Geomorphology 118 (2010) 167–175

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Geomorphology

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Relationship between the initiation of a shallow landslide and rainfallintensity—duration thresholds in Japan

Hitoshi Saito a,b,⁎, Daichi Nakayama a, Hiroshi Matsuyama a

a Department of Geography, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1, Minami-Osawa, Hachioji, Tokyo 192-0397, Japanb Research Fellow of the Japan Society for the Promotion of Science, Japan

⁎ Corresponding author. Department of GeographyEnvironmental Sciences, Tokyo Metropolitan UniversityTokyo 192-0397, Japan. Tel.: +81 42 677 1111x3871; fa

E-mail address: saitou-hitosi@ed.tmu.ac.jp (H. Saito)

0169-555X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.geomorph.2009.12.016

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 September 2009Received in revised form 15 December 2009Accepted 23 December 2009Available online 4 January 2010

Keywords:Shallow landslidesI–D thresholdRescalingQuantile regressionEast Asian summer monsoon

The empirical rainfall intensity and duration (I–D) threshold for the initiation of shallow landslide is newlydefined for Japan where heavy rainfalls frequently occur during the East Asian summer monsoon season. Therainfall causes sediment-related disasters annually. This paper presents an examination of 1174 rainfall-induced shallow landslides that occurred during 2006–2008. Their I–D conditions were analyzed objectivelyfrom rainfall data (Radar-Raingauge Analyzed Precipitation) to derive the I–D threshold using the quantile-regression method: I=2.18 D−0.26, where I is measured in millimeters per hour and D in hours, asmeasured from the beginning of rainfall to the landslide occurrence. Rainfall events are separated by theabsence of rainfall for 24 h. We then established a rescaled I–D threshold by dividing the rainfall intensity bythe mean annual precipitation (MAP), as IMAP=0.0007 D−0.21, where IMAP is the rescaled average per-hourrainfall intensity. These thresholds were defined by the second percentile regression line for D of 3–537 h.The new thresholds are considerably lower than those previously reported for the world, humid subtropicalregions, the Asian monsoon region, and Japan. The result suggests that Japan is highly prone to rainfall-induced shallow landslides because of its high-relief topography, geologic conditions, human interference,and rainfall characteristics during the East Asian summer monsoon season.

, Graduate School of Urban, 1-1, Minami-Osawa, Hachioji,x: +81 42 677 2589..

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Rainfall-induced landslides often cause considerable damage tosociety. To analyze the primary causes of landslides, it is necessary tounderstand the relation between rainfall and the initiation of landslides(Ibsen and Casagli, 2004; Hong et al., 2005; Guzzetti et al., 2007, 2008).Therefore,manystudies have developed rainfall thresholds for landslideinitiation using an empirical model or a physical (process-based)model(Onodera et al., 1974; Caine, 1980; Larsen and Simon, 1993;Montgomery and Dietrich, 1995; Crozier, 1999; Glade et al., 2000;Gabet et al., 2004; Aleotti, 2004; Chien-Yuan et al., 2005; Hong et al.,2005; Matsushi and Matsukura, 2007; Guzzetti et al., 2007, 2008;Marques et al., 2008; Cannon et al., 2008; Crosta and Frattini, 2008;Dahal and Hasegawa, 2008; Coe et al., 2008; Dahal et al., 2009; ChiangandChang, 2009). The empirical thresholds refer to statistical analysis ofthe relation between rainfall and landslide occurrence (Caine, 1980;Guzzetti et al., 2007, 2008; Dahal and Hasegawa, 2008). For example,Guzzetti et al. (2007) summarized rainfall, climatic variables, and theirempirically based thresholds for thewholeworld and various parts of it.

In the studies described above, rainfall intensity and duration,cumulative event rainfall, and antecedent rainfall were the mostcommonly investigated variables. Landslide initiation caused by heavyrainfall has been related to rainfall intensity and duration (I–D) (Caine,1980; Aleotti, 2004; Guzzetti et al., 2007, 2008; Cannon et al., 2008;Dahal and Hasegawa, 2008; Coe et al., 2008). Antecedent rainfall alsoplays an important role in landslide initiation (Guzzetti et al., 2007,2008; Dahal and Hasegawa, 2008). Although it is important to observenot only the amount of precipitation but also the (largely unknown)amount of water that infiltrates and moves into the ground (Caine,1980; Reichenbach et al., 1998), I–D thresholds are often used to predictlandslide occurrence and to warn appropriate authorities of potentiallandslide hazards (Keefer et al., 1987; Aleotti, 2004; Hong et al., 2005;Dahal and Hasegawa, 2008; Guzzetti et al., 2008; Coe et al., 2008;Cannon et al., 2008).

