10th Asian Regional Conference of IAEG (2015)

Geological and geomorphological features of landslides induced by 2011 Typhoon Talasin a granite porphyry area
Yasuto HIRATA (1) and Masahiro CHIGIRA (2)(1) Graduate School of Science, Kyoto University
E-mail:y.hirata@slope.dpri.kyoto-u.ac.jp(2) Disaster Prevention Research Institute, Kyoto University, Japan
Abstract
Typhoon Talas brought heavy rain in the Kii Peninsula, Japan on September 2-5,
2011, causing hundreds of debris avalanches and debris flows in granite porphyry areas in
the southeastern part of the peninsula. We made field investigation to clarify the geologicaland geomorphological background of the landslides, and found that most of the debris
avalanches contained a lot of boulders commonly larger than 1 m in diameter. Their source
materials involved many boulders of granite porphyry, some of which were in a weatheredzone in-situ and others were in debris on nearby sedimentary rocks. These landslides hadvolumes less than 50,000 m3 each and low equivalent frictional coefficients (0.20-0.46),
which are similar to landslides of grus. However, these landslides involved many big
spherical boulders, so that they were much more destructive than those of grus. The
boulders are corestones made by the spheroidal weathering of granite porphyry, which had
well-developed, high-angle columnar joints and sheeting joints near slope surfaces. The
granite porphyry is weathered from the joint surfaces, forming rindlets outside and sphericalcorestones inside. The weathering zones involving corestones form a thick mantle on
low-relief surfaces in higher elevations, which are incised by erosion and mass movements.
The rain-induced debris avalanches occurred near the margins of the thick weathering
mantle and went down to nearby valleys as debris flows.
Keywords: corestone, spheroidal weathering, debris avalanche

1.

weathered granite with micro-sheeting from the casestudy on landslides induced by the rainfall of 250
mm/ 5 hours in Hiroshima on June 1999. Tobe et al.
(2007) found that the landslide densities in granite
area was 293 /km2 and was ten times as large as thatin granodiorite area (13 /km2), even though both theareas had experienced a strong precipitation of about
200 mm/ 5hours in Obara village in 1972 and theysuggested that it results from difference in theweathering profiles of the two rocks.
On the other hand, landslides often occur inweathering mantles of granitic rocks that contains
boulders; for example, during the Uetsu disaster in1967 (Okuda et al., 1968). Debris-flows consisting of
spherical boulders would move rapidly with highinertia, destroying materials on the way, but this typeof landslide has not been studied sufficiently.
We report rain-induced landslides containing
many boulders of granite porphyry in association withthe geological and geomorphological background in
the Kii Peninsula, Japan (Fig. 1). The case study wemade was for hundreds of debris avalanches anddebris flows induced by Typhoon Talas in thesoutheastern part of the peninsula on September 2-5,
2011. Debris flows that included many large boulders

Introduction

Granitic rocks commonly weather deeper than 10
m and weathered granitic rocks are often subjected toinnumerable landslides induced by rainfall. A typical
weathering profile of granite in subtropical climate
consists of decomposed granite, which is also called
grus, with corestones in the lower part and saprolite
in upper part (Ruxton and Berry, 1957). The
corestones are formed by spheroidal weathering(Ollier, 1967). Another type of weathering profile
consists of saprolite in the upper part and a zone of
micro-sheeting joints in the lower part, lying on
bedrock with sheeting joint (Hashikawa andMiyahara, 1974). Sheeting joints, which are
low-angle and widely spaced, strike roughly parallel
to slope contours and dip gently downslope (Chigira,2001, Hamaoki et al., 2007). The micro-sheeting
joints are a several millimeters to centimeters spaced
and parallel to the sheeting joints (Chigira, 2001).
They are also called as laminations (Twidale, 1973).
These macro and micro-sheeting joints are thought tobe made by buttressed expansion mainly due tounloading (Folk and Patton, 1982).
Chigira (2001) pointed out the vulnerability of the1

10th Asian Regional Conference of IAEG (2015)

occurred and destroyed houses and infrastructures. 23
people were killed by the landslides and floods, mostof which occurred in Nachikatsuura Town from 2:30
to 3:00 a.m. on September 4 (Sato et al., 2013).
2.

both in Owase is relatively short (<10 yrs).
3.

