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

Geomorphology 118 (2010) 167–175

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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h

Relationship between the initiation of a shallow landslide and rainfall intensity—duration thresholds in Japan Hitoshi Saito a,b,⁎, Daichi Nakayama a, Hiroshi Matsuyama a a b

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

a r t i c l e

i n f o

Article history: Received 18 September 2009 Received in revised form 15 December 2009 Accepted 23 December 2009 Available online 4 January 2010 Keywords: Shallow landslides I–D threshold Rescaling Quantile regression East Asian summer monsoon

a b s t r a c t The empirical rainfall intensity and duration (I–D) threshold for the initiation of shallow landslide is newly defined for Japan where heavy rainfalls frequently occur during the East Asian summer monsoon season. The rainfall causes sediment-related disasters annually. This paper presents an examination of 1174 rainfallinduced shallow landslides that occurred during 2006–2008. Their I–D conditions were analyzed objectively from rainfall data (Radar-Raingauge Analyzed Precipitation) to derive the I–D threshold using the quantileregression method: I = 2.18 D− 0.26, where I is measured in millimeters per hour and D in hours, as measured from the beginning of rainfall to the landslide occurrence. Rainfall events are separated by the absence of rainfall for 24 h. We then established a rescaled I–D threshold by dividing the rainfall intensity by the mean annual precipitation (MAP), as IMAP = 0.0007 D− 0.21, where IMAP is the rescaled average per-hour rainfall 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 subtropical regions, the Asian monsoon region, and Japan. The result suggests that Japan is highly prone to rainfallinduced shallow landslides because of its high-relief topography, geologic conditions, human interference, and rainfall characteristics during the East Asian summer monsoon season. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Rainfall-induced landslides often cause considerable damage to society. To analyze the primary causes of landslides, it is necessary to understand the relation between rainfall and the initiation of landslides (Ibsen and Casagli, 2004; Hong et al., 2005; Guzzetti et al., 2007, 2008). Therefore, many studies have developed rainfall thresholds for landslide initiation 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; Chiang and Chang, 2009). The empirical thresholds refer to statistical analysis of the 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 their empirically based thresholds for the whole world and various parts of it.

⁎ Corresponding author. Department of Geography, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1, Minami-Osawa, Hachioji, Tokyo 192-0397, Japan. Tel.: +81 42 677 1111x3871; fax: +81 42 677 2589. E-mail address: [email protected] (H. Saito). 0169-555X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2009.12.016

In the studies described above, rainfall intensity and duration, cumulative event rainfall, and antecedent rainfall were the most commonly investigated variables. Landslide initiation caused by heavy rainfall 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 also plays an important role in landslide initiation (Guzzetti et al., 2007, 2008; Dahal and Hasegawa, 2008). Although it is important to observe not 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 predict landslide occurrence and to warn appropriate authorities of potential landslide 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 Japanese archipelago is characterized by its high-relief topography and complex geological conditions. Heavy rainfalls frequently occur in Japan, especially during the summer monsoon season (Matsumoto, 1989, 1993; Matsumoto and Takahashi, 1999), causing sedimentrelated disasters such as shallow landslides and debris flows (Japan Sabo Association, 2001). Many studies have therefore analyzed the relation between rainfall and landslide initiation in Japan using empirical and physical models, e.g. the tank model, which is a conceptual rainfall–runoff model (Suzuki et al., 1979), the effective