Japan is situated in the East Asian monsoon region. The Japanesearchipelago is characterized by its high-relief topography andcomplex geological conditions. Heavy rainfalls frequently occur inJapan, especially during the summer monsoon season (Matsumoto,1989, 1993; Matsumoto and Takahashi, 1999), causing sediment-related disasters such as shallow landslides and debris flows (JapanSabo Association, 2001). Many studies have therefore analyzed therelation between rainfall and landslide initiation in Japan usingempirical and physical models, e.g. the tank model, which is aconceptual rainfall–runoff model (Suzuki et al., 1979), the effective

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rainfall-hourly rainfall relation (Ministry of Construction, 1984), themodified effective rainfall (Yano, 1990; Hiura et al., 2005), Soil WaterIndex (Okada et al., 2001), the rainfall index Rf (Sasaki et al., 2001),and the new rainfall index R′ (Nakai et al., 2006). For example,Onodera et al. (1974) investigated slope failures caused by heavyrainfall in Chiba and Kanagawa prefectures, and proposed threeempirical thresholds (lower, intermediate, and upper) to predictslope failures from cumulative event rainfall and maximum rainfallintensity.

Hong et al. (2005) identified the I–D threshold of landslides inShikoku Island, Japan. They assessed the activity of four crystalline-schist landslides using on-site monitoring of rainfall. Their resultsindicate that rainfall thresholds are applicable as an empiricalstandard to evaluate landslide hazards quantitatively. Matsushi andMatsukura (2007) discussed the initiation of shallow landslides in theBoso Peninsula, Japan, based on the I–D relation, geotechnicalproperties of hillslope materials and slope hydrological processes.

Dahal et al. (2009) compared I–D values that triggered shallowlandslides on Shikoku with I–D thresholds proposed by previousstudies. They found that the I–D values were slightly higher than thethreshold established by Larsen and Simon (1993) for the humidtropical region and those by Dahal and Hasegawa (2008) for the Asianmonsoon region (Table 1).

However, these studies in Japan analyzed only limited locationsand events (e.g., a mountainous debris torrent, a landslide event, oran individual rainfall event). Few studies have addressed theregional relation between the initiation of landslides and rainfalls inJapan, although landslides are densely distributed throughout Japanboth spatially and temporally. Since rainfall-induced shallow land-slides frequently occur in Japan, the relation between landslideinitiation and I–D conditions is important for both scientific and socialinterest.

The objective of this study is to analyze the rainfall conditionsresponsible for shallow landslides, and to establish regional I–Dthresholds for all of Japan. Shallow landslides reflect not only rainfallconditions but also topographic, geologic, and other circumstances.Among these, rainfall conditions are a primary trigger and areinfluenced by regional climatic systems such as the East Asianmonsoon. This study therefore specifically investigates the effect ofrainfall conditions on shallow landslides.

This examination included 1174 rainfall-induced shallow landslidesthat occurred throughout Japan during 2006–2008 to determine the I–Dand IMAP (I rescaled by mean annual precipitation: MAP) –D thresholdsusing the quantile-regression method (Koenker and Hallock, 2001;Koenker, 2009). New thresholds were compared with those that hadbeen proposed for the world (Caine, 1980; Guzzetti et al., 2008), forhumid (sub)tropical and Asian monsoon regions (Chien-Yuan et al.,

Table 1I–D threshold equations for the world, humid tropical regions, and Japan.

Reference Area Eq

Caine (1980) World I=Jibson (1989) World I=Guzzetti et al. (2008) World I=

World I=World I=

Guzzetti et al. (2008) Cfa I=Cfa I=

Larsen and Simon (1993) Puerto Rico I=Chien-Yuan et al. (2005) Taiwan I=Cannon et al. (2008) Southern California I=Dahal and Hasegawa (2008) Nepal Himalaya I=

Jibson (1989) Japan I=Hong et al. (2005) Shikoku Island, Japan I=This study Japan I=

Cfa corresponds to the climate of humid subtropical east coast in Köppen's system.

2005; Guzzetti et al., 2008; Dahal and Hasegawa, 2008), and for Japan(Jibson, 1989; Hong et al., 2005).

