RELATION BETWEEN DISTANCE FROM EARTHQUAKE SOURCE FAULT AND SCALE OF LANDSLIDE TRIGGERED BY RECENT TWO STRONG EARTHQUAKES IN THE NIIGATA PREFECTURE, JAPAN Baator Has 1*, Yasuo Ishii 2, Kiyoteru Maruyama 3, Soki Suzuki 4, Hideki Terada 5 ABSTRACT Two strong earthquakes, the earthquake (M6.8) in 24 and the -offshore earthquake (M6.8) in 27 struck the Niigata Prefecture, eastern Japan, caused a large number of landslides. To clarify the relationship between landslides scale and the distance from the earthquake source fault, we interpreted and analysed the landslides induced by these two earthquakes. In the case of the earthquake, 362 landslides, a total area of 5.15 km 2 were interpreted in 1,3 km 2 area. In the case of the -offshore earthquake, 18 landslides with total area of.6 km 2 were recognized in 421 km 2 area. The results showed that most of landslides occurred on the hanging-wall side of the source fault, and landslides scale tends to decrease with increasing of distance from fault rupture. Key Words: Earthquake-induced landslide, earthquake, -offshore earthquake, Source fault INTRODUCTION Two strong earthquakes, the earthquake (M6.8) in 24 and the -offshore earthquake (M6.8) in 27 struck Niigata Prefecture, eastern Japan, caused serious damage to the central Niigata region. According to the Japan Meteorological Association (JMA), the earthquake recorded maximum seismic intensity of 7 and the -offshore earthquake recorded 6-upper. A large number of slope failures and landslides induced by these earthquakes (Ministry of Land, Infrastructure, Transport and Tourism, MLIT, 25; 27), especially by the earthquake, some large landslides blocked the Imo River in 52 palaces, caused serious damage to the mountainous area. Although several studies have revealed some geological and geomorphological characteristics of the landslides induced by the earthquakes (e.g. Yagi et al., 27; Sato et al., 28), however there are few studies focused on the relation between the earthquake-induced landslides and the earthquake source 1 Researcher, Snow Avalanche and Landslide Research Center, PWRI, 2-6-8 Nishiki-cho, Myoko-shi, Niigata Prefecture, Japan, 944-51 (*Corresponding Author; Tel:81-255-72-4131; Fax:81-255-72-9629 Email:hasupa55@pwri.go.jp) 2 Director, Snow Avalanche and Landslide Research Center, PWRI, 2-6-8 Nishiki-cho, Myoko-shi, Niigata Prefecture, Japan, 944-51 3 Senior Researcher, Snow Avalanche and Landslide Research Center, PWRI, 2-6-8 Nishiki-cho, Myoko-shi, Niigata Prefecture, Japan, 944-51 4 Engineer, Shikoku Branch, Nippon Koei, 4-4 Marunouchi, Takamatsu-shi, Kagawa Prefecture, Japan, 76-33 5 Director, Research Center for Risk Mangament, National Institute for Land and Infrastructure Management, MLIT, Asahi 1-banchi, Tsukuba, Ibaraki Prefecture, Japan, 35-84 -412-
fault. In this study, to reveal the relationship between scale of earthquake-induced landslides and the earthquake source fault, we interpreted these landslides by using aerial photographs and field investigations. Then we analysed landslides frequency, distribution and scale with the distance from the source fault. Meanwhile, we analysed the characteristics of landslides distribution with the epicentre of the mainshock, seismic intensity and peak ground acceleration (PGA). STUDY AREAS Fig.1 showed the two study areas. These areas located in central Niigata Prefecture, including the focal regions of Fig.1 Location of study areas the earthquake and the -offshore earthquake and those nearby areas, The study area for the earthquake belongs to Uonuma hills which ranging above sea level of 3-7 m, while the study area for the -offshore earthquake belongs to Higashikubiki hills which ranging 1-3 m above sea level. The two areas are characterized by a NNE-SSW directed mountain ranges. The geology of these regions are mainly occupied by from Neogene to Quaternary sedimentary rocks, mainly consists of sandstone, mudstone, siltstone and alternation of sandstone and mudstone. The geological structure of this region is characterized by a series of sub-parallel of NNE-SSE anticlines and synclines (Kobayashi et al., 1995; Yanagisawa et al., 1986). METHOD Showing as Fig.2, we measured the distance from source fault as the shortest distance from the fault rupture. The source fault models used in this study were showed as Table 1. The focal depth is according to JMA. Fig.2 Distance from the source -413-