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rainfall-hourly rainfall relation (Ministry of Construction, 1984), the modified effective rainfall (Yano, 1990; Hiura et al., 2005), Soil Water Index (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 heavy rainfall in Chiba and Kanagawa prefectures, and proposed three empirical thresholds (lower, intermediate, and upper) to predict slope failures from cumulative event rainfall and maximum rainfall intensity. Hong et al. (2005) identified the I–D threshold of landslides in Shikoku Island, Japan. They assessed the activity of four crystallineschist landslides using on-site monitoring of rainfall. Their results indicate that rainfall thresholds are applicable as an empirical standard to evaluate landslide hazards quantitatively. Matsushi and Matsukura (2007) discussed the initiation of shallow landslides in the Boso Peninsula, Japan, based on the I–D relation, geotechnical properties of hillslope materials and slope hydrological processes. Dahal et al. (2009) compared I–D values that triggered shallow landslides on Shikoku with I–D thresholds proposed by previous studies. They found that the I–D values were slightly higher than the threshold established by Larsen and Simon (1993) for the humid tropical region and those by Dahal and Hasegawa (2008) for the Asian monsoon region (Table 1). However, these studies in Japan analyzed only limited locations and events (e.g., a mountainous debris torrent, a landslide event, or an individual rainfall event). Few studies have addressed the regional relation between the initiation of landslides and rainfalls in Japan, although landslides are densely distributed throughout Japan both spatially and temporally. Since rainfall-induced shallow landslides frequently occur in Japan, the relation between landslide initiation and I–D conditions is important for both scientific and social interest. The objective of this study is to analyze the rainfall conditions responsible for shallow landslides, and to establish regional I–D thresholds for all of Japan. Shallow landslides reflect not only rainfall conditions but also topographic, geologic, and other circumstances. Among these, rainfall conditions are a primary trigger and are influenced by regional climatic systems such as the East Asian monsoon. This study therefore specifically investigates the effect of rainfall conditions on shallow landslides. This examination included 1174 rainfall-induced shallow landslides that occurred throughout Japan during 2006–2008 to determine the I–D and IMAP (I rescaled by mean annual precipitation: MAP) –D thresholds using the quantile-regression method (Koenker and Hallock, 2001; Koenker, 2009). New thresholds were compared with those that had been proposed for the world (Caine, 1980; Guzzetti et al., 2008), for humid (sub)tropical and Asian monsoon regions (Chien-Yuan et al.,

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. A threshold is the minimum or maximum level of some quantity needed for a process to take place or for a state to change (Reichenbach et al., 1998; Guzzetti et al., 2007; Dahal and Hasegawa, 2008). For rainfallinduced landslides, a threshold is definable by rainfall conditions that are likely to trigger landslides. The I–D thresholds are usually obtained by drawing minimum-level lines to the rainfall intensity (Y-axis) and duration condition that causes landslides (X-axis) shown in Cartesian semi-logarithmic, or double logarithmic coordinates (Guzzetti et al., 2007). Caine (1980) first established a global I–D threshold for shallow landslides and debris flows based on 73 cases. The threshold curve is expressed as −0:39

I = 14:82D

ð0:167bDb240Þ;

ð1Þ

where D is expressed in hours, and I is expressed in millimeters per hour (Table 1). Recently Guzzetti et al. (2007) reviewed rainfall thresholds worldwide; Guzzetti et al. (2008) proposed a new threshold curve as I = 2:20D

−0:44

ð0:1bDb1; 000Þ;

ð2Þ

using 2626 rainfall events associated with shallow landslides and debris flows. They also obtained thresholds for six different climate regions of Köppen's system: Cfa, Cfb, Csa, Csb, Cwa, and H. Their global threshold is lower than those of Caine (1980) or of other previous studies, which is attributable to their larger dataset. Regional I–D thresholds were also identified in various parts of the world: Taiwan (Chien-Yuan et al., 2005), the Nepal Himalayas (Dahal and Hasegawa, 2008), Colorado and southern California (Coe et al., 2008; Cannon et al., 2008), as well as Italy particularly and Europe in general (Aleotti, 2004; Guzzetti et al., 2007). The I–D threshold curve for debris flows in Taiwan (Chien-Yuan et al., 2005) is expressed as I = 115:47D

−0:80

ð1bDb400Þ;

ð3Þ

and that for landslides in the Nepal Himalayas (Dahal and Hasegawa, 2008) is −0:79

I = 73:90D

ð5bDb720Þ:

ð4Þ

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

Area

Equation

Range (h)

Number in Fig. 7

Caine (1980) Jibson (1989) Guzzetti et al. (2008)

World World World World World

I = 14.82D− 0.39 I = 30.53D− 0.57 I = 2.20D− 0.44 I = 2.28D− 0.20 I = 0.48D− 0.11

0.167 b D b 240 0.5 b D b 12 0.1 b D b 1000 0.1 b D b 48 48 ≤ D b 1000

1 2-W 3-1 3-2 3-3

Guzzetti et al. (2008) Larsen and Simon (1993) Chien-Yuan et al. (2005) Cannon et al. (2008) Dahal and Hasegawa (2008)

