10th Asian Regional Conference of IAEG (2015)

Rain-induced rock avalanches with a sliding surface along an out-of-sequence thrust
Noriyuki ARAI(1), Masahiro CHIGIRA(2)(1) Graduate School of Science, Kyoto University, Japan
E-mail:arai@slope.dpri.kyoto-u.ac.jp(2) Disaster Prevention Research Institute, Kyoto University, Japan
Abstract2011 Typhoon Talas induced a large numbers of rock avalanches in the Kii Peninsula,where is underlain by Jurassic-lower Miocene accretionary complexes. We performed
geological investigation in the Akatani area, where two huge rock avalanches with volumesof 2 million and 8 million m3 occurred, and we found that these two rock avalanches had
their sliding surfaces along a large-scale out-of-sequence thrust (OST) extending more than
5 km. This OST had been exposed on riversides by long-term river incision and overlying
dip slopes had started to deform and finally failed catastrophically, being triggered by theheavy rainstorm. This finding suggests that locating a large-scale OST is essentially
important to predict and interpret potential sites of rock avalanche as well as finding agravitational slope deformation using high-resolution DEMs.

Keywords: rock avalanche, accretionary complex, out-of-sequence thrust

Kimura (1998, 2000) reviewed the out-of-sequencethrust (OST) in Japan and emphasized the importanceof OST in accretionary complexes for landslides.
However, the relationship between rock avalanches
and OST has not been well understood. We
performed thorough geological mapping around hugerock avalanches and examined the relationshipbetween geological structures and two landslides. As
a result, we found that two landslides had theirsliding surfaces along a large-scale OST extending
more than 5 km.

１．Introduction
Recently, extreme weather related to global
warming frequently occurs with many record-setting
rainfall events all over the world, which have induced
rock avalanches. Examples of recent rain-induced
rock avalanches with tens or more than a hundred of
fatalities are a rock avalanche in Philippine Leyte in2006 (Evans et al., 2007; Guthrie et al., 2009), a rockavalanche in Shiaolin village, Taiwan by 2009
Typhoon Morakot (Tsou et al., 2011), and rockavalanches induced by 2011 Typhoon Talas in the Kii
Mountains, Japan (Chigira et al., 2013).
The area of landslide distribution by 2011
Typhoon Talas, which is our study area, was thesame area of 1889 Totsukawa rainstorm disaster,which induced many large rock avalanches. Hirano etal.,(1984) reviewed the Totsukawa disaster, andemphasized that many rock avalanches occurred on
the dip slopes. Murata and Chigira (2000) performed
geological investigation around the two rockavalanche sites (Shiono and Kawarabi-Hinoseyama(Same location with the Akatani-E of this paper))which occurred during the Totsukawa rainstorm
disaster and confirmed that Akatani-E landslide had
occurred on a dip slope but that the slip plane had agentler dip angle than the dips of bedding planes.

Fig.1 Locality map of the study area. The
rectangle refers to the location of studied area.

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10th Asian Regional Conference of IAEG (2015)

Fig.2 Akatani-E landslide

Fig.3 Akatani landslide

The study area is in the catchment of the Kawarabi
River, which is a tributary of the Kumano Riverflowing down to the south in the central part of theKii Mountains (Fig.1). Summit elevations in the
study area are 800 m - 1,000 m, and the riverbed of
the Kawarabi River is at an elevation of
approximately 400 m. In this area, two huge rockavalanches (Akatani-E, Akatani) with volumes of 2
million and 8 million m3 were induced by 2011
Typhoon Talas (Chigira et al., 2013). Figures 2 and 3
show these two rock avalanches we studied and Table
1 shows the numerical data of the two rockavalanches (Chigira et al., 2013).

instead of engineering geological maps. Previous
geologic maps in the study area are thus not reliable
enough to interpret the engineering geologicalcharacteristics of gravitational slope deformations
and landslides.
In order to clarify the geological background ofrock avalanches, we made field survey and weinvestigated the characteristics of faults, such as
attitude, width, material (fault breccia, fault gougeetc.), and sense, and thoroughly traced major faults inthe steep mountains as much as possible. In order todecrease the possibility that we miss the traces ofmajor faults, we made continuous outcrop sketches
on a scale of 1/1000 along the major rivers that crossthe general trend of geological structures. We usedbase maps with a scale of 1:5000 or 1:10000, which
we made from 1-m resolution DEMs obtained by
Nara Prefecture and the Kinki Regional Development
Bureau, Ministry of Land, Infrastructure, Transport
and Tourism.