MethodWe interpreted aerial photos of on a scale of 1:
20,000 taken in September 7 or 8 by the Ministry ofLand, Infrastructure, Transport and Tourism, Japan(MILT) or GeoEye, Inc., comparing with Google
Earth images taken on April. Then, we plotted all thelandslides on a GIS-based map, drawing polygons forlandslide scars or source area, and for transporting ordepositional areas, separately. Geological boundaries
are basically based on the seamless digital geologicalmap of Japan 1:200,000 (Geological Survey of Japan,2012), Miura (1999), and Kawakami and Hoshi(2007). Rainfall distribution maps were made from1-km mesh hourly precipitation data ofradar-AMeDAS provided by MILT. For the statisticalanalysis of landslides, we used 10m-mesh DEMs
supplied by MILT. Landslide densities are calculated
for each cell as the number of landslide sources in a
1-km2 circle with the center on that cell.
We made field investigations mainly in
Nachikatsuura Town, measured dimensions of sourceareas by a laser distance measuring instrument, andobserved landslide materials and weathering grade.
The weathering grades of granite porphyry areclassified into fresh, slightly weathered, moderately
weathered and highly weathered. Slightly weatheredrock is intact but it is discolored on its surface part.
Moderately weathered rock has a wider area of rockthan a grus in an outcrop. When an area of grus islarger than rock, it is classified as highly weatheredrock.

Geological and climatic setting

The southeast part of the Kii Peninsula isunderlain by the Miocene Kumano Acidic Rocks(KARs) on higher elevations and by the Miocene
Kumano group or Cretaceous Shimanto Supergroup(both sedimentary rocks) on lower elevations (Fig. 1).
KARs, which spreads in an area of 600 km2, intruded
into or partially covered the sedimentary rocks(Aramaki, 1965), and formed a caldera (Miura, 1999).
KAR is now separated into the northern mass and thesouthern mass. Granite porphyry occupies 80% of
KARs.
The stricken area is one of the regions that have alarge amount of rainfall (2500 mm/yr) in Japan, butlittle damage had been caused by landslides for over
several decades. Sato et al. (2013) reported there wasonly one disaster with debris-flows that damaged tohouses in Nachikatuura Town from 1944 to 2011.
However according to Seta et al. (1983), there were
debris-flow disasters in 1693, 1846 and 1929. The
debris-flows contained large boulders up to 2-20 m,
killing 22 people in total. In Shigu, one debris-flow
occurred with three fatalities in October 1997.
Wakatsuki et al. (2014) calculated return periods ofrainfall during the 2011 event and showed that thereturn period for 1 hour rainfall is 100-160 yrs andthat for 6 hours rainfall is 240-330 yrs in
Nachikatsuura and Shingu, but the return period for

4.

Result4.1 Weathering of granite porphyry
Granite porphyry has columnar joints andsheeting joints. Columnar joints are almost verticaland each column has 1-6 m diameters. On large
outcrops like a quarry we can observe that the rock
body of granite porphyry is intersected by low-angle
sheeting joints, which are subparallel to the slopes
near the ground surface. The sheeting joints are
undulating and anastomosing, and essentially do notdisplace columnar joints except for loosened rockmasses (Fig. 2). The intervals of sheeting joints are
2-5 m and tend to be smaller toward slope surfaces.
Dips of sheeting joints were mainly 20-30° with anaverage of 26° from the data obtained at 21 locations.
Spheroidal weathering is a common weathering
scheme for the granite porphyry in the study area. A
block of granite porphyry is weathered from the joint
surfaces to form a spheroidal corestone surrounded byrindlets (Photo. 1). The surface part of a corestone isa brown layer surrounding the inner whitish-grayFig. 1 Geological and topological map in thesoutheastern part of the Kii Peninsula.

