Comparison between the two triggered landslides in Mid-Niigata, Japan by July 13 heavy rainfall and October 23 intensive earthquakes in 2004

Recent DOI 10.1007/s10346-007-0093-0 Received: 19 January 2007 Accepted: 30 August 2007 Springer-Verlag 2007 Hiromitsu Yamagishi. Junko Iwahashi Comparison between the two triggered landslides in Mid-Niigata, Japan by July 13 heavy rainfall and October 23 intensive earthquakes in 2004 Abstract On July 13, 2004, heavy rainfalls because of the intensive activities of the rain front occurred in the Mid-Niigata Region, Japan. They were as much as 400 mm in 24 h, bringing about serious flooding by breaking the river banks. The heavy rainfalls also triggered more than 3,500 landslides. Three months later, the southern region of Mid-Niigata was attacked by an earthquake of magnitude 6.8 on the Richter scale on October 23, 2004. The main earthquakes were followed by intensive aftershocks, which continued until December 2004. By these earthquakes, variable landslides of more than 4,400 also occurred in the hilly and mountainous areas. Namely, different triggers brought about the variable landslides in the hilly mountains whose features are very similar in geological and geomorphological points of view. Therefore, these two events are very useful for clearing the difference in features of the landslides between the two. We have been researching on both landslides in the field just after both occurred and later analyzing air photographs using the geographic information system (GIS). In this paper, we describe the comparison in the distribution features using GIS analytical data between the heavy rainfall-induced and the intensive earthquake-induced landslides. Keywords. Heavy rainfalls. Intensive earthquakes. GIS analyses. Mid-Niigata Introduction In 2004, ten typhoons landed, and heavy rainfalls gave serious damages throughout Japan. In particular, the Mid-Niigata region (Fig. 1a) was damaged by not only the July 13 heavy rainfalls but also the October 23 intensive earthquakes (Fig. 1b). The heavy rainfalls claimed the lives of 13 people by flooding of the two rivers Kariyata-gawa and Ikarashi-gawa, both of which are branch tributaries of the Shinano-gawa (the longest river in Japan). They also claimed two human lives by the landslides, which counted more than 3,600 (Yamagishi et al. 2005). Three months later, intensive earthquakes attacked the southern area (Fig. 1b) of the heavy-rainfall areas (Fig. 1a). A total of more than 60 persons were killed directly and indirectly by the earthquakes. It was an outstanding characteristic that very strong shaking attacked the hilly Tertiary mountainous areas, which occupy typically the west part of northeast Japan. Namely, important lifelines such as roads and rice fields within hilly mountainous areas were seriously damaged. These earthquakes also triggered many and several types of landslides (Sassa 2005; Sato et al. 2005 and others). It is also characteristic that both of the landslides caused by the heavy rainfalls and earthquakes attacked the similar geological and geomorphological conditions. In this paper, therefore, we discuss the comparison in patterns and landslide density between the different-triggered landslides using the geographic information system (GIS) and associated technology. On comparison, we have analyzed the landslide distribution in the areas affected by the July 13 heavy rainfalls (blue line area in Fig. 2) and the October 23 intensive earthquakes (red line in Fig. 2) by interpretation of aerial photographs taken after each event covering the areas abovementioned. Precipitation on July 13, 2004 According to Ushiyama (2004), the rainfall began in the evening of July 12, 2004, and reached its peak in the early morning of July 13. Figure 3 presents the heavy rainfall area on July 12 to 13 when flooding and landsliding took place. The maximum rainfalls during the 24 h by Automated Meteorological Data Acquisition System and other stations in Tochio City was recorded to be 421 mm. Landslide patterns and distribution by heavy rainfall on July 13, 2004 Many landslides have been inventoried through aerial photographs of Asia Aerial Service (Yamagishi and Ayalew 2005). Particularly, the Izumozaki (Nishiyama Hills) and Tochio areas (Higashiyama Hills) recorded in total more than 3,000 landslides. In particular, the Izumozaki area is characterized by more than 500 landslides recorded. Most of them are shallow-seated slides that are subdivided into two types: the planar type (Fig. 4a) and spoon type (Fig. 4b). In addition, some of the slides are deep-seated ones, most of which are associated with mudflows spreading long from the scarps (Fig. 5a). In addition, deep-seated slides with debris stopped at the foot of the scarp are recognized in places (Fig. 5b). Within these landslides, we have been checking about 10% sites of the landslides and mudflows by actual researching using the researching card (Fig. 6) for each landslide site (Yamagishi and Ayalew 2005). In this format, we checked the style of failure, scar pattern (surface and rock slides, flow sliding, gully erosion), type of slip plane (planar type, spoon type,), initiation point above or below knick points (erosion front), scale (width, depth, length), original slope inclination, geology (rock facies, structure, surface materials), shape of the head of the landslide in plan and crosssectional views, characteristics of the original slope (terrace scarp, old slide, valley wall, basin flank, sea cliff, artificial slope), slope vegetation (artificial, natural), prevention facilities, hydrology (water discharge), deposited materials (rock, gravel, sand), and consequent damages. The accumulated collection data according to the format allow building statistical relationships in Yamagishi and Ayalew (2005). For an example of the analyses, the scar widths are found to have a linear relationship with the corresponding length and depth of scars (Fig. 7). GIS analyses of the landslides caused by the July 13 heavy rainfalls We have made basic data by converting to the vector data models from the inventory maps of the landslides and mudflows inter-