Cfa Cfa Puerto Rico Taiwan Southern California Nepal Himalaya

I = 10.30D− 0.35 I = 6.90D− 0.58 I = 91.46D− 0.82 I = 115.47D− 0.80 I = 14.0D− 0.5 I = 73.90D− 0.79

0.1 b D b 48 0.1b D b 1000 2 b D b 312 1 b D b 400 0.167 b D b 12 5 b D b 720

3-4 3–5 4 5 6 7

Jibson (1989) Hong et al. (2005) This study

Japan Shikoku Island, Japan Japan

I = 39.71D− 0.62 I = 1.35 + 55D− 1.00 I = 2.18D− 0.26

0.5 b D b 12 24 b D b 300 3 b D b 537

2-J 8

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

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Similar local scale I–D thresholds for Japan were also proposed (Table 1), such as −0:62

I = 39:71D

ð0:5bDb12Þ

ð5Þ

by Jibson (1989), and −1:00

I = 1:35 + 55D

ð24bDb300Þ

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 active tectonic belt, where four major plates (Pacific, Eurasian, Philippine Sea, and North American) interact. The island chain is characterized by a narrow and elongated shape, with mountains and hills occupying a large share of the land (Fig. 1). The mountain ranges are often bordered by faults with high vertical displacement rates, or are heavily 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 than that of other parts of the world (Katsube and Oguchi, 1999; Kawabata et 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 Kyushu Islands (western Japan) are characterized by high MAP values because heavy rainfalls frequently occur during the East Asian summer monsoon season (Matsumoto, 1989; Matsumoto and Takahashi,

169

1999). Two factors, the polar front and typhoons, account for such heavy 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 between August and October, and often cause heavy rainfall. Fig. 2 shows that MAP is also high on the northwest (Sea of Japan) side of Honshu Island. In these areas, however, the high annual precipitation is explained by heavy snowfall during the winter monsoon season (Matsuyama, 1998; Shimamura et al., 2006). Matsumoto (1993) examined the global distribution of daily maximum precipitation records, noting that most of the Japanese Islands and their surroundings have experienced a daily precipitation of more than 300 mm at least once since the beginning of modern meteorological observations. Some Japanese meteorological stations have recorded daily precipitation of more than 1000 mm. Sustained maximum daily rainfall at this level has seldom been recorded in Europe and North America. The combination of such heavy rainfall and steep topography in Japan results in widespread hillslope failures and landslides (Oguchi et al., 2001a).

3.2. Landslide data This study analyzed 1174 rainfall-induced shallow landslide events that occurred during 2006–2008 (Fig. 3). We collected landslide disaster data with the courtesy of the Erosion and Sediment Control Department, River Bureau, Ministry of Land, Infrastructure, Transport and Tourism, Japanese Government. The collected data include precise addresses of sites or villages where shallow landslides occurred. To acquire geographic coordinates (latitude and longitude) of these places, we used the CSV Address Matching Service (Center for Spatial Information Science at the

Fig. 1. Elevation of Japan. Source: Digital Map 1 km Grid (Elevation), the Geographical Survey Institute of Japan.

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

University of Tokyo, 2009). The data also include the exact or approximate time of landslide occurrence. A clear latitudinal gradient was found in the distribution of shallow landslides (Fig. 3) which roughly corresponded to the distribution of MAP (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).

data have precise site information (e.g., sometimes only the name of a village). Okada et al. (2001) also reported that the Analyzed Precipitation with a spatial resolution of 5 km is appropriate for analyzing the relation between the initiation of landslides and rainfall conditions.

3.3. Rainfall data

Both I and D were defined as the average rainfall intensity (mm h− 1) and the duration (h) from the beginning of a rainfall event to landslide occurrence. Following methods used by the Ministry of Construction (1984), we defined one rainfall event as the rainfall period delimited by a non-rainfall period of more than 24 h. We first plotted I–D values in double-logarithmic coordinates, and defined the rainfall threshold as the level above which one or more than 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 are constants, was used to determine the threshold, as in many previous studies (Table 1, Caine, 1980; Aleotti, 2004; Chien-Yuan et al., 2005; Hong et al., 2005; Guzzetti et al., 2007, 2008; Cannon et al., 2008; Coe et al., 2008; Dahal and Hasegawa, 2008). The quantile-regression method (Koenker and Hallock, 2001; Koenker, 2009) was adopted to determine the I–D threshold objectively. 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 percentiles following the method described for Guzzetti et al. (2007, 2008), using the R package (ver. 2.8.0, R Development Core Team, 2009; Koenker, 2009). We emphasized the 50th and 2nd percentile regressions among the analyses. The 50th percentile regression line was used to