Table 1 Parameters of source fault models Earthquake Length(km) Width(km) Strike (degree) Dip (degree) Fault Type Focal depth Reference 24 16 N36E 53NW Reverse 13 Hikima &Koketsu (25) -offshore 32 24 N34E 36SE Reverse 17 Koketsu (28) Based on interpretation of landslides from aerial photographs taken immediately after the earthquakes (the case of earthquake is interpreted by MLIT, 25), we measured the maximum length, width of landslides, and calculated distance from the fault rupture and epicentre by using the spatial analysing function of GIS. We also analysed the landslides frequency, envelope of landslides distribution with the seismic intensity and PGA, variation of landslides scale with the distance from the fault rupture and epicentre. CHARACTERISTICS OF LANDSLIDE DISTRIBUTION AND FREQUENCY Landslides distribution Table 2 showed the result of landslides interpretation of the two earthquakes. In the case of the earthquake, a total of 362 landslides interpreted in 1,3 km 2 area (study area A in Fig.1). Total area of landslides is about 5.15 km 2, and ratio of landslide area (landslides area divided by the study area) is about.5%. In the total landslides, 2 landslides occurred inside the rectangle area of the source fault, occupied 55.2% of the whole landslides. A number of 35 landslides, 96.7% of whole landslides occurred on the hanging-wall side of the source fault. In the case of the -offshore earthquake, 18 landslides interpreted in 421 km 2 area (study area B in Fig.1), the landslides area is about.6 km 2, and the ratio of landslide area is about.2%. In the total 18 landslides, 9 were located inside of the rectangle of the source fault. However, whole landslides belong to the hanging-wall side of the source fault because of the footwall located in sea bottom. Table 2 Distribution of landslides induced by the and -offshore earthquake Earthquake total number landslides induced by earthquake rectangle of source fault source fault inside outside hanging-wall foot wall number % number % number % number % 362 2 55.2 162 44.8 35 96.7 12 3.3 -offshore 18 9 5. 9 5. 18 1. - - Landslides distribution with seismic intensity and PGA Fig.3 showed the landslides distribution with seismic intensity (based on JMA) and PGA (based on data of the KiK-NET and K-NET of NIED, 29) of the mainshock of the earthquakes. In the case of the earthquake, landslides located in the area where the -414-
seismic intensity larger than 5-upper (5+ in Fig.3a), and more than 95% of landslides distributed in the area where the seismic intensity larger than 6-lower (6+ in Fig.3a). As showed in Fig.3b, the whole landslides distributed in the area where the PGA larger than 5 gal. However, in the case of the -offshore earthquake, landslides occurred in the area where the seismic intensity larger than 6-lower (6- in Fig.3c) and the PGA larger than 2 gal (Fig.3d). Landslides frequency Fig.4 showed the frequency of maximum length and width of landslides. In the case of earthquake Fig. 3 Landslides distribution with the seismic intensity and PGA of the two earthquakes (Fig.4a), the highest frequency of maximum length is in the range of 1-2 m, included 176 landslides, while the second frequency is in the range of smaller than 1 m (125 landslides). The landslides those maximum lengths are smaller than 2 m occupied 83.1% of the whole number. On the other hand, the highest frequency of maximum width is in the range of smaller than 1 m, included 251 landslides, and the cumulative percentage of the landslide width smaller than 2 m is 92.8% of the whole number of landslides. In the landslides induced by the -offshore earthquake, shown as Fig.4b, the highest frequency of maximum length is in the range of 5-1 m, included 7 landslides. The landslides those maximum lengths are smaller than 1 m occupied 72.2% of the whole Number of landslide 3 25 2 15 1 5 (a) 69.3 125 251 34.5 92.8 83.1 176 85 98.6 95. 43 21 98.9 1 97.8 1 1. 98.9 number of L. number of W. cumulative per. L. cumulative per. W. 4 4 4 1. <1 1-2 2-3 3-4 4-5 5< 12 1 8 6 4 2 15 12 9 6 3 (b)-offshore 61.1 4 11 22.2 9 1. 72.2 7 4 94.4 94.4 1 1. number of L. number of W. cumulative per. L. cumulative per. W. <5 5-1 1-15 15-2 2-25 12 1 8 6 4 2 Cumulatiove percentage(%) Maximum length and width of landslide (m) Maximum length and width of landslide (m) Fig. 4 Maximum length and width frequency of landslides induced by earthquake -415-