2. I–D thresholds in previous studies

In this section, we review previously proposed I–D thresholds. Athreshold is theminimum ormaximum level of some quantity neededfor a process to take place or for a state to change (Reichenbach et al.,1998; Guzzetti et al., 2007; Dahal and Hasegawa, 2008). For rainfall-induced landslides, a threshold is definable by rainfall conditions thatare likely to trigger landslides. The I–D thresholds are usually obtainedby drawing minimum-level lines to the rainfall intensity (Y-axis) andduration condition that causes landslides (X-axis) shown in Cartesiansemi-logarithmic, or double logarithmic coordinates (Guzzetti et al.,2007).

Caine (1980) first established a global I–D threshold for shallowlandslides and debris flows based on 73 cases. The threshold curve isexpressed as

I = 14:82D−0:39ð0:167bDb240Þ; ð1Þ

whereD is expressed in hours, and I is expressed inmillimeters per hour(Table 1). Recently Guzzetti et al. (2007) reviewed rainfall thresholdsworldwide; Guzzetti et al. (2008) proposed a new threshold curveas

I = 2:20D−0:44 ð0:1bDb1;000Þ; ð2Þ

using 2626 rainfall events associated with shallow landslides anddebris flows. They also obtained thresholds for six different climateregions of Köppen's system: Cfa, Cfb, Csa, Csb, Cwa, and H. Theirglobal threshold is lower than those of Caine (1980) or of otherprevious studies, which is attributable to their larger dataset.

Regional I–D thresholds were also identified in various parts of theworld: Taiwan (Chien-Yuan et al., 2005), the Nepal Himalayas (Dahaland Hasegawa, 2008), Colorado and southern California (Coe et al.,2008; Cannon et al., 2008), as well as Italy particularly and Europe ingeneral (Aleotti, 2004; Guzzetti et al., 2007). The I–D threshold curvefor debris flows in Taiwan (Chien-Yuan et al., 2005) is expressed as

I = 115:47D−0:80ð1bDb400Þ; ð3Þ

and that for landslides in the Nepal Himalayas (Dahal and Hasegawa,2008) is

I = 73:90D−0:79ð5bDb720Þ: ð4Þ

uation Range (h) Number in Fig. 7

14.82D−0.39 0.167bDb240 130.53D−0.57 0.5bDb12 2-W2.20D−0.44 0.1bDb1000 3-12.28D−0.20 0.1bDb48 3-20.48D−0.11 48≤Db1000 3-3

10.30D−0.35 0.1bDb48 3-46.90D−0.58 0.1b Db1000 3–591.46D−0.82 2bDb312 4115.47D−0.80 1bDb400 514.0D−0.5 0.167bDb12 673.90D−0.79 5bDb720 7

39.71D−0.62 0.5bDb12 2-J1.35+55D−1.00 24bDb300 82.18D−0.26 3bDb537

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Similar local scale I–D thresholds for Japan were also proposed(Table 1), such as

I = 39:71D−0:62ð0:5bDb12Þ ð5Þ

by Jibson (1989), and

I = 1:35 + 55D−1:00ð24bDb300Þ ð6Þ

by Hong et al. (2005) and Guzzetti et al. (2007).

3. Study area and data collection

3.1. Study area

Japan includes large and small islands, mainly Hokkaido, Honshu,Shikoku, and Kyushu Islands (Fig. 1). It is situated along an activetectonic belt, where four major plates (Pacific, Eurasian, PhilippineSea, and North American) interact. The island chain is characterizedby a narrow and elongated shape, with mountains and hills occupyinga large share of the land (Fig. 1). The mountain ranges are oftenbordered by faults with high vertical displacement rates, or areheavily deformed by tight folds with a short wavelength (Kaizuka,1987; Research Group for Active Faults of Japan, 1991). Consequently,the local topographic relief in Japan is generally much greater thanthat of other parts of the world (Katsube and Oguchi, 1999; Kawabataet al., 2001; Oguchi et al., 2001a,b; Saito et al., 2009).