We used Radar-Raingauge Analyzed Precipitation data (hereinafter designated as Analyzed Precipitation), obtained using the RadarAMeDAS (Automated Meteorological Data Acquisition System) of the Japan Meteorological Agency. Data were produced using radar estimates and observation by raingauges densely distributed all over Japan. The temporal resolution is 1 h, with spatial resolution of 5 km (1988–2000), 2.5 km (2001–2005), or 1 km (2006 to date). Not only the resolution, but also the quality of the data has improved since 2006 because of the usage of more radar and raingauge data. For this reason, this study specifically examines the shallow landslide events that occurred during 2006–2008. The Japan Meteorological Agency and many previous studies have already verified the accuracy of the Analyzed Precipitation (Yamamoto, 1991; Forecast Division, Forecast Department of the Japan Meteorological 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 of 1989–2008. The ratio of the former to the latter is 0.93 (averaged over the land), indicating that MAP in 2006–2008 was close to the average condition of the past 20 years. In this study, we changed the resolution of the Analyzed Precipitation from 1 to 5 km before analysis because not all landslide

4. Identification of I–D thresholds

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

determine the general trend of rainfall I–D conditions associated with shallow landslides. The rainfall I–D threshold was then determined by the 2nd percentile regression line, based on Guzzetti et al. (2007, 2008). A limitation of regional I–D thresholds is that the threshold determined for a specific region cannot be exported easily to neighboring regions or similar areas because not only morphological

and lithological differences but also because of meteorological and climatological variation (Jakob and Weatherly, 2003) other than I and D of individual rainfall events (Guzzetti et al., 2007, 2008). To offset the latter effects, it is important to rescale rainfall intensity using MAP (Aleotti, 2004; Guzzetti et al., 2007, 2008; Dahal and Hasegawa, 2008). Therefore, we divided I by MAP to obtain rescaled I (IMAP) and analyzed IMAP–D conditions and thresholds by adopting the same procedure 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 in Japan (circles) and quantile-regression lines in double-logarithmic coordinates. Values of D range from 3 to 537 h, and I from 0.17 to 32.6 mm h− 1. Visual inspection of Fig. 5 reveals that many shallow landslides occur when rainfall persists for 10–200 h from the beginning of rainfall. The 50th percentile regression line −0:45

I = 22:1D

ð3bDb537 hÞ

ð7Þ

describes the trend of the relation between the initiation of shallow landslides and rainfall conditions. Eq. (7) shows that, with an increase in rainfall duration, the intensity that is likely to initiate shallow landslides decreases. The same applies to the other quantileregression 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 Fig. 4. Monthly frequency of rainfall-induced shallow landslide events that occurred during 2006–2008.

−0:26

I = 2:18D

ð3bDb537 hÞ:

ð8Þ

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in lower quantile-regression lines (Fig. 6). The 50th percentile regression line is expressed as −0:52

IMAP = 0:0114D

ð3bDb537 hÞ:

ð9Þ

As in Fig. 5, a decrease in IMAP with increasing D is apparent for all quantile-regression lines, with exponent values of − 0.18 to −0.64. The IMAP–D threshold for Japan (2nd percentile regression line) is −0:21

IMAP = 0:0007D

ð3bDb537 hÞ:

ð10Þ

This equation indicates that rainfall intensities of 0.2–0.6×10− 3 of MAP have the potential to initiate shallow landslides. 6. Discussion 6.1. Comparison with previously proposed I–D thresholds

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

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 to initiate shallow landslides. For a longer duration (e.g., N100 h), rainfall intensity of about 0.5 mm h− 1 also has the potential to cause landslides. 5.2. IMAP–D conditions and threshold Fig. 6 presents IMAP–D conditions in Japan. The range of rescaled rainfall intensity is 8.60 × 10− 5 to 1.00 × 10− 2 (h− 1) of MAP. The rescaling slightly reduced the variation of rainfall intensity and result