number. The whole landslides ranged smaller than 1 m in the maximum width. Compared to the earthquake, the landslides induced by the -offshore earthquake showed fewer in number and smaller in scale. RELATIONSHIP BETWEEN SCALE OF LANDSLIDES AND DISTANCE FROM FAULT RUPTURE To clarify the frequency and size of landslides with the source fault, we analysed the variation of landslides frequency, and scale (maximum length, maximum width, area) with distance from fault rupture. Landslides frequency with distance from the fault rupture Fig.5 showed the frequency of landslides with distance from the fault rupture. In the case of earthquake (Fig.5a), the highest frequency of landslides occurred within the distance of 5-1 km from the fault rupture, included 17 landslides. Within 2 km from the fault rupture, 99.7% of the whole landslides occurred. In the case of -offshore earthquake (Fig.5b), same as the earthquake, highest frequency of landslides occurred within the distance of 5-1 km from the fault rupture, included 8 landslides. The cumulative percentage of landslides occurred within 2 km from the fault rupture is 88.9%. Number of landslide 3 25 2 15 1 5 (a) 77 Number of landslide cumulative percentage 21.3 17 68.2 96.1 11 Fig. 5 landslide frequency with the distance from the source fault 99.7 13 1. <5 5-1 1-15 15-2 2-25 Distance from the fault rupture(km) 1 12 1 8 6 4 2 15 12 9 6 3 (b) -offshore Number of landslide cumulative percentage. 8 44.4 4 66.7 4 88.9 94.4 1. 1 1 5< 5-1 1-15 15-2 2-25 25< Distance from the fault rupture(km) 12 1 8 6 4 2 Cumulatiove percentage(%) Landslides scale with distance from the fault rupture Fig.6 showed the landslides scale with distance from the fault rupture. In the earthquake, the landslide occurred within about 22 km from the fault rupture, while the -offshore earthquake is about 28 km. Showed by envelope of maximum length of landslides with distance from the fault rupture in Fig.6a, the maximum length of landslides that occurred on the hanging-wall side is larger than that on the foot wall (only in earthquake). Another tendency of the both earthquakes is that the maximum length of landslides decreases with increase of the distance from fault rupture. The same tendency is also seen in maximum width and area of landslides with distance from the fault rupture shown as Fig.6b and Fig.6c. -416-
Length of landslides (m) Width of landslide (m) Area of landslides (1 3 m 2 ) 7 6 5 4 3 2 1-3 -25-2 -15-1 -5 5 1 15 2 25 3 (b) 6 -offshore 5 4 3 2 1-3 -25-2 -15-1 -5 5 1 15 2 25 3 (c) 2 -offshore 16 12 8 4 (a) -offshore -offshore -offshore -3-25 -2-15 -1-5 5 1 15 2 25 3 Foot wall Hanging-wall Fig. 6 Variation of landslide scale with the distance from fault rupture F Distance from fault rupture (km) Length of landslides (m) Width of landslide (m) Area of landslides (1 3 m 2 ) 7 6 5 4 3 2 1 4 3 2 1 -offshore 5 1 15 2 25 3 35 (b) 6 5 -offshore 5 1 15 2 25 3 35 (c) 2 -offshore 16 12 8 4 (a) 5 1 15 2 25 3 35 Distance from epicenter (km) Fig. 7 Landslide size with the distance from epicentre A B LANDSLIDES SCALE WITH THE DISTANCE FROM EPICENTER Fig.7 showed the maximum length and width of landslides with distance from epicentre of the mainshock of each earthquake. In the case of the earthquake, landslides occurred within 28 km from the epicentre, the maximum length of landslides decreases with increase of the distance from the epicentre, but larger landslides exist in far from epicentre showing as A in the Fig.7a. Tendency of the maximum width and area of landslides shows as same as the maximum length (Fig.7b and Fig.7c.) However, in the case of -offshore earthquake, landslides occurred within about 3 km from the epicentre, but the tendency of landslides scale decreasing tendency with increase of distance from the epicentre is not clear. DISCUSSION AND CONCLUSIONS Based on above analysis, we revealed some features of landslides triggered by the two earthquakes. Even though the geological and geomorphological conditions are similar, the -417-
number and size of landslides induced by the two earthquakes are different. The fewer number and smaller scale of landslides induced by the -offshore earthquake compared to that by the earthquake (see Table 2, Fig.3), maybe related to the difference of focal depth (see Table 1), because of ground strong motion attenuated with the distance from hypocenter (e. g. Lee et al., 23). Also, source fault and half of the fault rectangle -offshore earthquake located in sea bottom (see Fig.1) maybe another considerable reason responsible for the differences. The result that most of the landslides occurred in the area where PGA larger than 5 gal, seismic intensity larger than 5-upper, coincided with the tendency of landslides induced by earthquakes in Tertiary region in eastern Japan (Abe et al., 26), indicated that larger landslides occur in a limited area related to PGA and seismic intensity. The features of frequency and scale of landslides induced by two earthquakes showed that hanging-wall side of reverse fault is an important factor for earthquake-induced damage prediction including slope failures. The results coincided with previous study of reverse fault earthquakes, such as the Chi-Chi earthquake of Taiwan (Wang et al., 22) and 25 