Heavy rainfall occurs frequently in Japan, where MAP is 500–7000 mm (Fig. 2). The southern part of Honshu, Shikoku, and KyushuIslands (western Japan) are characterized by highMAP values becauseheavy rainfalls frequently occur during the East Asian summermonsoon season (Matsumoto, 1989; Matsumoto and Takahashi,

Fig. 1. Elevation of Japan. Source: Digital Map 1 km Grid (E

1999). Two factors, the polar front and typhoons, account for suchheavy rainfalls (Mizukoshi, 1965; Okuta, 1968, 1970; Oguchi et al.,2001a). The polar front persists over Japan generally in June and July,the main rainy season (Bai-u). Typhoons usually hit Japan betweenAugust and October, and often cause heavy rainfall. Fig. 2 shows thatMAP is also high on the northwest (Sea of Japan) side of HonshuIsland. In these areas, however, the high annual precipitation isexplained by heavy snowfall during the winter monsoon season(Matsuyama, 1998; Shimamura et al., 2006).

Matsumoto (1993) examined the global distribution of dailymaximum precipitation records, noting that most of the JapaneseIslands and their surroundings have experienced a daily precipitationof more than 300 mm at least once since the beginning of modernmeteorological observations. Some Japanese meteorological stationshave recorded daily precipitation of more than 1000 mm. Sustainedmaximum daily rainfall at this level has seldom been recorded inEurope and North America. The combination of such heavy rainfalland steep topography in Japan results in widespread hillslope failuresand landslides (Oguchi et al., 2001a).

3.2. Landslide data

This study analyzed 1174 rainfall-induced shallow landslideevents that occurred during 2006–2008 (Fig. 3). We collectedlandslide disaster data with the courtesy of the Erosion and SedimentControl Department, River Bureau, Ministry of Land, Infrastructure,Transport and Tourism, Japanese Government.

The collected data include precise addresses of sites or villageswhere shallow landslides occurred. To acquire geographic coordinates(latitude and longitude) of these places, we used the CSV AddressMatching Service (Center for Spatial Information Science at the

levation), the Geographical Survey Institute of Japan.

Fig. 2. Distribution of mean annual precipitation (MAP) compiled from the Radar-Raingauge Analyzed Precipitation during 2006–2008.

170 H. Saito et al. / Geomorphology 118 (2010) 167–175

University of Tokyo, 2009). The data also include the exact orapproximate time of landslide occurrence.

A clear latitudinal gradientwas found in the distribution of shallowlandslides (Fig. 3) which roughly corresponded to the distribution ofMAP (Fig. 2). Fig. 4 shows that most landslides occurred during June–September, i.e. the summer monsoon season in Japan (Matsumoto,1989; Matsumoto and Takahashi, 1999).

3.3. Rainfall data

We used Radar-Raingauge Analyzed Precipitation data (hereinaf-ter designated as Analyzed Precipitation), obtained using the Radar-AMeDAS (Automated Meteorological Data Acquisition System) of theJapan Meteorological Agency. Data were produced using radarestimates and observation by raingauges densely distributed allover Japan. The temporal resolution is 1 h, with spatial resolution of5 km (1988–2000), 2.5 km (2001–2005), or 1 km (2006 to date). Notonly the resolution, but also the quality of the data has improved since2006 because of the usage of more radar and raingauge data. For thisreason, this study specifically examines the shallow landslide eventsthat occurred during 2006–2008.

The Japan Meteorological Agency and many previous studies havealready verified the accuracy of the Analyzed Precipitation (Yama-moto, 1991; Forecast Division, Forecast Department of the JapanMeteorological Agency, 1995; Makihara et al., 1996; Makihara, 1996,2007; Shimpo, 2001a,b). To evaluate rainfall conditions during 2006–2008, we compared MAP during 2006–2008 (Fig. 2) with that of1989–2008. The ratio of the former to the latter is 0.93 (averaged overthe land), indicating that MAP in 2006–2008 was close to the averagecondition of the past 20 years.

In this study, we changed the resolution of the AnalyzedPrecipitation from 1 to 5 km before analysis because not all landslide

data have precise site information (e.g., sometimes only the name of avillage). Okada et al. (2001) also reported that the AnalyzedPrecipitation with a spatial resolution of 5 km is appropriate foranalyzing the relation between the initiation of landslides and rainfallconditions.

4. Identification of I–D thresholds

Both I andDwere defined as the average rainfall intensity (mm h−1)and the duration (h) from the beginning of a rainfall event to landslideoccurrence. Following methods used by the Ministry of Construction(1984), we defined one rainfall event as the rainfall period delimited bya non-rainfall period of more than 24 h.