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

The new I–D thresholds for Japan were compared with those that were proposed earlier (Figs. 7 and 8; Tables 1 and 2). These studies determined I–D and IMAP–D thresholds using methods that mutually differed. However, most thresholds were determined as the lower boundary of rainfall conditions, permitting a direct comparison of these thresholds (e.g. Aleotti, 2004; Guzzetti et al., 2007, 2008; Cannon et al., 2008; Dahal and Hasegawa, 2008). For this study, I and D were defined as the average rainfall intensity (mm h− 1) and the duration (h) from the beginning of a rainfall event, which was delimited by a non-rainfall period of more than 24 h, to landslide occurrence. Fig. 7 shows that the new I–D threshold for Japan is lower than other global, regional, and local thresholds. In particular, it is significantly lower than the thresholds of Jibson (1989) for Japan and Hong et al. (2005) for Shikoku. The new threshold is also lower than 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 (black and 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); 0.1 b D b 1000; 3-2, Guzzetti et al. (2008), 0.1 b D b 48. 3-3, Guzzetti et al. (2008), 48 ≤ D b 1000; 3-4, Guzzetti et al. (2008), Cfa (climate of humid subtropical east coast in Köppen's system), 0.1 b D b 48; 3-5, Guzzetti et al. (2008), Cfa, 0.1 b D b 1000; 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).

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

6.2. Factors affecting I–D conditions in Japan

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 (black and 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) 0.1b D b 1000; 3-2, Guzzetti et al. (2008) 0.1 b D b 48; 3-3, Guzzetti et al. (2008) 48 ≤ D b 1000; 7, Dahal and Hasegawa (2008); 9, Cannon (1988) 2 b D b 24; 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) Mild mid-latitude climates.

et al., 2005; Dahal and Hasegawa, 2008). The difference is greatest for shorter durations and decreases with increasing duration, which is attributed to the larger dataset used for this study: it includes both large 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 shows that the new IMAP–D threshold is 10 to 103 times lower than those in other studies, such as that for the world (Jibson, 1989; Guzzetti et al., 2008), central and southern Europe, and the mild mid-latitude climate (Guzzetti et al., 2007), Italy (Bacchini and Zannoni, 2003; Aleotti, 2004), the Nepal Himalayas (Dahal and Hasegawa, 2008), and Japan (Jibson, 1989). The difference between our I–D (or IMAP–D) threshold and that of Guzzetti et al. (2008) is particularly large for shorter duration (D ≤ 48); for longer durations, it is small. The results presented above indicate that rainfall intensity with a potential to initiate shallow landslides in Japan is the lowest in the world. This is important information for assessing landslide hazards in

Fig. 5 shows that shallow landslide events are mainly associated with rainfall durations of 10–200 h. In particular, many shallow landslides also occurred when rainfall persisted for more than 100 h. These durations are longer than those of similar climatic regions such as Cfa (humid subtropical east coast, Fig. 8a in Guzzetti et al., 2008). This result is partly attributable to the definition of the rainfall duration used in our study. In Japan, however, rainfall durations sometimes exceed a week during the summer monsoon season. Although some other studies defined rainfall thresholds for durations longer than 100 h (Figs. 7 and 8), the landslide events for these longer durations are few (e.g. Aleotti, 2004; Chien-Yuan et al., 2005; Guzzetti et al., 2007, 2008). Shallow landslide events occur predominantly during the summer monsoon season in Japan (Fig. 4). Fig. 3 depicts that shallow landslide events are mainly distributed in southwestern Japan, where heavy rainfall is frequented by the polar front and typhoons (Mizukoshi, 1965; Okuta, 1968, 1970; Matsumoto, 1989, 1993; Matsumoto and Takahashi, 1999), resulting in a high MAP (Fig. 2). Occurrence of shallow landslides therefore corresponds to climatic characteristics of Japan. Factors other than climate are also responsible for the low I–D thresholds for Japan. Japan, located in an active tectonic belt, is characterized by high-relief mountainous and hills (Fig. 1, Japan Sabo Association, 2001; Oguchi et al., 2001a,b). Katsube and Oguchi (1999) reported that the hillslope angle in the Japanese mountains tends to be around 35°, which is sufficiently steep to initiate shallow landslides (Yanai, 1989; Iida, 1999; Saito et al., 2009). Surveys of hillslopes in steep ranges in central Japan revealed that most hillslope units were created by shallow and deep landslides (Oguchi, 1996; Katsube and Oguchi, 1999). Because of the frequent landslide occurrence, sediment yields from Japanese mountains were equivalent to the world maximum (e.g. Yoshikawa, 1974; Ohmori, 1983). Therefore, the I–D thresholds reflect the topography of Japanese mountains and hills. Geologic influences on frequent landslides in Japan have also been reported; 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 a humid Japanese climate, favoring landslides and debris flows. Furthermore, rocks in Japanese mountains are often deformed by mass rock creep that forms folds, faults and fractures (Chigira, 1992),