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Fig. 2 Sketch of excavation walls of granite porphyry. Most of the narrow lines seem like sheeting joints.
portion. Rindlets are separated from the corestone.
They are also called as shells or scales (Ollier, 1967).
When we closely look at the rindlets, we know thatthey are not like a spherical shell but a contact lens.
The study area has low-relief surfaces on top of
the mountains, where we observe thick highly-moderately weathered rock up to 100 m of depth(Fig. 3). These weathering mantles have many
spheroidal corestones embedded in grus. These
low-relief surfaces may be paleo-surfaces, below
which granite porphyry has been weathered for a longtime without denudation.

rainfall for the 5 hours was 200-470 mm in granite
porphyryarea.
The
precedingcumulative
precipitation before the intense rainfall was 400-800
mm for 4 days.
Landslides occurred at high ratio in granite
porphyry area where 5-hour cumulative rainfall
exceeded 250 mm. Landslide densities were 1-4 /km2
in the local areas of 330 km2 (Fig. 5). The maximum
density of landslides was about 18 /km2 in the
Nachigawa watershed. The mean slope of sourceareas in granite porphyry area was 33° (1σ ± 7°), on
the other hand, the mean slope of the local areas ingranite porphyry was 31° (1σ ± 10°).

4.2 Distribution of landslides
Overlaying the geological map with the landslide
distribution suggests that most of the landslides are
related with granite porphyry in the survey area (Fig.
4). 623 scars are plotted on granite porphyry area; 54
are on its surrounding slopes on sedimentary rocks;
22 are on the other acidic rocks; and 11 are on
sedimentary rocks far from granite porphyry area.
During the heavy rainfalls from 11 p.m. on
September 3 to 4 a.m. on September 4, the maximum
hourly rainfall was over 80 mm and cumulative

4.3 Feature of landslides in and around the granite
porphyry
In the field survey, we found that large boulders(> 1 m in diameter) of granite porphyry were
included in the deposits of most landslides, whose
sources were located in the weathered granite
porphyry area or along the boundary between granite
porphyry and sedimentary rock (Fig. 6).
(1) Debris avalanches of weathered granite
porphyry
Within granite porphyry area, some debris-flows
including the boulders moved downward over 1 km
length. One of the largest debris avalanches andfollowing debris flows ran 2.4 km and resulted incatastrophic damage to the Iseki area in
Nachikatsuura Town (Fig. 6a). Its source consisted of
two adjacent planar scars. The west portion has ahorizontal length of 90 m, an average width of 50 m
and an average depth of 10 m. The east portion is 50
m long, 35 m wide and 5 m deep on average. Their
total volume was estimated to be 50,000 m3. The
landslide source scars had spheroidally weatheredcorestones in the upper part and slightly weathered
granite porphyry with columnar joints in the low part.
This landslide may have occurred by the sliding ofrock columns along the axis of the columns. Where
the debris flow went out to a wide valley bottom at
Iseki (Fig. 6), the debris flows spread over the full
width of a few tens of meters and deposited the

Photo. 1 Spheroidal weathering of granite porphyry.

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10th Asian Regional Conference of IAEG (2015)

Fig. 3 Weathering mantle in granite porphyry. (a) Plane view. (b) Cross section along the profile
line.
boulders with the maximum diameter of 5 m. On top
layer, granitic sand and fine driftwood was seen to fillof the original ground surface with angular gravel
the space between spherical boulders with diametersof 0.5-3 m. The size of the granite porphyry boulders
became smaller than 1.5 m further downstream withoccasionally big boulders up to 3 m of diameter.
There were shallow small landslide scars where
corestones failed downslope near the edge of gentle
ridges with thick weathering mantles. One example isshown in Fig. 6b; it had a horizontal length of 20 m,
an average width of 20 m and an average depth of 0.5

Fig. 5 Landslide densities and total local areas ingranite porphyry as a function of the 5 hour
cumulative rainfall.

Fig. 4 Distribution of source areas and rainfall
cumulated for 5 hours during this 2011 event.

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m (Fig. 7b). The lower part of the scar showed rockcolumns with discoloration. In the upper part,corestones embedded in grus, where was the main
portion of the landslide.
Rock avalanches of granite porphyry with sliding
along sheeting joints occurred there (Photo 2). This
landslide slope consisted of moderately to highly
weathered granite porphyry in the higher portion andslightly weathered in the middle to lower. Rockblocks separated by the columnar and sheeting joints
were observed to have rotated in a slump manner on
the side scarp of the landslide, which suggests that
displacement along the sheeting joints had alreadyoccurred before the 2011 event.