northern Pakistan earthquake (Sato et al., 27). The feature of larger and more landslides occurred on hanging-wall side compared to that on foot wall, maybe related to the hanging-wall/foot wall effects of strong ground-motion by reverse fault earthquake (e.g. Abrahamson and Somerville, 1996). However, the landslides with the distance from epicentre did not clearly show the tendency of landslides scale and frequency decrease with increase of the distance. Consequently, compared to the distance from epicentre, the distance from fault rupture should be used as an index for area setting of earthquake-induced landslide evaluation nearby reverse fault. In this study, the landslides occurred within distance of 28 km form the fault rupture (Fig. 6) and more than 8% of landslides occurred within the distance of 2 km. This result showed that refer to distance from fault rupture, the landslides concentrated in narrower region compared to that distance from epicentre in the cases of moderate magnitude earthquakes. However, larger area could be estimated in the case of landslides induced by larger magnitude earthquakes (e.g. Keefer, 1984). According to results of analysis of landslides with the distance from fault rupture, we concluded as below. 1) In the two cases of reverse fault earthquake, most landslides occur on the hanging-wall compared to that on the footwall. 2) The scale of landslides decreases with increasing of the distance from fault rupture. ACKNOWLEDGMENT The authors gratefully acknowledge the Yuzawa Sabo Office, MLIT, Japan, for providing landslides data sets of the earthquake. REFERENCES Abe, S., Takahashi, A., Ogita, S., Komatsu, J., Moriya, H. and Yoshimatsu, H. (26). The geological and geomorphological characteristics of landslide topography and seismic intensity of earthquake in Tertiary region, Journal of the Japan Landslide Society, Vol.43 (3): 155-162. (In Japanese) Abrahamson, N. A. and Somerville, P. G. (1996). Effects of the hanging-wall and footwall on -418-
ground motions recorded during the Northridge earthquake, Bull. Seismo. Soci. America, Vol.86 (1B): pp.s93-99. Hikima K. and Koketsu K. (25). Rupture processes of the 24 (mid-niigata prefecture) earthquake, Japan: A series of events in a complex fault system, GRL, Vol.32 (L1833): 1-5. Keefer, D. (1984): Landslides caused by earthquakes Geological Society of America Bulletin, Vol. 95, pp.46-421. Kobayashi, I., Tateishi, M., Yoshimura, T., Ueda, T. and Kato, H. (1995). Geology of the Kashiwazaki district, with geological map at 1:5,, Geological survey of Japan, pp.12 (in Japanese with English abstract) Koketsu, K. (28). Profile of the source fault of -offshore earthquake, Niigata Prefecture, 27, Seismo, Vol. 12(2): 6-7.. (In Japanese) Lee, W. H. K., Kanamori, H., Jennings, P. C. and Kiinger, C (23). International handbook of earthquake and engineering seismology, International Association of Seismology and Physics of the Earth's Interior, Part B, Academic Press, pp.1 MLIT (25), Slope failures and of the earthquake, Niigata Prefecture, 24, in homepage of Yuzawa Sabo office, MLIT: http://www.hrr.mlit.go.jp/yuzawa/sabo/chuetsu/pamphlet/pamphlet1.pdf MLIT (27). Information of slope failures by -offshore earthquake, Niigata Prefecture, homepage of Sabo Division, MLIT: http://www.mlit.go.jp/river/sabo/h19dosha/7gatu/niigatajishinnsaigaijyouhou7315.pdf NIED (29. The Strong motion measurement net of NIED(K-NET, KiK-net)homepage, http://www.kyoshin.bosai.go.jp/kyoshin/quick Sato, H. P., Hasegawa, H., Fujiwara, S., Tobita, M., Koarai, M., Une, H. and Iwahashi, J. (27). Interpretation of landslide distribution triggered by the 25 Northern Pakistan earthquake using SPOT5 imagery, Landslides, Vol.4: 113-122. Sato, H. P., Koarai, M., Une, H., Iwahashi, J., Miyahara, B. and Yamagishi, M. (28). GIS analysis on geomorphological features of slope failures triggered by the Niigata -oki earthquake in 27, Journal of the Geographical Survey Institute, No.114: 91-12. (In Japanese) Wang, W. N., Nakamura, H., Tsuchiya, S., Chen, C. C. (22). Distributions of landslides triggered by the Chi-chi earthquake in Central Taiwan on September 21,1999, Journal of the Japan Landslide Society, Vol.38 (4): 318-326. Yagi, H., Yamazaki, T. And Atsumi, M. (27). GIS analysis on geomorphological features and soil mechanical implication of landslides caused by 24 Niigata earthquake, Journal of the Japan Landslide Society,Vol.43(5): 294-36. (In Japanese with English abstract) Yanagisawa, Y., Kobayashi, I., Takeuchi, K., Tateishi, M., Chihara, K. and Kato, H. (1986). Geology of the Ojiya district, with geological map at 1:5,, Geological survey of Japan, pp.177 (In Japanese with English abstract) -419-