10th Asian Regional Conference of IAEG (2015)

Structural control of landslide in the accretionary complexes of the Southwest Japan

Hidehisa NAGATA(1) and Satoru KOJIMA(2)(1) Fu Sui Do co. ltd.,Japan
E-mail: nagatah@concerto.plala.or.jp(2) Department of Civil Engineering, Gifu Univ., Japan

Abstract
Relationship between landslide mechanisms and geological structure in the accretionary
complexes of the Southwest Japan was examined. The analyzed region includes southern
Kyusyu, Shikoku, Kii Peninsula and Akaishi Mountains. This region is geologically composedof Sanbagawa metamorphic belt, Chichibu belt as a Jurassic accretionary complex andShimanto belt as a Cretaceous to Neogene complex in the Pacific Ocean ward.
Gently dipping strata more than slope tend to cause landslides of dip slope slide. In
Chichibu belt in Shikoku and Kii Peninsula, this type of landslides is often recognized. Some
numbers of cases are found in the Shimanto belt in Kyushu, Kii Peninsula and the foot of
Akaishi Mountains. Although steeply dipping strata more than slope are originally stable,
combined fault or cleavage planes in accretionary complex play important role to the instabilityof slope. Wedge-shaped slides and slides along the daylight low-angled fault often occur in the
Shimanto belt in Shikoku and Kii Peninsula. Gravitational buckling in nearly vertical strata is
responsible for landslides of toppling or slide along the hinge plane. In Akaishi Mountains
affected by collision of Izu-Ogasawara arc in 15Ma, both the Chihibu and Shimanto belts havealmost vertical structure of strata. Therefore, gravitational deformation is frequently developed
and landslides also occur in this region.
Deep-seated rapid landslides in non-metamorphic accretionary complexes including
Chihibu and Shimanto belts are dominant induced by earthquake or heavy rainfall. While, in
Sanbagawa belt as a metamorphic accretionary complex, deep-seated slow landslides are
dominant. This is not structural but rock control, that is schists are more ductile than the
non-metamorphic sedimentary rocks.

Keywords: structural control, outfacing dip slope, accretionary complex, Outer Zone ofSouthwest Japan

1.

Introduction

landslide of outfacing dip slope is one of the typicallandslides controlled by geological structures. Basedon the relationship between dips of slope and strata
Suzuki (2000) subdivides the outfacing dip into three
types; daylighting dip, parallel dip, and hangnail dip(Fig. 1).
The Outer Zone of Southwest Japan is alandslide-prone area as well as the coastal area along
the Sea of Japan (Fig. 2). In the latter, most oflandslides are distributed in the Neogene
volcano-sedimentary rocks, whereas in the former,
landslides occur in the Mesozoic to Paleogene
accretionary complexes. Although structurally

Stratified rock mass like sedimentary rocks hasmechanical anisotropy; it is strong to the normal
direction to planar structure but weak to the parallelone. In case that the planar structures dip to the same
direction of the slope, the structure is called theoutfacing dip (cataclinal) structure. The outfacing dip
structure tends to be unstable slope contrasting to theinfacing dip structure that the planar structure isdipping to the opposite direction of the slope. Many
landslides induced by earthquakes and heavy rainfalls
have occurred in the outfacing dip slope. The

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10th Asian Regional Conference of IAEG (2015)

Fig.1 Relationship between attitude ofstrata and ground surface (Suzuki, 2000).
Fig.3 Schematic profile showing theformation process of accretionary complex(Kojima et al., 2015)

controlled landslides are recognized in both areas,rock strength, dimensions of landslide and movement
characteristics are different. In this paper, wesummarize the structural controlled landslides in theaccretionary complexes and examine the mechanisms
in detail and characteristics of regions.
2.

Sanbagawa metamorphic belt is mainly composed of
crystalline schists (pelitic, psamitic, mafic andsiliceous) associated with small amount of ultramafic
rocks and weakly metamorphosed greenstones. The
Chichibu belt consists of sandstone, mudstone,
greenstones, limestone, chert, and melanges. The
Shimanto belt is mainly composed of sandstone andmudstone with small amount of blocks of greenstones,
chert, and limestone. Reflecting the subduction, theseaccretionary complexes have basically continental
ward (north to west) dipping structures of strata andfaults (Fig. 3). Therefore, north facing or west facing
slopes tend to be outfacing and east facing or south
facing slopes are infacing. However, exceptional
structures are often developed by deformation likefolding after the accretions.