２．Method
In the study area, several geological surveys had
been carried out and geologic maps were published,but they are not reliable enough for landslide study.
Their surveys were intended to clarify the geologicalhistory rather than to make a geological map withhigh accuracy. Consequently, they were mainly basedon the data along major rivers, and the traces ofgeological features on mountain ridges had not been
well specified. In addition, Crush zones of faults,
which are very important for slope stability, had notbeen well characterized and traced on geologic maps,
because the most important features of faults were
supposed to be displacement of beds and because thephysical characteristics of their crush zones had
secondary meaning in the common geologic maps

３．Result
Three kinds of units are distributed in the study
area (Fig.4). One is the mixed rock of allochthonous
blocks (green rocks and chert etc.) and sandstone
blocks in argillaceous matrix. The second is themixed rock of sandstone blocks in argillaceous
matrix without allochthonous blocks. The third iscoherent sequences and the broken formations. The
strata in the study area belong to the Miyama
Formation in the northern Zone of Shimanto Belt
(Kishu Shimanto Research Group, 1986, Kumon etal., 1988). The broken formations show lenticular
beds and boudinage, divided by small faults.
We found that the beds in the study area were cut
by a large low-angle thrust fault, which we name the
Kawarabi thrust (Fig.4). The Kawarabi thrustgenerally trends from NE-SW to ENE-WSW and dip
to the northwest from 29° to 40°, being traced morethan 5 km. It has a brittle crush zone, of which width
exceeds 1m locally (Fig.4). We defined the width offault as the sum of the width of gouge and fault
breccia and plotted the mean of the minimum and theTable 1
Akatani-E and Akatani landslides
Chigira et al.,(2013)2

10th Asian Regional Conference of IAEG (2015)

Fig.4 Geologic map of the study area. Arrows with numbers 1 and 2 show the position of Fig.6 and Fig.7.
maximum in each outcrop. The fault breccia
generally consists of rock fragments and clayeymaterials. Figures 6 and 7 show the pictures of the
Kawarabi thrust taken at the places shown in Fig.4(arrows 1 and 2). Because the Kawarabi thrust has abrittle crush zone and extends longer than 5 km, it
may be a large-scale out-of-sequence thrust in
accretionary complexes (Kimura, 1998; 2000).
The crush zone of the fault shown in Fig.6 has a
black gouge layer wider than 5cm along the top andbottom of the 1.5-m wide crush zone, and the innerzones of the crush zone is composed of clayey fault
breccia. Along the base of the hanging wall of thefault, seepage water flowed out constantly,
precipitating brown iron hydroxide. This suggeststhat the fault would prohibit groundwater flow andwould cause pore water pressure build up during
rainstorms. At the outcrop of Fig.7, the fault has a
70-cm wide crush zone consisting of clayey breccia
and a few layers of 3-5cm thick black gouge along
the main shear planes. The relationship between themain shear plane and Riedel shear plane (Kano etal.,1991; Onishi and Kimura, 1995) suggested thatthe footwall displaced toward N26°E with 10° plunge.
Similar sense has been obtained from other outcropsof this fault.
Besides the Kawarabi thrust, the study area hasother minor low-angle faults and high-angle faults
trending NW-SE or NE-SW. They have various
widths as is shown in Fig.10: The width of a fault isthe sum of the width of gouge and fault breccia. The
mean of the minimum and the maximum of a fault
width is plotted for 78 outcrops. The Kawarabi thrust,which is not included in this data, had an
extraordinary width of crush zone up to 1.5m and an
average of 50cm. One fault in Fig.10 had a 1.3-m
width crush zone, but it is an NE-SW trending highangle fault.
We found that the Kawarabi thrust occupied the
major parts of the detachment surfaces of the
Akatani-E and Akatani rock avalanches (Fig.2 and 3)
as follows. Just after the landslide in 2011, undulating
slickenside surface exposed in the source area of the
Akatani-E landslide, which surface was confirmed tobe the fault surface of the Kawarabi thrust by thefollowing geological mapping. The 2011 Akatani
landslide, on the other hand, left so large amount ofdebris in the source area that the base detachment
surface could not be observed just after the event. A

Fig.5 A slope map made from the 1-m DEMs
obtained by airborne laser scanner surveys made on
7 and 23 September, 2011 after the 2011 Kii
disaster by Nara Prefecture and the Kinki Regional
Development Bureau, Ministry of Land,
Infrastructure, Transport and Tourism.

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10th Asian Regional Conference of IAEG (2015)

Fig.6 Kawarabi thrust (Arrow 1 in Fig.4). Looking to
the northwest. The scale bar is 70cm long

Fig.7 Kawarabi thrust (Arrow 2 in Fig.4). Looking
to the southwest. The scale bar is 100cm long.

Fig.8 Sketch of Kawarabi thrust at Arrow 2 in Fig.4 and stereographic projection of the plane structures ofthe fault. The arrow indicates the slip vector.