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

Area

Equation

Range(h)

Number in Fig. 8

Jibson (1989) Guzzetti et al. (2008)

World World World World

IMAP = 0.02D− 0.65 IMAP = 0.0016D− 0.40 IMAP = 0.0017D− 0.13 IMAP= 0.0005D− 0.13

0.5 b D b 12 0.1 b D b 1,000 0.1 b D b 48 48 ≤ D b 1,000

2-W 3-1 3-2 3-3

Cannon (1988) Bacchini and Zannoni (2003) Aleotti (2004)

Dahal and Hasegawa (2008)

San Francisco Cancia, Dolomites, Italy Piedmont, Italy Piedmont, Italy Central and Southern Europe Mild mid-latitude climates Nepal Himalaya

D = 46.1–3.6 ⁎ 103IMAP + 7.4 ⁎ 104 ⁎ (IMAP)2 IMAP = 0.74D− 0.56 IMAP = 0.76D− 0.33 IMAP = 4.62D− 0.79 IMAP = 0.0064D− 0.64 IMAP = 0.0194D− 0.73 IMAP = 1.10D− 0.59

2 b D b 24 0.1 b D b 100 2 b D b 150 2 b D b 150 0.1 b D b 700 0.1 b D b 700 5 b D b 720

9 10 11-1 11-2 12-1 12-2 7

Jibson (1989) This study

Japan Japan

IMAP = 0.03D− 0.63 IMAP = 0.0007D− 0.21

1 b D b 12 3 b D b 537

2-J

Guzzetti et al. (2007)

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Such deformations also favor landslides (Chigira, 1992; Chigira and Kiho, 1994). Human-related factors also account for landslides in Japan. Many mountainous and hilly areas in Japan have been developed for human use. The resultant disturbances such as road construction often engender shallow landslides (Ayalew and Yamagishi, 2005). In summary, high-relief topography, geologic conditions, human disturbances and rainfall characteristics during the East Asian summer monsoon season together account for the notably low I–D threshold for Japan. In other words, Japan is highly prone to shallow landslides compared 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 characterized by long rainfall duration, such as that longer than 100 h. However, I–D plots represent only average conditions of the rainfall event, and do not necessarily reflect high rainfall intensity at the time of shallow landslide occurrence. Fig. 9 depicts the relation between peak rainfall intensity and rainfall duration from the beginning of a rainfall event to shallow landslide occurrence. Although I and D show clear negative correlations (Figs. 5 and 6), the peak rainfall intensity is high for both short and 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 for initiation of shallow landslides from both short and long rainfall events. Antecedent rainfall plays an important role in the gradual saturation of the soil (Guzzetti et al., 2007, 2008; Dahal and Hasegawa, 2008). Figs. 5 and 9 show that several shallow landslides occur during low-intensity long-term rainfall events. Some rainfall events in the summer monsoon season in Japan are characterized by their lowintensity but long duration. The role of antecedent rainfall in triggering landslides in Japan therefore seems clear and important. The observations described above point to some limitations of I–D analysis in relation to landslide initiation in humid climatic regions such as Japan. Further studies are necessary to establish local-scale I–D thresholds considering the variation of rainfall intensity, antecedent rainfall conditions and other variables such as topography and geology.