2011 landslide crown and that trees on the slopes
above this old scarp had been bent, which suggeststhat slow slope movement had been continuing before
the 2011 landslide.
On the slopes surrounding granite porphyry body,debris of granite porphyry is usually lying on darkgray clayey materials, in which we found visible
pyrite. These clayey materials are probably due to thehydrothermal alteration caused by the intrusion of the
granite porphyry into the shale of the Mitsuno
Formation, the top of the Kumano Group.
4.4 Mobility of debris-flows
To evaluate the mobility of landslides, equivalentfrictional coefficient (H/L) was calculated from therelative height (H) and horizontal distance (L)between the crown of a scar and the tip ofdepositional area interpreted from the aerial photos.
The equivalent friction coefficients of all 75 debris
flows which moved over 500 m are estimated to be
0.34 ± 0.08 on the topographical map. We also
estimated volumes of some debris-avalanches to befrom 102 to 5 × 104 m3 in the field, and theirequivalent friction coefficients were 0.20-0.46 (Fig.
8).

(2) Debris avalanches of talus deposit
Landslides of thin talus deposit occurred near theboundary between granite porphyry and the KumanoGroup. For example, landslides in the south of Mt.
Myoho were shallow and tabular in shape (Fig. 6c).
Figure 7c shows this type of landslide, of whichsource area had a horizontal length of 20 m, anaverage width 10 m and an average depth 1 m. Debris
slid down on mudstone, which was altered to clay andvery weak. We found a small old scarp above theFig. 6 Landslide distribution on a geological map in Nachikatsuura Town. (a)-(e) indicates the landslide locationin Fig. 7. Bedding plains plotted in blue and geological boundaries were modified from Mizuno (1957).

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Fig. 7 Photos and cross sections of typical source areas in Nachikatsuura Town in Fig. 6. (a) and (b) Landslides
of in-situ weathered granite porphyry. (c) and (d) Those of talus deposit. (e) landslide on a boundary between
granite porphyry and sedimentary rocks.
5.

of the Kii Peninsula strongly suggests that they areaffected by bedrock geology in a geological settingthat granite porphyry with intensive columnar joints
intruded into sedimentary rocks. Landslides occurredin a concentrated manner on the granite porphyryslopes and on sedimentary rock slopes near thecontact with the granite porphyry.
The granite porphyry in the study area has
well-developed columnar joints and has sheeting
joints near the slope surfaces, which joints gave thelandslide susceptible features to this granite porphyry.
Sheeting joints are well known in granite rock body
(Ollier, 1965), but probably not known from granitic
rocks with intense columnar joints. However, wefound that sheeting joints are well developed in thisrock type and several rockslides occurred withdetachment surfaces along sheeting joints. This typeof slide could be triggered by earthquakes as well as
by rainfall. Aizawa et al. (2009) reported that large
rock-falls in this granite porphyry were caused not
only by heavy rainfall but by small earthquakes (anintensity of 4 on the Japanese seven-stage seismic
scale) or even in normal period.
Chemical weathering from the surfaces of theseDiscussion

Shallow landslides have been commonly believedthat their occurrence sites are not controlled bybedrock geology except for special bedrock types likeunwelded ignimbrite (Yokota and Iwamatsu, 2000) or
deeply weathered massive granite (Chigira, 2001),
but the experience of the 2011 rainstorm in the south

Photo. 2 Right side wall of a landslide scar. (a)Middle-highly weathered rock mass with sheeting
joints. The depth of the scar is about 9 m. (b) Close
image of the square b.

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during heavy rainstorms. This is probably the reason
why landslides occurred intensively on sedimentaryrocks near the igneous contacts.
The mobility of landslides of the study site iscompared with that of landslides induced by the 1999
Hiroshima rainstorm, which resulted in a similar
mobility as follows. The landslide mobility iscommonly represented by H/L, which is known as anequivalent coefficient of friction. When comparing
the coefficients of the landslides in the study site andthose of weathered granite during 1999 Hiroshima
rainstorm disaster (Kaihori, 2001), they are plotted ona similar trend (Fig. 8). In the case of weathered
granite in Hiroshima, however, corestones wererarely made and microsheeting developed instead(Chigira, 2001). On the other hand, the landslides inour study site involved many big spherical boulders,which probably had higher kinetic energy in motion
and struck objects strongly, so that they were much
more destructive than those of grus. Round shape ofrocks could have led the landslides to high mobility.