Geology and geologic structure of the Outer
Zone of Southwest JapanThe Outer Zone of Southwest Japan bounded on
the north by the Median Tectonic Line (MTL) and on
the east by the Itoigawa-Shizuoka Tectonic Line(ISTL) is the area of accretionary complexes formedby successive subduction of oceanic plate from theJurassic to Neogene time. The ages of accretion are
successively younging to the oceanward; The
Sanbagawa metamorphic belt, the Jurassic Chichibu
belt, and the Cretaceous Shimanto belt occur from
north to south. The Shimanto belt is subdivided into
the Cretaceous Northern Shimanto belt and the
Paleogene to Neogene Southern Shimanto belt. The

Fig.2 Index map also
shown the tectonic setting(after GSJ, 2014) andlandslide distribution (after
Inokuchi et al. 2011).
MTL: Median Tectonic
Line,
ISTL: Itoigawa Shizuoka
Tectonic Line,Sb; Sanbagawa
metamorphic belt,
Ch: Chichibu belt,
Sn; Northern Shimanto
belt,
Ss: Southern Shimanto belt.

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10th Asian Regional Conference of IAEG (2015)3.

Geologic structures and landslides in theOuter Zone of Southwest Japan

3.1

KyusyuIn Kyusyu Island, inclination of strata tends tobecome gentle to the oceanward. Namely, The rocksof Sanbagawa and Chichibu belts dip vertically or
steeply, Northern Shimanto belt dip moderately, andSouthern Shimanto belt dip gently; moreover, thethrusts stacking the strata are more gentle.
Under such geological condition, heavy rain in2012 brought five deep-seated rapid landslides of 0.3
to 3×106 m3 on the slopes of the Mimikawa Valley
composed of sedimentary rocks of the Northern
Shimanto belt (Chigira, 2009). Among these, in the
Matsuo-shinbashi landslide, although bedding planesof rock is hangnail dip, minor fault planes obliquely
cutting the bedding is daylighting dip. Shimato
landslide has the daylighting sliding plane originatedfrom thrust.

Fig.4 Stereo projection of bedding planes in
the Chichibu belt and schistosity in the
Sanbagawa belt along Miyagawa River, Kii
Peninsula.
Moderate to gentle inclined mainly
northward beddings are dominant in the
Chichibu belt. In the Sanbagawa belt, dipsof schistosity is gently inclined southward.
According to mapping, The Chichibu belt
has imbricated structure with open folding.
On the contrary, in the Sanbagawa belt,
tight recumbent foldings are recognized.

3.2

Shikoku
In Shikoku Island, to the contrary in Kyusyu,
strata of the Shimanto belt tend to dip steeply,
whereas the Sanbagawa and Chichibu belts often
show gently inclined bedding or schistosity. Fold
structures with low-angle axial planes are also
developed in the Sanbagawa belt. Distribution area ofSanbagawa crystalline schists has daylighting slopes
in places, accordingly, it becomes to landslide-prone
area. Also in the Chichibu belt, Sasaki and Yokoyama(2013) revealed that the Taninouchi landslide has the
low angled daylighting structure (partly infacing) bydetailed mapping. Although the Shimanto belt is low
frequency area of landslide, large landslides occur
occasionally because of the long term loosening ofrock mass. The Kanagi landslide (8.5×106 m3)
probably induced by the 1707 Hoei earthquake
collapsed in the toppled area of originally steeply
inclined Southern Shimanto strata (Chigira, et al.,1998).

Daylighting dip area
Hangnail dip area

Fig.5 Stereo projection of dip vectors in the
Akatani landslide (Yokoyama et al., 2013).
The main slide plane is along the thrustplane. Bedding plane, cleavage plane, andminor fault planes are scattered. Greenlarge circle shows the the slope angle. Red
small circle indicate assuming friction
angle (20 degrees).

3.3

Kii Peninsula
Geologic structures similar to the Shikoku Island
are developed in the Kii Peninsula. Namely, gently ormoderately inclined schistosity and bedding planes
are dominant in Sanbagawa and Chichibu belts.
Figure 4 shows a stereo projection of schistosity in
Sanbagwa belt and bedding plane in Chichibu belt
along the Miyagawa River flowing in the eastern partof the Kii Peninsula (after Nagata et al., 2010). On thecontrary, moderate to steep dip of beddings areprevailing in the Shimanto belt.
In the Chichibu belt, Kasugadani landslide
induced by 2004 heavy rain which has the parallel dip
structure (Nagata et al., 2010) and prehistoriclandslide of Kasagi area inferred the collapse of
daylighting dip structure (Kojima et al., 2015). In

the Shimanto belt, many of the large rapid landslides
caused by 2011 heavy rain were wedge type failure
formed by combined fault fracture zone and hangnail
dipping bedding plane (Chigira et al., 2013). Akatani
landslide, which is one of the largest landslide
induced by the 2011 rainfall, has parallel dipped main
sliding plane and hangnail dipped bedding plane (Fig.
5; Yokoyama et al., 2013).

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3.4

Akaishi Mountains
Structural trend were bent into N-S direction by
the collision of Izu-Ogasawara arc after 15Ma. Dipsof the strata are generally steep in every geologic belt.
According to such structural characteristics, slide-typelandslides are few in main part of Akaishi Mountains.
On the other hand, rapid landslides with precursory
buckling or toppling (bending toward valleyside) ofstrata often occur. The Akakuzure landslide (Chigira
and Kiho, 1994), which is not certain that it failed all
at once, and the Oyakuzure landslide (1.2×108 m3) are
typical examples of this type of landslide. Rocks ofboth landslides are mainly composed of phyllitic
argillaceous rocks of Shimanto belt.
4.