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

7. Conclusions The empirical I–D thresholds for initiating shallow landslides in Japan were determined and compared with previously proposed global, regional, and local thresholds. We examined 1174 rainfallinduced shallow landslides that occurred during 2006–2008 using rainfall data of the Radar-Raingauges Analyzed Precipitation. The I–D thresholds were identified quantitatively using the quantile-regression method, which is robust and resistant to errors and outliers. To compare the new I–D threshold with those of other studies, we rescaled I by dividing it by MAP. The results indicate that rainfall intensities of 1.64–0.42 mm h− 1 have the potential to initiate shallow landslides in Japan, with rainfall duration of 3–537 h. This threshold is lower than those reported in almost all previous studies, meaning that Japan is highly prone to landslides. The low threshold reflects highrelief topography, geologic conditions, human interference, and shortbut-heavy, or gentle-but-long rainfall events that occur during the East Asian summer monsoon season. Acknowledgments We thank the Erosion and Sediment Control Department, River Bureau, Ministry of Land, Infrastructure, Transport and Tourism, Japanese Government, for allowing us to use their landslide disaster data. We also thank Profs. Takashi Oguchi, David Alexander, and Nel Caine for their valuable comments. This study was partially supported by a Grant-in-Aid for JSPS Research Fellows, the Ministry of Education, Culture, Sports, Science and Technology, Japanese Government (No. 20-6594). References Aleotti, P., 2004. A warning system for rainfall-induced shallow failures. Engineering Geology 73, 247–265. Ayalew, L., Yamagishi, H., 2005. The application of GIS-based logistic regression for landslide susceptibility mapping in the Kakuda–Yahiko Mountains, Central Japan. Geomorphology 65, 15–31. Bacchini, M., Zannoni, A., 2003. Relations between rainfall and triggering of debris-flow: case study of Cancia (Dolomites, Northeastern Italy). Natural Hazards and Earth System Sciences 3, 71–79. Caine, N., 1980. The rainfall intensity–duration control of shallow landslides and debris flows. Geografiska Annaler. Series A. Physical Geography 62, 23–27. Cannon, S., 1988. Regional rainfall–threshold conditions for abundant debris-flow activity. In: Ellen, S.D., Wieczorek, G.F. (Eds.), Landslides, floods, and marine effects of the storm of January 3–5, 1982, in the San Francisco Bay Region, California: US Geological Survey Professional Paper, vol. 1434, pp. 35–42. Cannon, S., Gartner, J., Wilson, R., Bowers, J., Laber, J., 2008. Storm rainfall conditions for floods and debris flows from recently burned areas in southwestern Colorado and southern California. Geomorphology 96, 250–269. Center for Spatial Information Science at the University of Tokyo, 2009. CSV Address Matching Service. . Available at http://newspat.csis.utokyo.ac.jp/geocode/. Chiang, S., Chang, K., 2009. Application of radar data to modeling rainfall-induced landslides. Geomorphology 103, 299–309. Chien-Yuan, C., Tien-Chien, C., Fan-Chieh, Y., Wen-Hui, Y., Chun-Chieh, T., 2005. Rainfall duration and debris-flow initiated studies for real-time monitoring. Environmental Geology 47, 715–724. Chigira, M., 1992. Long-term gravitational deformation of rocks by mass rock creep. Engineering Geology 32, 157–184. Chigira, M., 2001. Micro-sheeting of granite and its relationship with landsliding specifically after the heavy rainstorm in June 1999, Hiroshima Prefecture, Japan. Engineering Geology 59, 219–231. Chigira, M., 2002. Geologic factors contributing to landslide generation in a pyroclastic area: August 1998 Nishigo Village, Japan. Geomorphology 46, 117–128. Chigira, M., Kiho, K., 1994. Deep-seated rockslide-avalanches preceded by mass rock creep of sedimentary rocks in the Akaishi Mountains, central Japan. Engineering Geology 38, 221–230. Chigira, M., Oyama, T., 2000. Mechanism and effect of chemical weathering of sedimentary rocks. Engineering Geology 55, 3–14. Chigira, M., Yokoyama, O., 2005. Weathering profile of non-welded ignimbrite and the water infiltration behavior within it in relation to the generation of shallow landslides. Engineering Geology 78, 187–207. Chigira, M., Nakamoto, M., Nakata, E., 2002. Weathering mechanisms and their effects on the landsliding of ignimbrite subject to vapor-phase crystallization in the Shirakawa pyroclastic flow, northern Japan. Engineering Geology 66, 111–125. Coe, J., Kinner, D., Godt, J., 2008. Initiation conditions for debris flows generated by runoff at Chalk Cliffs, central Colorado. Geomorphology 96, 270–297.

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