10th Asian Regional Conference of IAEG (2015)

Validation of topographical and geological conditions involved in shallow landslide using
statistic methods
Yuichiro NISHIKANE(1), Takeshi KAWAGOE(1), Takuya URAKOSHI(1)and Tomokazu ISHIHARA (1)(1) Railway Technical Research InstituteE-mail:Nishikane.yuichiro.48@rtri.or.jp
Abstract
It has been reported in the previous studies that topographical and geologicalconditions are involved in the shallow landslide. Some of the conditions have been
explained qualitatively and the applicability of them has not been shown statistically.
Therefore, the authors conducted statistical surveys using topographical and geological datain order to validate the applicability of the topographical and geological conditions todetection of unstable slopes reported in the previous studies. First, the authors reviewed the
topographical and geological conditions involved in shallow landslides reported in theprevious studies. Secondly, the authors conducted field observations in Chuetsu, Niigata
Pref. and Hofu, Yamaguchi Pref., Japan where a number of shallow landslides recently
occurred. In both the areas, the authors checked whether or not each landslide site andnon-landslide site satisfied the topographical and geological conditions. Additionally, theauthors obtained topographical and geological data in the study areas by map interpretation,
and applied Hayashi’s quantification method II to the obtained data. As a result of theanalysis, some of the conditions were confirmed to contribute to discrimination between thelandslide sites and the non-landslide sites. Finally, the following conditions were confirmed
to be involved in shallow landslides in the study areas: 1) convex break of slope, 2) valley
slope, 3) angle of over 40 degrees and 4) spring.

Keywords: shallow landslide, topographical and geological condition, field observations, mapinterpretation, Hayashi’s quantification method II

1.

Introduction

2.

The occurrence of the shallow landslide isaffected by topographical and geological conditions.
Therefore, these conditions are important clues todetecting unstable slopes susceptible to shallow
landslides. Various conditions involved in shallow
landslide have been reported in the literatures basedon the analysis of the past slope disasters (e.g.,
Suzuki, 1997). Some of the conditions have been
explained qualitatively, and the applicability of themhas not been shown statistically. Therefore, in thisstudy, the authors conducted statistical surveys using
topographical and geological data in order to assess
the conditions involved in the shallow landslide.
These data were obtained by means of fieldobservation and map interpretation in the study areas
where a number of shallow landslides recently
occurred.

Review of the previous studies

The authors reviewed the topographical andgeological conditions involved in shallow landslides
reported in the previous studies. These conditions
were categorized in the view of the following two
points (Table 1).
[1] The acquisition method of topographical andgeological data
- The conditions are classified into two
categories based on the acquisition method of
topographical and geological data; one is mainly
obtained by map interpretation, and the other isobtained by field observation. Some of the conditions
are classified into both the categories.
[2] The relationship between shallow landslide andthe conditions- The conditions are classified into three groups,
namely “factor”, “sign” and “result” based on therelationship with shallow landslide. The conditions

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10th Asian Regional Conference of IAEG (2015)
Table. 1 Topographical and geological conditions
involved in the shallow landslide
Area

Topographical andgeological conditions

Angle of over 30 degrees
Concave slope
Valley slope
Whole slope
Outfacing
Weathered soil
Tree lean
Convex break of slope
Steep rock slope
Source area Spring
Crack on slope
Soil creep
Fan-like gentle slope
Moving area beneath a steep escarpment/Settled area Mound beneath a steep
escarpment(a) The acquisition method
Reference

1
2
2
3
3
4
2
5
3, 6
6
4

Map
interpretation
○
○
○

○
○

Field
observation
○
○
○
○
○
○
○
○
○
○
○

(b)The relationship withshallow landslide
Factor

Sign

○
○
○
○
○

Result
○
○

○
○
○
○

○
○
○

5

○

○

5

○

○

1: Yasue and Okubo (1974); 2: Hatano (1974); 3: Chigira (2002); 4: Sasaki et al. (1994); 5: Suzuki (2000); 6: Kato
(2008)

which are susceptible to cause the occurrence ofshallow landslide are classified as “factors”. The ones
which are recognized before the occurrence ofshallow landslide are classified as “signs”. The ones
which are recognized after the occurrence of shallow
landslide are classified as “results”.
3.

*The shown areas are extracted from the study areas.

