10th Asian Regional Conference of IAEG (2015)

Georisks along active faults of Median Tectonic Line in Shikoku, Southwest Japan

Shuichi HASEGAWA (1), Eiji TAMURA (2), Daisuke KANBARA(2)(1) Kagawa University, Japan
E-mail:hasegawa@eng.kagawa-u.ac.jp(2) Yonden Consultants Co. Inc., Japan
E-mail:e-tamura@yon-c.co.jp
Abstract
The Median Tectonic Line (MTL) is one of the most predominant and active faults
in Japan. The MTL marks striking contrast in topography between the mountains andalluvial plains in Shikoku, Southwest Japan. The active faults of the MTL are inferred tocause magnitude 7 to 8 class earthquakes on the basis of the fault lengths and the amount ofdisplacement. These topographic contrast and nearby strong earthquakes are the primarycause of landslides along the MTL. Geological investigations have revealed that large-scalelandslides have occurred since Early Pleistocene along the fault scarps of the MTL in
Shikoku. Recently, many small landslides have occurred due to slope cutting and groundexcavation for tunneling during the construction of the expressways. Geotechnicalinvestigations on these landslides have revealed that smectite-bearing clay-rich zones are
the origin of sliding surfaces. The smectite is inferred to be formed by hydrothermal
alteration which is originated from Middle Miocene volcanism. This hydrothermal
alteration also yield fine-grained pyrites which are the main cause of acid soil after slope
cutting. The rockmass types of tunnels composed of the Cretaceous Izumi Group is divided
into four types from view-points of landslide and hydrothermal alteration. Experience fromthe Median Tectonic Line in Shikoku provides us typical georisks along major active faults.
Keywords: Median Tectonic Line, active fault, hydrothermal alteration, landslide, georisk

1.

Introduction

the mountains and the alluvial plains in Shikoku. This
topographic contrast is the primary cause oflandslides along the MTL.
The georisks associated with the MTL are active
faulting and source of big earthquakes, large-scale
unstable fault scarps, fracture and hydrothermal
alteration zones, and large-scale landslides. The
Median Tectonic Line in Shikoku provides us typicalgeorisks along major active faults.

Japan is located in the Circum-Pacific “Ring offire" where seismic and volcanic activities occur
constantly. Japan has suffered great damages from themassive inter-plate earthquakes produced by platesubduction (such as the 2011 Tohoku Earthquake) andthe inland crustal earthquakes caused by active
faulting (such as the1995 Kobe Earthquake).
In southwest Japan, Nankai Trough earthquakes
whose magnitudes are measuring 8 to 9 haveoccurred every 100 to 150 years along the Nankai
trough on the boundary megathrust between Eurasia
and the Philippine Sea plate (Fig. 1). The Median
Tectonic Line (MTL) is one of the most predominant
and active faults in Japan. It forms an active fault
system with predominantly right-lateral displacementin the Quaternary in Shikoku and western Kii,
southwest Japan, owning to oblique subduction of thePhilippine Sea plate. The active fault planes of theMTL are usually steep and north dipping. Thus, theMTL have marks a topographical boundary between

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10th Asian Regional Conference of IAEG (2015)

2.

MTL. These rocks indicate K-Ar dating of about
13 to 15Ma, with many of them concentrated around
14Ma (Shinjoe and Sumii, 2000 etc.).

Geological setting

2. 1 Median Tectonic Line (MTL)The Median Tectonic Line (MTL) is the main
topographical boundary in north Shikoku, and it runs
from Naruto city on the north of Tokushima city toMatsuyama city, trending N75 ° E. The MTL ismarked by remarkable contrast in topography
between the mountains and plains. The active faultsof the MTL run along the foot of the Sanuki Range ineastern Shikoku and along the foot of Shikoku Range
in central and western Shikoku. The fault scarps
reach up to 1000 meters in height in central Shikoku.
The MTL forms a boundary between the Upper
Cretaceous Izumi Group on the north and theSambagawa metamorphic rocks on the south (Fig. 1).
The distinct fracture zone of the MTL reaches 80
meters into the bedrock, and hydrothermal alteration
zones are widely distributed in the bedrock(Hasegawa et al., 2001).