Fig.9 The relationship between the Akatani landslide and the Kawarabi thrust. Modified from Chigira et al.,(2013).
few years later, most of the debris on the detachment
surface had been removed during rainfall events anda slickenside surface exposed at the base of thelandslide scar, which coincided with the trace of the
mapped Kawarabi thrust. Chigira et al., (2013)
estimated that Akatani rock avalanche had occurredunder the wedge failure among the two high angle
faults with NW-SE strike and small fault with E-W
strike and north dip. Our new finding that the Akatani
landslide had its base detachment surface, which was

the Kawarabi thrust, indicate that the detachment
surface was box-shaped in cross section (Fig.9).
Landslide deposit of the Akatani-E landslide had
been washed away by the Kawarabi River to expose
its internal materials, which had a large amount of
black clayey materials. They must be derived fromthe crush zone of the Kawarabi thrust.
４．Discussion

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10th Asian Regional Conference of IAEG (2015)

Fig.10 Histogram of the fault widths
The trace of the Kawarabi thrust enlightens its rolein the formation of the rock avalanches and preceding
gravitational slope deformation. The trace of the
Kawarabi thrust is shown in Fig.5 on a slope image
and geologic cross sections including this fault are
shown in Fig.11. Geological cross sections along
the slope lines of the Akatani (D-D’) and Akatani-E(B-B’) landslides and subparallel nearby lines suggestthat the Kawarabi thrust, which was the detachment
surface of both the landslides, had been almost
exposed at the foots of these two landslides. This isdue to the incision of meandering rivers and suggeststhat the materials on the weak thrust had been
undercut. In addition, the Akatani and Akatani-E
landslides had been preceded by gravitational slope
deformation, which were indicated by small scarps in
the upper parts of the landslides (Chigira et al., 2013).
Furthermore, lower parts of both the landslides had
been failed before the 2011 events. These two slopes
had been thus under unstable conditions. The small
scarps of gravitational slope deformation werelocated near the extention of the Kawarabi thrust,which suggests that the deformation started when thethrust became near the surface of the slope foot byriver incision. Yokoyama, O. et al., (2014)
investigated the Akadani (Akatani of this paper) rockavalanche and reported that the landslide occurredalong the many minor faults trending NE-SW and dip
36°. We infer that these minor faults are branches of
the Kawarabi thrust. Yokoyama, S. et al., (2013)
performed geological investigation around the
Akatani area and reported that the Akatani landslide
occurred along the composite failure planes of
sub-unit thrust and several dip faults. However,comparing their description on the detachment
surface and our observation suggests that theirsub-unit thrust may correspond to the high-angle fault
in this paper.
As is seen in the geologic cross sections along
A-A’ and C-C’ lines where no rock avalanche
occurred, the level of the river has not reached the
Kawarabi thrust there. Along section A-A’, the rockmass of the hanging wall of the thrust mainly

Fig.11 Geologic cross sections. Legend is thesame with that of Fig.4.
composed of sandstone, which is not completely cutby the river. Sandstone has rigidities several times
larger than those of mudstone in the Shimanto belt
and a sandstone bed acts as a competent bed tostabilize a slope.
From the above-mentioned discussion, theKawarabi thrust is interpreted as a major factor
controlling the rock avalanches as well as preceding
gravitational slope deformation. This thrust has acrush zone, which is up to 1.5 m thick and iscomposed of clayey fault breccia with gouge layers.
These fault materials are very weak and impermeable,
so the fracture zone is expected to prevent the
groundwater filtration and build up the pore waterpressure. Geometrically, rock avalanches occurred
where the fault dip downslope and cut by river
incision. These facts strongly suggest that locatinga large-scale low-angle-thrust is essentially importantto predict potential sites of rock avalanches and tointerpret the internal structure of preceding
gravitationally deformed slopes. In addition, thecombination of low-angle thrust faults and high-angle
faults may be a common basic cause of gravitationalslope deformation and catastrophic failure by
rainstorms in mountains of accretionary complexes.
５．Conclusion
2011 Typhoon Talas induced a large numbers of5

10th Asian Regional Conference of IAEG (2015)

rock avalanches in the Kii Peninsula, where isunderlain by Jurassic-lower Miocene accretionary
complexes. We performed geological investigation in
the Akatani area, where two huge rock avalancheswith volumes of 2 million and 8 million m3 occurred,
and we found that these two rock avalanches had
their sliding surfaces along a large-scale low-angle
fault extending more than 5 km. This fault dips
downslope and had been exposed at riversides bylong-term river incision hence its hanging wall beds
became gravitationally destabilized and started todeform and finally failed catastrophically during therainstorm. Our finding suggests that locating alarge-scale low-angle-thrust is essentially importantto predict potential sites of rock avalanches as well asinterpreting the internal structure of gravitationally
deformed slopes. The combination of low-angle
thrust faults dipping downslope and high-angle faults
trending slope lines may be a common basic cause ofgravitational slope deformation and catastrophic
failure in mountains of accretionary complexes.

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Acknowledgment
We thank the Kinki Regional Development Bureau,
Ministry of Land, Infrastructure, Transport andTourism and Nara Prefecture for providing thehigh-resolution DEM data and giving us thepermission to go into the construction sites.
Discussion with Yuki Matsushi in the field and in thelaboratory is highly acknowledged.
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