Fig. 1 The selected
non-landslide sites

Study areas

The study areas having a size of 5 km × 5 km
each are located in Chuetsu, Niigata Pref. and in Hofu,
Yamaguchi Pref., respectively in Japan. In Chuetsu, aseries of earthquakes including three large ones
whose magnitude were more than 6.2 on the Japanese
Meteorological Agency scale occurred on 23 October
2004 (Sidle et al., 2005). The earthquakes triggered anumber of shallow landslides in Chuetsu (Yagi, 2005).
Additionally, this area had experienced heavy
rainfalls before the earthquake in 17 July 2004. The
heavy rainfalls also triggered some shallow landslides
in Chuetsu (Iwahashi et al., 2008).
In Hofu, heavy rainfalls observed on 21 July 2009.
The daily rainfall in Hofu reached 275 mm. The
heavy rain induced a number of shallow landslides
and mud flows (Fukuoka et al., 2009; Wakatsuki et al,2010).
The study area in Chuetsu is located in a hillregion with the altitude of 300-350m, and mostly
consists of alternate layers of Miocene-Pliocene
sandstones and mudstones. The study area in Hofu islocated in a mountain region with the altitude of250-400m, and mostly consists of late Cretaceous
granitic rocks.
4.

landslide

sites

and

order to compare the topographical and geologicalsettings between the landslide sites and thenon-landslide sites. The non-landslide sites wereselected by generating random numbers on thecoordinates on maps of the study areas excluding
areas of landslides, pond and artifact. 400
non-landslide sites were randomly selected in eachstudy area. The locations of the landslide sites and thenon-landslide sites in a part of the study area areshown in Fig. 1.
5.

Field observation

5.1 Methods
The authors conducted the field observation andchecked whether or not each landslide site andnon-landslide site in both the study areas satisfied the
topographical and geological conditions reviewed in
Chapter 2.
5.2 Results
The result of the check of the conditions is shownin Table 2. The check was conducted at 44 landslide
sites and 36 non-landslide sites in Chuetsu, and 5
landslide sites and 2 non-landslide sites in Hofu
(Photo. 1). Because of the situations of vegetation andlocation, the authors could not check whether
respective sites satisfied all the topographical andgeological conditions. The “satisfaction ratio” in the
Table 2 is the ratio of sites where each condition was

Selection of the landslide sites and thenon-landslide sites

Landslide sites in the study areas were selected byinterpreting aerial photos taken before and after thedisasters. The number of the selected landslide sites
was 106 in Chuetsu and 193 in Hofu.
Additionally, non-landslide sites were selected in2

10th Asian Regional Conference of IAEG (2015)

Table. 2 Result of the filed observation in Chuetsu and in Hofu
Area

Topographical and geological conditions

Angle of over 30 degrees
Concave slope
Valley slope
Tree lean
Convex break of slope
Steep rock slope
Source area Spring
Crack on slope
Soil creep
Moving area/ Fan-like gentle slope beneath a steep escarpment
Settled area Mound beneath a steep escarpment
Whole slope

Checked
Chuetsu Hofu
44
5
25
4
40
5
8
4
42
4
11
0
22
4
5
4
2
3
23
5
21
5

Landslide site
Recognized
Satisfaction
Chuetsu Hofu
ratio
43
3
93.9
3
0
10.3
20
5
55.6
5
0
41.7
37
3
87.0
4
0
36.4
10
1
42.3
0
0
0.0
1
0
20.0
10
3
46.4
1
0
3.8

Non-landslide site
CheckedRecognized
Satisfaction P-value*
Chuetsu Hofu Chuetsu Hofu
ratio
36
2
22
2
63.2 < 0.001
34
2
6
0
16.7
0.72
34
2
7
1
22.2 < 0.001
17
2
9
0
47.4
0.4
35
2
8
1
24.3 < 0.001
35
2
4
0
10.8
0.068
15
1
1
0
6.3
0.015
9
0
0
0
0.0
1
9
0
1
0
11.1
1
25
2
1
0
3.7 < 0.001
26
2
0
0
0.0
1

* P-value was calculated by the Fisher's exact test (two-sided test).

satisfied, defined by Equation (1). It was calculated
both in the landslide site and the non-landslide site.
R = 100 Nsatisfied / Nchecked