3. Active faulting of the MTL
The rate of right-lateral slip of the fault during thelate Quaternary is estimated to be several meters per
1000years in Shikoku (Okada, 1980).
The MTL active faults were previously thought tobe unruptured at least in the past 1000 years based on
the absence of historical evidence for destructive
earthquakes. However, trench excavations show thatpart of the MTL ruptured most recently during or
after the 16th century A.D. with at least 6.9 ± 0.7 mof slip along the main fault trace (Tsutsumi andOkada, 1996).
The MTL probably does not rupture along its
entire length in a single earthquake but instead
consists of multiple “earthquake segments” that
rupture independently of one another. The recurrence
interval and surficial offset for surface-rupturing
earthquakes at four individual sites on the MTL in
Shikoku Island are 1000–3000 years and 5–8 m,
respectively (Tsutsumi et al., 1996).
The active faults of the MTL are inferred to cause
magnitude 7 to 8 class earthquakes on the basis of thefault lengths and the amount of displacement. Fault
segmentation models have been developed by using
various types of information about faults, including
both geological and geophysical data, in order toestimate the size of future cascade earthquakes(Okada, 1992; Ikeda et al., 2009).

2. 2 Bedrocks
The Sanuki Range in eastern Shikoku and hills tothe north of the MTL in central and eastern Shikoku
are underlain by the Upper Cretaceous Izumi Group).
The Izumi Group consists mostly of interbedded
sandstone and shale, with subordinate felsic tuffs andconglomerates. The Sambagawa metamorphic rocks
are widely distributed in the southern Shikoku Range
and are mainly composed of pelitic schists andgreenschists.
In Shikoku, not only are the intrusive rocks ofrhyolite of the middle Miocene distributed along theIyomishima

Fig.1 Geology and active faults along the median Tectonic Line in Shikoku210th Asian Regional Conference of IAEG (2015)

around the rhyolite intrusive. X-ray diffraction
reveals that greenschists or rhyolites is characterizedby containing more swelling clay mineral of smectite
and a chlorite-smectite mixed-layer mineral.
Smectite and chlorite-smectite mixed-layer mineral
are located mainly near the rhyolite which haveprovided slip surface of landslide (Fig.3).
In the east region of Shikoku (Loc. B at Donariin Fig.1) intrusive rhyolite is not observed along theMTL, however, hydrothermal alteration occur
similarly as observed in the west and central regionsof Shikoku. Unexpected landslides occurred on a cut
slope 5 years earlier. The occurrence of such an
unexpected landslide is considered to be caused by
the smectite-bearing altered sandstone, which is an
expansive clay mineral with weak strength due tohydrothermal alteration, and the presence of fissure
dipping towards the face of the slope (Tamura et al.,2007).

4. Hydrothermal alteration along the MTL
4.1 Hydrothermal alteration of fault gouge along
the MTL
Hasegawa et al. (2001) reported that the faultgouge along the intrusive rock of the MTL contains
tridymite, dolomite and magnesite, as well as
smectite and illite-smectite mixed-layer mineral andreported that those mineral composition had been
produced by hydrothermal alteration. Thishydrothermal alteration also yield fine-grained pyrites
which are the main cause of acid soil after slope
cutting.
4.2 Landslides due to hydrothermal alteration on
cut slopes
Tamura et al. (2007) have reported bedrock
deterioration by hydrothermal alteration on the cut
ground slope along the MTL (Fig.1). The bedrocks ofthe cut slope at Iyomishima is composed mainly ofpelitic schist of the Sanbagawa belt. Rhyolite with awidth of about 2 m, belonging to the Ishizuchi groupof the middle Miocene intrudes into the lowest part ofcut slope (Fig. 2).
On this cut slope, an altered clay zone is observed

Ge

Fig.2 Geology of cut slope at Iyomishima,Ehime prefecture(modified after Tamura et al., 2007a)Fig.3 Rock mass classification and distribution
clay minerals of cut slope at Iyomishima(modified after Tamura et al., 2007a)

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5.

Large-scale landslides along the MTL

Geological investigations have revealed thatlarge-scale landslides along the fault scarps of theMTL have been occurring since early Pleistocene in
Shikoku (Hasegawa, 1991, Hasegawa, 1992,
Hasegawa & Sawada, 1999, 2001). Typical examplesof the Early Pleistocene rock mass slide in Sanuki
Range is the Kirihata Hill rock mass slide situatedalong the Chichio fault (Fig.1, Fig. 4, Fig. 5). It isinferred that big earthquakes due to the fault activity
caused the rock mass slides is the fault scarps in thepast and slid on/in the Quaternary sediments. Theirfindings are as follows:i) Large-scale rock mass slides along active faultsof the MTL have occurred from Early Pleistocene(about 1Ma) to Holocene. The masses are highly
dissected by erosion in case of older formative ages
are older.
ii) The masses have been displaced right-laterally
by active fault of the MTL. More the masses aredissected, more are the displacements by the active
fault. The average displacement rate attains
5-10m/kiloyears at the Ikeda faultand 2-3m/kiloyears
at the Chichio fault (Hasegawa and Sawada, 1999).
iii) The rock slide masses of the Izumi Group are
extremely loose and possess abundant open cracks
with lost cohesion. They have preserved original
stratifications, but some of them resemble debris.
iv) This sliding zone material composed of“clay-matrix brecciated layer (CBL)” indicates thatthe rock mass slid rapidly on this clay-rich brecciatedlayer as lubricant layer. This clay-matrix layers can beinferred to be caused by the hydraulic fracturingv) The Middle Miocene hydrothermal alterationalong the MTL has produced smectite and smectite
bearing mixed-layer minerals in bedrocks as fault
gouges or clay veins and might have become ageological factor to cause the landslides.