We first plotted I–D values in double-logarithmic coordinates, anddefined the rainfall threshold as the level above which one or morethan one shallow landslide can be triggered (Guzzetti et al., 2007,2008). A threshold curve in the form of I=α D−β, where α and β areconstants, was used to determine the threshold, as in many previousstudies (Table 1, Caine, 1980; Aleotti, 2004; Chien-Yuan et al., 2005;Hong et al., 2005; Guzzetti et al., 2007, 2008; Cannon et al., 2008; Coeet al., 2008; Dahal and Hasegawa, 2008).

The quantile-regression method (Koenker and Hallock, 2001;Koenker, 2009) was adopted to determine the I–D thresholdobjectively. The data that were used might contain some errors.Therefore, it is important to employ statistical methods (e.g.quantiles) that are robust and resistant against errors and outliers(Wilks, 2006). We performed quantile regressions in the 2nd, 5th,10th, 20th, 30th, 40th, 50th, 60th, 70th, 80th, and 90th percentilesfollowing the method described for Guzzetti et al. (2007, 2008), usingthe R package (ver. 2.8.0, R Development Core Team, 2009; Koenker,2009). We emphasized the 50th and 2nd percentile regressionsamong the analyses. The 50th percentile regression line was used to

Fig. 3. Distribution of 1174 rainfall-induced shallow landslide events that occurred during 2006–2008.

171H. Saito et al. / Geomorphology 118 (2010) 167–175

determine the general trend of rainfall I–D conditions associated withshallow landslides. The rainfall I–D threshold was then determined bythe 2nd percentile regression line, based on Guzzetti et al. (2007,2008).

A limitation of regional I–D thresholds is that the thresholddetermined for a specific region cannot be exported easily toneighboring regions or similar areas because not only morphological

Fig. 4. Monthly frequency of rainfall-induced shallow landslide events that occurredduring 2006–2008.

and lithological differences but also because of meteorological andclimatological variation (Jakob andWeatherly, 2003) other than I andD of individual rainfall events (Guzzetti et al., 2007, 2008). To offsetthe latter effects, it is important to rescale rainfall intensity usingMAP(Aleotti, 2004; Guzzetti et al., 2007, 2008; Dahal and Hasegawa,2008). Therefore, we divided I by MAP to obtain rescaled I (IMAP) andanalyzed IMAP–D conditions and thresholds by adopting the sameprocedure as that used for the non-rescaled one.

5. Results

5.1. I–D conditions and threshold

Fig. 5 depicts I–D conditions associated with shallow landslides inJapan (circles) and quantile-regression lines in double-logarithmiccoordinates. Values of D range from 3 to 537 h, and I from 0.17 to32.6 mm h−1. Visual inspection of Fig. 5 reveals that many shallowlandslides occur when rainfall persists for 10–200 h from thebeginning of rainfall.

The 50th percentile regression line

I = 22:1D−0:45ð3bDb537 hÞ ð7Þ

describes the trend of the relation between the initiation of shallowlandslides and rainfall conditions. Eq. (7) shows that, with an increasein rainfall duration, the intensity that is likely to initiate shallowlandslides decreases. The same applies to the other quantile-regression lines in which the value of the exponent (β) is between−0.16 and −0.53 (Fig. 5).

The I–D threshold for Japan (2nd percentile regression line) is

I = 2:18D−0:26ð3bDb537hÞ: ð8Þ

Fig. 5. I–D conditions of shallow landslides in Japan (circles) and quantile regressionlines (2nd, 5th 10th, 20th, 30th, 40th, 50th, 60th, 70th, 80th, and 90th percentiles frombottom to top). The 2nd percentile regression line depicts the I–D threshold in thisstudy.

172 H. Saito et al. / Geomorphology 118 (2010) 167–175

This threshold indicates that for rainfall events of shorter duration(e.g. b 10 h), a rainfall intensity of 2.0 mm h−1 has the potential toinitiate shallow landslides. For a longer duration (e.g., N100 h), rainfallintensity of about 0.5 mm h−1 also has the potential to causelandslides.