Topographical and geological information ofslopes where the landslide sites and the non-landslide
sites located was acquired using 1:25,000
topographical maps and published geological maps.
The topographical information of the landslide site
was interpreted using topographic maps drawn before
the landslides occurred. The items of the
topographical information were described below
based on the topographical classification reported byOnda et al. (1996) and Suzuki (2000).
[1] Vertical cross-section shape of slope (Fig. 2(a))[2] Horizontal cross-section shape of slope (Fig. 2(b))[3] Slope angle[4] Location on vertical cross-section of slope (Fig.
2(c))
The items of geological information interpretedusing 1:50,000 geological maps were the followings.
[5] Rock facies[6] Distance from a syncline axis or an anticlinal axis
[7] Relationship between the direction of dip of slope
and that of geological structure (Fig. 3)
The information on topography and geology ofthe landslide site was obtained from the top oflandslide. The items of geological information [6] and[7] were not obtained in Hofu because information on
the geological structure such as the direction of
bedding plane and the fold structure could not(1)

where R is the “satisfaction ratio” (%) ; Nsatisfied, thenumber of sites where each condition was satisfied;Nchecked, the number of sites where the satisfaction of
the condition could be checked.
The authors carried out the Fisher's exact test to
examine the significant difference between thesatisfaction ratio of the landslide site and that of the
non-landslide site. The Fisher's exact test (Fisher,1935) is a statistical significance test used in theanalysis of contingency tables, and is valid for all
sample size. As a result of the test, the significant
difference at 5 % significant level was confirmed in
the satisfaction ratios of the following conditions: [1]angle of over 30 degrees, [2] valley slope, [3] convex
break of slope, [4] spring and [5] fan-like gentle slope
beneath a steep escarpment.
6.

Mapinterpretation
quantification method II

andHayashi’s

6.1 Acquisition of topographical and geologicalinformation(a) Chuetsu(b) Hofu (valley wall)
Photo. 1 Shallow landslides in Chuetsu and in Hofu

3(c) Hofu (near the crest)10th Asian Regional Conference of IAEG (2015)(a) Category score in Chuetu(a) Vertical cross-section shape of slope

Convex

Rectilinear

Topographical and geological condition
Item (explanatory variable)
Category
Convex
Vertical cross-section shape
Rectilinearof slope
Concave
Ridge
Horizontal cross-section
Straight
shape of slope
Valleyθ < 2020 ≤ θ < 30
Slope angle θ (°)
30 ≤ θ < 4040 ≤ θ
Crest
Location in vertical direction
Upper slopeof slope
Lower slope
L < 300
Distance from a syncline axis
300 ≤ L < 600
L (m)600 ≤ L
Orthogonal
Relationship between the
direction of dip of slope andDiagonalthat of geological structure
ParallelConcave(b) Horizontal cross-section shape of slope

Ridge

Straight

Valley*Arrow shows a direction of water flow.

-1

Category score-0.5 00.5

1-1Category score-0.5 00.5

1

83/ 10222/ 215
1/ 83
21/ 7448/ 260
37/ 66
3/ 60
8/ 133
26/ 11369/ 94
3/ 54
87/ 235
16/ 11137/ 7723/ 7946/ 244
46/ 11732/ 113
28/ 170(b) Category score in Hofu
Topographical and geological condition
Item (explanatory variable)
Category
Convex
Vertical cross-section shape
Rectilinearof slope
Concave
Ridge
Horizontal cross-section
Straight
shape of slope
ValleyCrest
Location in vertical direction
Upper slopeof slope
Lower slope(c) Location on vertical cross-section of slope
Crest
Upper slope
Convex break of slope

Lower slope
Vertical cross-section

Fig. 2 The classification of topographicalinformation

Number*75/ 8684/ 218
34/ 96
4/ 11854/ 167
135/ 115
7/ 64
146/ 200
40/ 136* Number of landslide sites/ non-landslide sites

Fig. 4 Result of Hayashi’s quantification method
II

Direction of dipof slopeθNumber*variable and an explanatory variable by Kan (1993).
As the procedure of Hayashi’s quantification method
II, the authors followed Kan (1993), and Kan andFujikoshi (2011).