Fig. 4 Geological profile along the Median Tectonic
Line in Shikoku (Hasegawa et al., 2006)

Fig. 5 Kirihata hill as an ancient mega-landslide
occurred at 1 Ma along the Chichio fault of theMedian Tectonic Line in Shikoku (Hasegawa, 1991).

Fig. 6 Sliding zone material composed of“clay-matrix brecciated layer (CBL)” (Hasegawa andSawada, 2001)

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6. Rockmass characterization of expressway
tunnels along the MTL

ground is composed of cracked rock mass, butshows small convergence during tunnelling. The third
H-type ground is composed of altered rock mass andshows very large convergence during tunnelling even
if rock mass at face seems to be sound. The fourthHS-type ground is composed of altered and cracked
rock mass and shows very large convergence during
tunnelling.
These four types of tunnel show distinct
characteristics during tunnelling. Especially, it is clearthat the large convergence tends to appear in H-typeand HS-type tunnel ground. X-ray diffraction analysis
suggests that smectite-bearing clay veins originatedfrom hydrothermal alteration and have caused large
convergence during tunnelling. Therefore, it is very
important to confirm the presence of the swelling clay
mineral (smectite etc.) before and during tunnel
construction.

As national highways which connect Tokushima,
Takamatsu and Matsuyama are usually crowded, newexpressways have been constructed by Japan
Highway Public Corporation since 1985. Three
expressways have been constructed in North Shikoku.
They are Tokushima, Takamatsu and Matsuyama
expressways (Fig.7).
Most of three tunnels of three expressways areconstructed in the sedimentary rocks of theCretaceous Izumi Group. Moreover, Tokushima andMatsuyama Expressways has been constructed along
active faults of the Median Tectonic Line (MTL).
These tunnel constructions have revealed thedifficulty in predicting the ground condition before
and during tunnelling, because the convergences
during tunnelling do not correspond to lithology orface observation. Hasegawa et al. (2006) and Yoshidaet al. (2006) have re-examined the tunnelling data andhave clarified the rock mass characteristics of theIzumi Group near the MTL. These papers proposes
the rock mass characterization of the Izumi Group forthe tunnel construction on the base of the damage
history of both rock mass sliding and hydrothermal
alteration (Table 1).
This classification was confirmed by the rockmass grades and convergences during tunnelling (Fig.
8, Fig.9, Fig10). The first N-type ground is composedof sound rock mass and shows very small
convergence during tunnelling. The second S-type①Tokushima Highway ②Takamatsu highway③Kochi Highway ④Matsuyama Highway

Fig.7 Geological map and expressway of Shikoku region (Hasegawa et al., 2006)

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Table 1 Classification of tunnel ground in the Izumi
Group from rock mass slide and hydrothermalalteration (Hasegawa et al., 2006).

7,000
6,000

Hydrothermal alteration

Landslide

H-typeＮ-type×

5,000○Length(m)×D-grade
Ｃ等級
C-grade
Ｄ等級

4,000
3,000
2,000
1,000
0
N-typeNタイプ

S-typeHS-typeN-typeS-type150150050050 100 150 200 250 300 350 400
Convergence(mm)50 100 150 200 250 300 350 400Convergence(mm)1500050 100 150 200 250 300 350 400
Convergence(mm)150

N-typeD-gradeFrequency

Frequency

Frequency

5050050 100 150 200 250 300 350 400
Convergence(mm)HS-typeD-grade100

10050000050 100 150 200 250 300 350 400
Convergence(mm)H-typeD-grade100

500150S-typeD-grade100

05000150

100

Frequency
50

HS-typeC-gradeFrequency

100

Frequency

Frequency
50D150H-typeC-grade100

0HS-type

S-typeC-grade100

C

H-type150
Ｎ-typeC-gradeHSタイプ
HS-typeFig. 8 Length of rock mass grades in each groundtypes in tunnels of Tokushima, Takamatsu andMatsuyama expressways (Hasegawa et al., 2006).