5.2. IMAP–D conditions and threshold

Fig. 6 presents IMAP–D conditions in Japan. The range of rescaledrainfall intensity is 8.60×10−5 to 1.00×10−2 (h−1) of MAP. Therescaling slightly reduced the variation of rainfall intensity and result

Fig. 6. IMAP–D conditions of shallow landslides in Japan (circles) and quantile regressionlines (2nd, 5th 10th, 20th, 30th, 40th, 50th, 60th, 70th, 80th, and 90th percentiles frombottom to top). The 2nd percentile regression line depicts the IMAP–D threshold in thisstudy.

in lower quantile-regression lines (Fig. 6). The 50th percentileregression line is expressed as

IMAP = 0:0114D−0:52ð3bDb537 hÞ: ð9Þ

As in Fig. 5, a decrease in IMAP with increasing D is apparent for allquantile-regression lines, with exponent values of −0.18 to −0.64.

The IMAP–D threshold for Japan (2nd percentile regression line) is

IMAP = 0:0007D−0:21ð3bDb537 hÞ: ð10Þ

This equation indicates that rainfall intensities of 0.2–0.6×10−3 ofMAP have the potential to initiate shallow landslides.

6. Discussion

6.1. Comparison with previously proposed I–D thresholds

The new I–D thresholds for Japan were compared with those thatwere proposed earlier (Figs. 7 and 8; Tables 1 and 2). These studiesdetermined I–D and IMAP–D thresholds using methods that mutuallydiffered. However, most thresholds were determined as the lowerboundary of rainfall conditions, permitting a direct comparison ofthese thresholds (e.g. Aleotti, 2004; Guzzetti et al., 2007, 2008;Cannon et al., 2008; Dahal and Hasegawa, 2008). For this study, I andD were defined as the average rainfall intensity (mm h−1) and theduration (h) from the beginning of a rainfall event, which wasdelimited by a non-rainfall period of more than 24 h, to landslideoccurrence.

Fig. 7 shows that the new I–D threshold for Japan is lower thanother global, regional, and local thresholds. In particular, it issignificantly lower than the thresholds of Jibson (1989) for Japanand Hong et al. (2005) for Shikoku. The new threshold is also lowerthan those for climatically similar regions such as humid tropical(Larsen and Simon, 1993), Cfa (humid subtropical east coast)(Guzzetti et al., 2008), and Asian monsoon regions (Chien-Yuan

Fig. 7. I–D thresholds determined by this study (red one) and those of various studies(presented in Table 1). Thick lines (black and gray): global thresholds. Thin lines (blackand gray): thresholds for humid (sub)tropics or Asian monsoon regions. Dashed line:other regional thresholds. Blue lines: thresholds for Japan. 1, Caine (1980); 2-J and 2-W,Jibson (1989); 3-1, Guzzetti et al. (2008); 0.1bDb1000; 3-2, Guzzetti et al. (2008),0.1bDb48. 3-3, Guzzetti et al. (2008), 48≤Db1000; 3-4, Guzzetti et al. (2008), Cfa(climate of humid subtropical east coast in Köppen's system), 0.1bDb48; 3-5, Guzzettiet al. (2008), Cfa, 0.1bDb1000; 4, Larsen and Simon (1993); 5, Chien-Yuan et al.(2005); 6, Cannon et al. (2008); 7, Dahal and Hasegawa (2008); 8, Hong et al. (2005).

Fig. 8. IMAP–D thresholds determined by this study (red) and those of various studies(presented in Table 2). Thick lines (black and gray): global thresholds. Thin lines (blackand gray): thresholds for humid (sub)tropics or Asian monsoon regions. Dashed line:other regional thresholds. Blue lines: thresholds for Japan. 2-J and 2-W, Jibson (1989);3-1, Guzzetti et al. (2008) 0.1bDb1000; 3-2, Guzzetti et al. (2008) 0.1bDb48; 3-3,Guzzetti et al. (2008) 48≤Db1000; 7, Dahal and Hasegawa (2008); 9, Cannon (1988)2bDb24; 10, Bacchini and Zannoni (2003); 11-1 and 11-2, Aleotti (2004); 12-1,Guzzetti et al. (2007) Central and Southern Europe; 12-2, Guzzetti et al. (2007) Mildmid-latitude climates.

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et al., 2005; Dahal and Hasegawa, 2008). The difference is greatest forshorter durations and decreases with increasing duration, which isattributed to the larger dataset used for this study: it includes bothlarge and small landslide events.