0≦θ< 30： Parallel30≦θ< 60： Diagonal60≦θ≦90： Orthogonal

Direction of dip ofgeological structure

6.3 Results of Hayashi’s quantification method II

Fig. 3 Relationship between the direction of dipof slope and that of geological structure

As a result of the selection of the explanatory
variables, “rock facies” and “distance from ananticlinal axis” in Chuetsu and “rock facies” and“slope angle” in Hofu were excluded. That is because
they did not satisfy the criterion of the correlation.
The result of the analysis with the exception of these
explanatory variables was shown in Fig. 4. The
category score shows the magnitude of thecontribution to the discrimination between thelandslide site and the non-landslide site. Categories
whose category score are positive are estimated tocontribute to landslide sites. In Chuetsu, “convex” inthe vertical cross-section shape of slope, “valley” inthe horizontal cross-section shape of slope and “over40 degrees” in the slope angle showed large
contribution to landslide sites. In Hofu, “convex” inthe vertical cross-section shape of slope and “valley”
in the horizontal cross-section shape of slope showedlarge contribution to landslide sites. The result ofanalysis in Chuetsu showed 84.6% classification
accuracy and that in Hofu showed 74.0%
classification accuracy. The classification accuracy

interpreted in the geological map.
6.2 Method of Hayashi’s quantification method II
The contribution of the condition todiscrimination between the landslide sites and thenon-landslide sites was analyzed by Hayashi’squantification method II (Hayashi, 1954) using theinterpreted topographical and geological data.
Hayashi’s quantification method II is a method ofmultivariate discrimination analysis to manipulatequalitative data as explanatory variables. In this study,
the presence or absence of landslide (landslide site or
non-landslide site) was used as the objective variable,
and the items of topographical and geologicalinformation were used as the explanatory variables.
Before the analysis, the explanatory variables wereselected based on the criterion of correlation amongexplanatory variables or that between an objective

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10th Asian Regional Conference of IAEG (2015)

valley slope
convex break of slope

springangle of over 40 degrees
Fig. 6 Topographical and geological conditions
which contribute largely to shallow landslide

Fig. 5 Location of landslide sites andtopographical conditions which contributelargely to shallow landslide in Chuetsu

distributed near the crests. Weathered granitic
bedrock was distributed in the lower slopes, anddecomposed granite soil was distributed near the
crests. It is considered that the reason why “slope
angle” was excluded from the explanatory variables
applied to the analysis is because a lot of shallow
landslides occurred at not only steep slopes but also
shelving slopes in Hofu.
Among the topographical conditions reported in
the previous studies, some conditions, which showed
the significant difference in the satisfaction ratios(Section 5.2), were confirmed to be involved inshallow landslide. Among the selected conditions,“valley slope” and “convex break of slope” were also
confirmed to contribute largely to shallow landslides
by Hayashi’s quantification method II in Chuetsu andin Hofu. Therefore, both the conditions would belargely involved in shallow landslide and important
clues to detecting unstable slopes.
“Angle of over 30 degrees” was picked out as thecondition involved in shallow landslide according to
the result of the review of the previous studies.
Furthermore, the slope angle which would contributeto shallow landslides in Chuetsu was over 40 degrees
as the result of Hayashi’s quantification method II.
These results verify that the angle of over 30 degrees,
especially over 40 degrees, is a condition involved inshallow landslide.
The conditions of “spring” and “fan-like gentle
slope beneath a steep escarpment” are also confirmed
to be involved in shallow landslide. The former
shows the “sign” of the occurrence of shallow
landslide, and the latter shows the “result” of that
(Table 1). Conditions which show the “sign” of theoccurrence of shallow landslide is important todetecting unstable slopes. Therefore, the springshould be checked in the field observation for
detecting unstable slopes.

was defined as the percentage of the number of sites
correctly classified to the total number. According to
Kan and Fujikoshi (2011), classification accuracy ofover 75% shows high accuracy. The classification
accuracy in Chuetsu was higher than the criterion
value and that in Hofu was near it.
7.

Discussions

The selection of the explanatory variable before
Hayashi’s quantification method II in both the areas
was thought to be reasonable, judging from theclassification accuracies. As a result of the analysis,
the conditions of “convex” in the verticalcross-section shape of slope and “valley” in thehorizontal cross-section shape of slope wereconfirmed to contribute largely to shallow landslides
both in Chuetsu and in Hofu. In Chuetsu, thecondition of “over 40 degrees” in the slope angle wasalso confirmed to contribute largely to shallow
landslides.
The condition of “convex” in the verticalcross-section shape of slope corresponded to theconvex break of slope in the field observation. The
positional relation between landside sites and the
topographical conditions which contribute largely toshallow landslides in Chuetsu is shown in Fig. 5. This
figure shows that most landslide sites locates atslopes with these conditions.
“Slope angle” was confirmed to contribute to thediscrimination between the landslide site and thenon-landslide site only in Chuetsu. In Hofu, a lot ofshallow landslides occurred at the lower slopes(valley walls) whose angles are around 40 degrees(Photo. 1(b)) and at gentle slopes near crests (Photo.
1(c)). Weathered granitic bedrock was distributed inlower slopes, and decomposed granitic soil was

8.