○: affected, ×: unaffectedRock massgrade

Hタイプ
H-typeFrequency○

Sタイプ
S-type50 100 150 200 250 300 350 400
Convergence(mm)00

50 100 150 200 250 300 350 400
Convergence(mm)50 100 150 200 250 300 350 400
Convergence(mm)Fig.9 Convergence of four types of tunnel ground in tunnels of Tokushima, Takamatsu and Matsuyama
expressways (Hasegawa et al., 2006).

N-type350○C-grade400△

D-grade350○○

C-gradeC-grade400△△

D-gradeD-grade350

Convergence(mm)250200150

HS-type○C-grade△D-grade

400
350300Convergence(mm)

300

300
Convergence(mm)H-type250200150

250200150

150

100

10050505050000

Thickness of overburden(m)050 100 150 200 250 300 350 400
Thickness of overburden(m)D-grade200

10050 100 150 200 250 300 350 400

C-grade

△250

1000

○300
Convergence(mm)400

S-type0050 100 150 200 250 300 350 400
Thickness of overburden(m)0 50 10 15 20 25 30 35 40
0 0 of 0overburden(m)0 0 0 0Thickness

Fig. 10 Convergences of four types of tunnel ground to thickness of overburden in tunnels of Tokushima,
Takamatsu and Matsuyama expressways (Hasegawa et al., 2006).

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10th Asian Regional Conference of IAEG (2015)

expressways.
Geotechnical investigations on these landslides
have revealed that smectite-bearing clay-rich zones
are the origin of sliding surfaces. The smectite isinferred to be formed by hydrothermal alteration
which is originated from Middle Miocene volcanism.
This hydrothermal alteration also yield fine-grained
pyrites which are the main cause of acid soil afterslope cutting.
The rockmass types of tunnels composed of theCretaceous Izumi Group is divided into four types
from view-points of landslide and hydrothermal
alteration. Experience from the Median Tectonic Linein Shikoku provides us typical georisks along major
active faults.

7. Acid soils on cut slops
Acid soils on cut slopes were rare in Shikoku, but
after the construction of expressway along the MTL,
poor vegetation on cut slopes have occurred. Typicalacid soil are observed on cut slope along the Minofault where about half of hydrothermally altered soilof interbedded sandstone and shale are less than 4 inpH. Surface of soil are covered by fine-grained
gypsum which suggests sulfuric acid.
The altered soil contains smectites and pyrites.
Observation by SEM have confirmed abundant cubic
pyrites which suggest the origin of hydrothermal
alteration (Fig.11). Although shale of the Izumi
Group contains framboidal pyrites, hydrothermallyoriginated cubic pyrites are inferred to have acted assource material of acid soils.

Acknowledgment
The authors thank to Prof. Atsumasa Okada ofRutsumeikan University, Dr. Yuki Ohno of Shikoku
Electric Power, Dr. Michiharu Ikeda of Shikoku
Research Institute, Dr. Yukinobu Yoshida of West
Nippon Expressway Engineering Kyushu Company,
Dr. Junji Uhicda of West Nippon Expressway
Company, Mr. Tomihiro Sawada of Sawa Soft
Science, Dr. Makoto Yanagida of Haishin Consultants,
Prof. Ranjan Kumar Dahal of Tribhuvan Universityand Ms. Seiko Tsuruta for their support during ourresearch.

framboidal pyrites
cubic pyrites

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Fig.11 Pyrite in altrated rock and altrated fault clay
along MTL
8. Conclusions
This paper reviewed Georisks along active faultsof Median Tectonic Line in Shikoku, Southwest
Japan.
The Median Tectonic Line (MTL) is one of themost predominant and active faults in Japan. The
MTL marks striking contrast in topography between
the mountains and alluvial plains in Shikoku,Southwest Japan. The active faults of the MTL are
inferred to cause magnitude 7 to 8 class earthquakes
on the basis of the fault lengths and the amount ofdisplacement.
These topographic contrast and nearby strong
earthquakes are the primary cause of landslides along
the MTL. Geological investigations have revealed
that large-scale landslides have occurred since Early
Pleistocene along the fault scarps of the MTL in
Shikoku. Recently, many small landslides haveoccurred due to slope cutting and ground excavation
for tunneling during the construction of the7

10th Asian Regional Conference of IAEG (2015)

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