The new threshold is also lower than the global thresholds of Caine(1980) and Jibson (1989), but resembles those of Guzzetti et al. (2008,line No. 3-2 in Fig. 7) for the rainfall duration of 3 to 48 h. Fig. 8 showsthat the new IMAP–D threshold is 10 to 103 times lower than those inother studies, such as that for the world (Jibson, 1989; Guzzetti et al.,2008), central and southern Europe, and the mild mid-latitudeclimate (Guzzetti et al., 2007), Italy (Bacchini and Zannoni, 2003;Aleotti, 2004), the Nepal Himalayas (Dahal and Hasegawa, 2008), andJapan (Jibson, 1989). The difference between our I–D (or IMAP–D)threshold and that of Guzzetti et al. (2008) is particularly large forshorter duration (D≤48); for longer durations, it is small.

The results presented above indicate that rainfall intensity with apotential to initiate shallow landslides in Japan is the lowest in theworld. This is important information for assessing landslide hazards in

Table 2IMAP–D threshold equations for the world, regional scales, and Japan.

Reference Area Equation

Jibson (1989) World IMAP=0.0Guzzetti et al. (2008) World IMAP=0.0

World IMAP=0.0World IMAP=0.0

Cannon (1988) San Francisco D=46.1–Bacchini and Zannoni (2003) Cancia, Dolomites, Italy IMAP=0.7Aleotti (2004) Piedmont, Italy IMAP=0.7

Piedmont, Italy IMAP=4.6Guzzetti et al. (2007) Central and Southern Europe IMAP=0.0

Mild mid-latitude climates IMAP=0.0Dahal and Hasegawa (2008) Nepal Himalaya IMAP=1.1

Jibson (1989) Japan IMAP=0.0This study Japan IMAP=0.0

Japan. In addition, these characteristics were determined using adataset larger than that of previous studies for Japan or the humid(sub)tropical and Asian monsoon region (Hong et al., 2005; Chien-Yuan et al., 2005; Dahal and Hasegawa, 2008; Guzzetti et al., 2008).

6.2. Factors affecting I–D conditions in Japan

Fig. 5 shows that shallow landslide events are mainly associatedwith rainfall durations of 10–200 h. In particular, many shallowlandslides also occurred when rainfall persisted for more than 100 h.These durations are longer than those of similar climatic regions suchas Cfa (humid subtropical east coast, Fig. 8a in Guzzetti et al., 2008).This result is partly attributable to the definition of the rainfallduration used in our study. In Japan, however, rainfall durationssometimes exceed a week during the summer monsoon season.Although some other studies defined rainfall thresholds for durationslonger than 100 h (Figs. 7 and 8), the landslide events for these longerdurations are few (e.g. Aleotti, 2004; Chien-Yuan et al., 2005; Guzzettiet al., 2007, 2008).

Shallow landslide events occur predominantly during the summermonsoon season in Japan (Fig. 4). Fig. 3 depicts that shallow landslideevents are mainly distributed in southwestern Japan, where heavyrainfall is frequented by the polar front and typhoons (Mizukoshi,1965; Okuta, 1968, 1970; Matsumoto, 1989, 1993; Matsumoto andTakahashi, 1999), resulting in a high MAP (Fig. 2). Occurrence ofshallow landslides therefore corresponds to climatic characteristics ofJapan.

Factors other than climate are also responsible for the low I–Dthresholds for Japan. Japan, located in an active tectonic belt, ischaracterized by high-relief mountainous and hills (Fig. 1, Japan SaboAssociation, 2001; Oguchi et al., 2001a,b). Katsube and Oguchi (1999)reported that the hillslope angle in the Japanese mountains tends tobe around 35°, which is sufficiently steep to initiate shallow landslides(Yanai, 1989; Iida, 1999; Saito et al., 2009). Surveys of hillslopes insteep ranges in central Japan revealed that most hillslope units werecreated by shallow and deep landslides (Oguchi, 1996; Katsube andOguchi, 1999). Because of the frequent landslide occurrence, sedimentyields from Japanese mountains were equivalent to the worldmaximum (e.g. Yoshikawa, 1974; Ohmori, 1983). Therefore, the I–Dthresholds reflect the topography of Japanese mountains and hills.