5Conclusions

10th Asian Regional Conference of IAEG (2015)
Hayashi, C. (1954): Multidimensional Quantification
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3,pp. 165-169.
Iwahashi, J., Yamagishi, H., Kamiya, I., Sato, H.
(2008): Discriminant analysis for landslides causedby the 2004 Niigata heavy rainfall in July and the
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Kan, T. (1993): Practice of multiple classification
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Kan, T. and Fujikoshi, Y. (2011): Discrimination
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Kato, N., Kikui, T., Miyase, M., Sakatani, Y., Nishi,
H. (2008): A risk assessment system of
sediment-related disaster using premonitory
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Society of Erosion Control Engineering, pp.11-19.
Onda, Y., Okunishi, K., Iida, T., Tsujimura, M. (Eds.)(1996): Hydrogeomorphology: The interaction ofhydrologic and geologic process, Kokon-shoin, pp.
177-189.
Sasaki, Y., Fujii, A., Asai, K., Nakamura, Y. (1994):
Mass Movement Process of Valley-Head Slope
Buried with Loam, Journal of the Japan Society ofEngineering Geology, Vol.35, No. 5, pp.27-39.
Sidle, R.C., Kamai, T., Trandafir, A.C. (2005):
Evaluation landslide damage during the 2004
Chuetsu earthquake, Niigata, Japan, Eos,Transactions American Geophysical Union, Vol. 84,
No. 13, pp. 133-136.
Suzuki, T. (1997): Introduction to map reading forcivil engineers, Volume 1 Geomorphological basis
for map reading, Kokon-shoin, 200 p.
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Yagi, H., Yamasaki, T., Iwamori, T., Atsumi, K.
(2005): Landslides triggered by the mid Niigata
prefecture earthquake in 2004 and theircharacteristics based on a detail mapping, Journal ofJapan Society of Engineering Geology, Vol. 48, No.
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Yasue, T., Okubo, S. (1974): A study of the behaviorof natural slope failure by field observation,
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Wakatsuki, T., Ishizawa, T., Uetake, M., Kawada, S.
(2010): Characteristics of debris flow and slope
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Natural disaster research Report, No. 44, pp. 39-51.

In this study, the authors validate some of the
topographical and geological conditions involved inshallow landslide reported in the previous studies.
These conditions are important clues to detecting
unstable slopes susceptible to shallow landslides.
The authors conducted the field observation in
Chuetsu and Hofu where a number of shallow
landslides recently occurred. In both the study areas,
the authors checked whether or not each landslide site
and non-landslide site satisfied the topographical andgeological conditions. Additionally, the authors
obtained topographical and geological data in the
study areas by map interpretation and applied
Hayashi’s quantification method II to the obtained
data. Consequently, the following conditions wereconfirmed to contribute largely to shallow landslides
in the study areas: 1) convex break of slope, 2) valley
slope, 3) angle of over 40 degrees and 4) spring (Fig.
6).
Acknowledgements
Mr. Masahiro Harada and Mr. Tsuyoshi Nakaharaof Kokusai Kogyo Co., Ltd. assisted the authors in
the map interpretation. Dr. Takehiro Ohta, Mr.
Atsushi Hasegawa and the authors’ colleagues ofRailway Technical Research Institute gave theauthors useful advices on this study. This paper was
revised with the constructive advice by two
anonymous reviewers. The authors are sincerely
grateful to them.

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Wang, G.H. (2009): Preliminary report on landslidedisaster in Hofu city, Yamaguchi Prefecture, Japan
triggered by the July 2009 Chugoku-Northern
Kyushu district heavy rainstorm, Journal of Japan
Society for Natural Disaster Science, Vol. 28, No. 2,
pp. 185-201.
Hatano, S.(1974): Recent geomorphology No. 8,
Soils and Foundations, Vol. 22, No. 11, pp. 50-58.

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