide (Nagata and Nozaki, 2007), were triggered in many places by the Mid-Niigata Prefecture earthquake. Lithological and structural complexity is one of the main factors that control landslide movement. In moderate mountainous areas in the Niigata Prefecture, where is geo-structurally active areas such as the MidNiigata region mainly composed of Neogene sedimentary rocks, geological structure has always influenced landslides. Many landslides in this region tend to occur along the axes of anticlines (Iwamatsu et al., 1975), possibly because of the weakened rock strength at the hinge of folds. A large amount of dip-slip sliding induced by the Mid-Niigata earthquake indicates that earthquake-induced landslides tend to be strongly influenced by the geological structure. Above all, the geological structure played a dominant role in the north-facing rock slide affected by strong ground motion. 4.2.2. Non-tectonic fault and crashed zone in west-facing slope The mechanism for landslides on the west-facing orthoclinal slope of Units A and B is different from that of Unit D. Excavations for countermeasures in Unit A and its vicinity extensively exposed the interior structure of the slope, which helped clarify the mechanism of landsliding. Fig. 14 is a sketch map of the cut-slope observed during the excavation of Unit A. We conducted field reconnaissance and observed of the cores drilled just after the earthquake in Unit A and drew this map and geological section (Figure 15). Although landslide debris moved by the earthquake had been completely excavated, a high angle normal fault (Figure 16) dipping to the west appeared at the head of the cut slope. The hanging wall of this fault was accompanied by heavy crashed zones “Cr” as shown in Fig. 16. A thick layer of pumiceous tuff was displaced more than 5 m, and a “tuff tail” was traced along the fault
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Fig. 14. Sketch map of cut-slope during the excavation of Unit A (after Nozaki and Has, 2013).
plane (see “b” in Figure 16). The displacement along the fault was less in the upper region (“a” in Figure 16) but became larger in the lower region. Thick crushed zones along the bedding-planes and/or beddingplane-slip faults also appeared in the middle and lower region of the slope (Figure 17). At the foot of the cut slope, part of their lower boundary was accompanied by a wedge (Figures 17 and 18a) along slip-planes with intercalated clay seams (Figure 18b). This crushed zone occasionally looked very loose (Figure 18c), part of which may have been caused by this event. We also observed siltstone blocks confined to the tuff layer (Figure 18d). These phenomena indicate that the structure of the hanging wall had been undoubtedly disturbed before this event, and part of the pre-existing slip-surface and/or slip-planes played a strong role in the occurrence of Units A and B by this earthquake. Another sharp fault also appeared on the cutting wall during the excavation along the ridge between Units A and D (Figure 19). The photograph shows a typical normal dip separation fault, and the right side is the hanging wall. The fault looked intact, but a thin crushed zone existed along the fault plane. Additionally, the hanging wall and foot wall of the fault looked intact, and no crushed zone was recognized. Therefore, a
tectonic fault was suggested. The upper central bushy region in Fig. 2 was the cut slope in Fig. 19, and the southern extension of this fault was considered to be connected to the normal fault in Unit A. However, on the northern extension of the outcrop (Figure 2), no such fault was confirmed despite the wide and extensive outcrop. This fault, and therefore the fault in the area of Unit A (Figure 16), was considered to be a non-tectonic fault and part of an old sliding surface (Nozaki and Has, 2013). It reasonably extends to west toward the natural outcrop shown in Fig. 6, suggesting that the whole structure was non-tectonic and was created by the old gravitational movement. Above all, in Unit A, the orthoclinal landslide occurred on a wedge of a pre-existing weak zone between the sharp normal fault and the bedding-slip-fault (Figure 20). This result indicated that the geological structure caused by older sliding played a dominant role in landsliding. 5. Conclusions Induced by the Mid-Niigata Offshore earthquake, a group of landslides occurred in the Hijirigahana Cape, mainly on the north-facing
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Fig. 15. Geological section of Unit A, I–I′ on Figure 14 (after Nozaki and Has, 2013).
Fig. 16. A sharp, nearly vertical normal fault appeared on the cutting wall during the excavation in Unit A. The black arrows indicate fault, and the white arrow indicates the movement of the hanging wall of the fault; Cr: crushed zone; Tf: tuff; S:scale, 4 m.
slope. Many translational landslides represented by Unit D that occurred on the north-facing slope slipped along the bedding planes, creating micro-steps. Most of them were reactivations of old landslides, except for the head region of Unit D, at which the rock mass thickly receded toward the ridge. Almost all slid masses were less than 2 to 3 m
thick because of the cataclinal structure and reactivation of thin old debris left on the gently tilted dip-slopes. The mechanism of landslides on the orthoclinal slope was rather complicated and interesting. The geological structure underneath the Unit A landslide on the steep slope showed interesting features. A
Fig. 17. View of the foot of the cutting wall of Unit A. Cr: crushed zone; Rd: road; S:scale, 3 m.
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Fig. 18. Features inside Unit A. Sis: siltstone; Cr: crushed zone; Tf: tuff. (a) sharp bedding plane as the boundary between the crushed zone and siltstone bed; (b) boundary between the crushed zone and siltstone bed; (c) loose siltstone blocks confined to the crushed zone; (d) siltstone block included in tuff strata.
non-tectonic normal fault, which looked like a typical tectonic fault, bedding-slip faults, which were flexural slips caused by folding and/or old primary landsliding, and broad zones of crushed rock, which were considered to be induced by older events, were found under Unit A.
This complicated geological structure was one of the main factors of this hazardous landslide of Units A and B. The rotational and translational earthquake-induced landslides in the Hijirigahana Cape strongly suggest that the geological structure
Fig. 19. A sharp fault appeared during the excavation in Unit A (see location in Figure 2, after Nozaki and Has, 2013).
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Fig. 20. Stereonet projection of Unit A. The black solid line indicates the normal fault that appeared at the head (see Figure 16). The gray solid line indicates the bedding plane. The black dashed line indicates the plane of slope, and the gray dashed line indicates the joint plane. The solid arrow shows the direction of landsliding.
played an important role in landslide occurrence under strong ground motion. This results implicated geological structure should be one of important factors for considering countermeasures to landslides in tectonically active mountainous regions like Japan.
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