Geologic influences on frequent landslides in Japan have also beenreported; rocks susceptible to landslides include granite (Chigira,2001), pyroclastic flow deposits (Chigira et al., 2002; Chigira, 2002;Chigira and Yokoyama, 2005), and sedimentary rocks (Chigira, 1992;Chigira and Oyama, 2000). Rocks can be well-weathered under ahumid Japanese climate, favoring landslides and debris flows.Furthermore, rocks in Japanese mountains are often deformed bymass rock creep that forms folds, faults and fractures (Chigira, 1992),

Range(h) Number in Fig. 8

2D−0.65 0.5bDb12 2-W016D−0.40 0.1bDb1,000 3-1017D−0.13 0.1bDb48 3-2005D−0.13 48≤Db1,000 3-3

3.6⁎103IMAP+7.4⁎10

4⁎(IMAP)2 2bDb24 9

4D−0.56 0.1bDb100 106D−0.33 2bDb150 11-12D−0.79 2bDb150 11-2064D−0.64 0.1bDb700 12-1194D−0.73 0.1bDb700 12-20D−0.59 5bDb720 7

3D−0.63 1bDb12 2-J007D−0.21 3bDb537

174 H. Saito et al. / Geomorphology 118 (2010) 167–175

Such deformations also favor landslides (Chigira, 1992; Chigira andKiho, 1994).

Human-related factors also account for landslides in Japan. Manymountainous and hilly areas in Japan have been developed for humanuse. The resultant disturbances such as road construction oftenengender shallow landslides (Ayalew and Yamagishi, 2005).

In summary, high-relief topography, geologic conditions, humandisturbances and rainfall characteristics during the East Asian summermonsoon season together account for the notably low I–D thresholdfor Japan. In other words, Japan is highly prone to shallow landslidescompared to other regions of the world.

6.3. Limitations of the rainfall I–D analysis

Figs. 5 and 6 show that I–D conditions for Japan are characterizedby long rainfall duration, such as that longer than 100 h. However, I–Dplots represent only average conditions of the rainfall event, and donot necessarily reflect high rainfall intensity at the time of shallowlandslide occurrence.

Fig. 9 depicts the relation between peak rainfall intensity andrainfall duration from the beginning of a rainfall event to shallowlandslide occurrence. Although I and D show clear negative correla-tions (Figs. 5 and 6), the peak rainfall intensity is high for both shortand long durations, with an average peak of 40.7 mm h−1 (Fig. 9).Heavy hourly rainfall frequently occurs in Japan (Onodera et al., 1974;Suda, 1991; Dahal et al., 2009), and such an event is critical forinitiation of shallow landslides from both short and long rainfallevents.

Antecedent rainfall plays an important role in the gradualsaturation of the soil (Guzzetti et al., 2007, 2008; Dahal and Hasegawa,2008). Figs. 5 and 9 show that several shallow landslides occur duringlow-intensity long-term rainfall events. Some rainfall events in thesummer monsoon season in Japan are characterized by their low-intensity but long duration. The role of antecedent rainfall intriggering landslides in Japan therefore seems clear and important.

The observations described above point to some limitations of I–Danalysis in relation to landslide initiation in humid climatic regions suchas Japan. Further studies are necessary to establish local-scale I–Dthresholds considering the variation of rainfall intensity, antecedentrainfall conditions and other variables such as topography and geology.

Fig. 9. Relation between rainfall duration and peak rainfall intensity from beginning of arainfall event to occurrence of a shallow landslide.

7. Conclusions

The empirical I–D thresholds for initiating shallow landslides inJapan were determined and compared with previously proposedglobal, regional, and local thresholds. We examined 1174 rainfall-induced shallow landslides that occurred during 2006–2008 usingrainfall data of the Radar-Raingauges Analyzed Precipitation. The I–Dthresholds were identified quantitatively using the quantile-regres-sion method, which is robust and resistant to errors and outliers. Tocompare the new I–D threshold with those of other studies, werescaled I by dividing it by MAP. The results indicate that rainfallintensities of 1.64–0.42 mm h−1 have the potential to initiate shallowlandslides in Japan, with rainfall duration of 3–537 h. This threshold islower than those reported in almost all previous studies, meaning thatJapan is highly prone to landslides. The low threshold reflects high-relief topography, geologic conditions, human interference, and short-but-heavy, or gentle-but-long rainfall events that occur during theEast Asian summer monsoon season.

Acknowledgments

We thank the Erosion and Sediment Control Department, RiverBureau, Ministry of Land, Infrastructure, Transport and Tourism,Japanese Government, for allowing us to use their landslide disasterdata. We also thank Profs. Takashi Oguchi, David Alexander, and NelCaine for their valuable comments. This study was partially supportedby a Grant-in-Aid for JSPS Research Fellows, theMinistry of Education,Culture, Sports, Science and Technology, Japanese Government (No.20-6594).

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