Fig. 8 Plots of volume vs equivalent frictionalcoefficient (H/L) of landslides.
two types of joints forms unstable structures onslopes. It forms large corestones embedded in grus,
leaving a thick weathering mantle on higher
elevations. This weathering profile is not stable on
slopes and landslides were induced by heavy rainfall
as much as 300 mm for 5 hours. Steep riverside
slopes, on the other hand, consist of weakly
weathered or fresh rocks and are stable and few
landslides occurred there.
Landslide densities in the granite porphyry areaof this study were up to 18 /km2 in the area with
rainfall of 350-400 mm for 5 hours and were muchless than the average landslide density of 293 /km2 in
granite areas with the rainfall of about 200 mm/5h of
the 1975 Nishimikawa rainstorm disaster (Tobe et al.,2007). This difference may be due to the difference inweathering profiles between these two rock types.
Typical weathering profile in our study site wascorestones embedded on grus and underlying rockcolumns, which are sheeted. The combination ofthese two joints fosters loosening of rocks, which
likely drains groundwater in the bedrock. On theother hand, the materials of landslides induced by the
Nishimikawa rainstorm were loosened surface soil
layer on much intact weathered granite; thepermeability contrast must be much larger than thecase of our study area.
Landslides that occurred on sedimentary rocks
near the contact with the granite porphyry are also
controlled by the geological history, which is ahydrothermal alteration during the igneous intrusion,and the resultant rock properties. The sedimentaryrocks near the contact with the granite porphyry in the
study area are generally hydrothermally altered to
become clayey materials, which is impervious andwouldprohibitgroundwaterflow.
Geomorphologically, the slope surfaces of granite
porphyry are generally higher than the juxtaposing
slopes of sedimentary rocks, on which boulders of
granite porphyry have accumulated. Debris are thus
lying on an impervious bed, which is preferable thatpore water pressure in the debris would increase

6.

Conclusions

In the southeastern part of the Kii Peninsula, over
700 of debris avalanches and following debris flows
were triggered in and near the granite porphyry areaby an intense rainfall of about 250 mm in 5 hours in
2011. The materials of the landslides included manylarge spherical boulders of granite porphyry, of whichshape probably enhanced the mobility duringtransportation. These spherical boulders are mostly
corestones made by the spheroidal weathering of
granite porphyry, which had well-developed
columnar joints and sheeting joints near groundsurface. The granite porphyry is weathered from thesejoints to make a corestone inside. Corestones arewidely distributed below low-relief surfaces in higher
elevations, where weathering must have continued fora long time. Landslide volumes were not so large (102
- 5 × 104 m3) and their equivalent friction coefficientswere 0.20-0.46, which is similar to those of debris
flows of grus. However, considering the inertia ofdebris flows suggests that the debris flows with large
corestones must be much higher than those of grus.
Weathering schemes of granitic rocks thus control thedestructive power of landslide-debris flows of
weathered granite.
The geological features of granite porphyry alsocharacterize density and distribution of landslides.
Landslide density in granite porphyry area was 1-4/km2 in the intense rainfall zone and 18 /km2 at themost. It is much sparser than a granite case which wasgiven less rainfall, probably owing to presence ofvertical joints. In addition, some sheeting joints
played a role of detachment surface of rockslide.
Landslides occurred on the Kumano Group near the7

10th Asian Regional Conference of IAEG (2015)

boundary with granite porphyry were mostly debris
slide on clayey materials made by hydrothermal
alteration, which occurred during the intrusion of
granite porphyry. Clayey materials prohibit
groundwater flow and may cause a buildup of porewater pressure to induce landslide.

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Acknowledgements
We are grateful to the Nachikatsuura Forestry
Corporative for giving us the permission to make an
investigation along their logging roads. We thank
AERO Asahi Corporation for providing us 1-m mesh
DEM, which was useful to distinguish low-relief
surface clearly.
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