Discussion

4.1 Relationship between geologic structures andinclination of ground surface
The daylighting dip slope has the gentler dip ofplanar structure than that of ground surface. Weakplane like shear zone and intercalated bed along theplanar structure becomes daylight and tensile strength
parallel to the planar structure is small. Thus thedaylighting dip slope tends to be unstable. Many
landslides of daylighting dip structure are recognizedin the area of Sanbagawa schists.
The parallel dip slope is more stable than the
daylighting dip slope. In case that the foot of slope iscut down, however, parallel dip slope also becomes
unstable similar to the daylighting dip slope. As theOuter Zone of Southwest Japan has been rapidly
uplifted in Quaternary time, the undercut slopes arecommon and unstable.
The hangnail dip slope is originally stablebecause the weak zone layer is not daylight. But, asshown above, many landslides are recognized even on
the hangnail dip slopes. The instability is caused bydaylighting gentle weak plane other than bedding
plane (Fig. 6b) or wedge shaped slide combined withthe other weak planes of outfacing dip (Fig. 6a).
Low-angle cleavage or fault planes were generally
formed during the development process ofaccretionary complexes (Fig. 3). Owing to that, there
is no small extent of slopes accompanied by weakplanes of daylighting or wedge shape forming.
In case of nearly vertical dip of planar structure,
another type of slope movement may occur: bucklingand valleyward bending. Weakened zone of rock mass
is so deeper in the higher level because of weatheringand stress release that the envelope of lower limit ofgravitational deformation will make low-angle weakplane(Fig. 6c). The plane changes into sliding plane in
the long term. Especially in steep slope, buckling andbending of strata begin much easily. Even in infacing
dip slope, precursory gravitational deformation andlandslide can form.
As indicated above, the structural control of

Fig.6 Schematic block diagrams and profile
showing the landslide mechanism on
hangnail dip slopes.
a: wedge shaped landslide slid along theintersection between bedding plane andweak plane like a fault fracture zone.
b: slide along the low-angle daylight weakplane.
c: formation of low-angle weak plane along
the lower limit of gravitational deformation(buckling).
landslides in accretionary complexes is not simple,
but most of the factors are related to the generationand growth of the complex. In other words, weakplanes such as cleavage and fault planes oblique to thebedding, fault planes formed during the stacking ofaccretionary wedge, fault planes accompanied byunequal subduction, and fault planes associated withthe uplifting of accretionary complexes make thelandsliding plane combined with bedding plane or
schistosity plane.
4.2
Structural control and rock control tolandslides
Landslides are not only controlled structurally.
Because cracky rock mass deforms more easily,landslides originated from buckling tend to occur inthe area of steeply inclined schists or phyllite. Anothertype of landslides controlled by weathering arerecognized in greenstones with poor stratification in
the Mikabu belt, a part of the Sanbagawa belt. Boththe lithology and structure give influence on the
distribution density, dimensions and movement
characteristics of landslides.
5.

Concluding remarks
We summarize structural control of landslide in
the Outer Zone of Southwest Japan from the4

10th Asian Regional Conference of IAEG (2015)(No.0421) at Miyagawa Village, Mie Prefecture,
Japan, Jour. Japan Landslide Soc., Vol.47, pp.
98-106.
Sasaki，M. and Yokoyama, S. (2013): Restudying of
surface boundary structure of Taninouchi landslidein the northern belt of Chichibu terrane, Research
Reports of Kochi University, Vol.62，pp.11-23．
Suzuki, T. (2000): Chapter 14: Slope development,
Introduction to map reading for constructionengineers, Vol.3, 751-776.
Yokoyama, S., Inokuchi, T., Nagata, H., Kato, H., andKimura, K. (2013): Geologic structural control of
the Akatani deep-seated catastrophic slope
movement, Nara Prefectutre. Abst. 52nd ann.
meeting Japan Landslide Soc., pp. 82-83.

viewpoint of relationship between the inclination ofplanar structure and ground surface. Though it is clearthat the daylighting and parallel dip slopes are
unstable as well, landslides often occur even in the
hangnail dip slope which is originally stable. Other
than the bedding and schistosity planes, weak planes
formed during accretion play an important role toform landslides in the accretionary complexes.
Geologic structures in accretionary complex are
furthermore complicated by the influence of post
tectonic
movement
includinggravitational
deformation, and thus landslides are formed in the(apparent) horizontal or infacing dip slopes. The
examination to geologic structure at landslide site isrequired in the future.
Acknowlegements
We appreciate Drs S. Yokoyama of Kochi
University and K. Nishiyama of Tokushima
University for teaching us the cases of landslide in
Kyusyu and Shikoku and discussing.
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