Fig. 1 Location maps of the Mid Niigata area. Affected area by the 2004 July 13 heavy rainfalls (left), affected area by the 2004 October 23 intensive earthquake (right) Recent preted from colored air photographs (Asia Air Survey, 1:8,000 on July 23th, and 1:15,000 on July 24th, both taken after the landsliding). As the base digital map, we used the digital elevation model (DEM) of GISMAP (Hokkaido Chizu), which is a 10-m-grid. Figure 8 shows the July 13 landslide distribution on the relief map from the DEM of 10-m grid of Izumozaki area. It was made from Spacial Analyst of ARCVIEW9.1. This figure indicates that most of the July 13 landslides are concentrated in the western steep slopes of a NNE SSW-trending ridge. The ridge is corresponding to an asymmetric geological anticline. Namely, the western wing shows steep dipping of Miocene strata, whereas the eastern wing shows gentle dipping strata (Kobayashi et al. 1993). Into Fig. 8, We added the old deep-seated landslide distribution maps (1:50,000) of Nagaoka and Takada provided by National Research Institute for Earth Science and Disaster Prevention (NIED), which are available from the website (http://www.bosai.go.jp/e/dosya. htm). Such deep-seated landslides are in the eastern gentle slopes as shown in Fig. 8. Landslide concentration and lithofaices In terms of lithofaices, the Izuozaki area (Nishiyama Hills) and Tochio area (Higashiyama Hills) are composed mostly of mud- Fig. 2 Researching areas of this paper. Blue lines encircles the July 13 rainfallinduced landslide areas, and red lines encircle the October 23 earthquake-induced landslide area Fig. 3 Map showing the precipitation (within 2 days) contour map from AMEDAS and other observation stations using GIS

Fig. 4 Shallow slide examples from Nishiyama Hills. a Planar-type surface failure in Washima Beach, b spoontype failure in Aida, Izumozaki (courtesy of Nakanihon Air Service) stone and sandstones both of which make up relatively hard Teradomari Formation in the lower and relatively soft mudstone and silt stones of the Pliocene to Pleistocene (Kobayashi et al. 1993). Most of the July 13 landslides are inventoried in the mudstone-rich zone rather than sandstone-rich zone (Fig. 9a). Figure 9b shows that the landslide numbers in the mudstone areas are much more than those in the sandstone areas. Comparison in concentration in 1961 and 2004 In the Izumozaki area (Nishiyama Hills), nearly the same precipitation induced the same number of landslides, which were inventoried by the 1962 air photograhs (1:20,000, black and white) shown in Fig. 10. The landslide concentration map indicates that the landslide distribution in 1961 does not coincide with that in 2004. Namely, in 2004, the landslides tended to occur in the other slopes of the 1961 landslides. The October 23 intensive earthquakes In the evening on October 23, intensive earthquake mostly attacked the Yamokoshi area, which is in southern area of the Tochio area damaged by July13 heavy rainfalls. The hypocenter of the main shock was located in 37 17 4 N in latitude, 138 52 2 E in longitude with a depth of 13 km and showed magnitude of 6.8 on the Richter scale (Fig. 11). In Kawaguchi town, south of the Yamakoshi area, the earthquake marked seismic intensity 7 of the Japanese Meteorological Agency scale, which is the maximum degree of this scale in Japan. The main shock was followed by a number of large aftershocks within 2 h after the main shock. The maximum acceleration exceeded 1,000 gal, 7 km away from the epicenter. These earthquakes were related to geological folding structures trending in NNE SSW directions, some of which are associated with active faults and folding (Hirata et al. 2005; Yanagisawa et al. 1986; Kobayashi et al. 1991). Landslide patterns and distribution caused by the intensive earthquakes on October 23, 2004 By the earthquakes, many landslides were induced (Figs. 12 and 13) because the earthquake area is the hills and mountains. According to Sekiguchi and Sato (2006), total major landslides are counted to be more than 4,400. The air photograph interpretation and field researching have revealed that the landslides are classified into the following three types as shown in the reference by Yamagishi et al. (2005). By these large-scale landsliding, along the Imogawa tributary, more than ten landslide dams were formed. We have classified the landslides into (1) deep-seated slides (Fig. 12a), (2) long-run slide (Fig. 12b), and (3) shallow slides (Fig. 12c). According to Sekiguchi and Sato (2006), the deep-seated landslides are more than 10 m deep and several tens of meters wide and long and reached to 4.5% in the total landslides. The long-run slides are characteristic of flowing down as debris flow or mudflow from ponds and along rivers up to 1 km. However, this type occupies only 1.4% of the total landslides. Finally, the shallow slides, which are less than 10 m deep and several meters to tens of meters wide, are much more small scale compared with the other two types. However, this type occupies more than 92% of the total landslides. GIS analyses of the landslides caused by the October 23 intensive earthquake Geomorphologically, the Yamakoshi area is mountainous areas of 200 to 600 m high, and between the ridges, many gentle slopes Fig. 5 a Deep-seated slide whose debris spread overlaying the road. b Deep-seated slide whose debris moved downward but stopped at the foot

Landslide Research Card Name of researcher Sketching Site Name: No: Generating date Y M D Map Name: Research date Y M D (day of Week) Outline of failure Scar Shape of failure Pattern Type of surface failure Scar Yes1 No0 1Surface failure 1Spoon type Channel Yes1 No0 2Rock failure 2Planar type Erosion Yes1 No0 3Rapid sliding 3Others Deposit Yes1 No0 4Gully erosion Generation point Scale Inclination of slope Geology Erosion front Width m 1gentle 2steep Rock facies 1upper Depth m Original slope Structure 2just on Length m Failure slope Surface material 3lower Original slope Slope Vegetation Shape of head in plan Shape of head in cross Prevention 1Terrace scarp 1Sea cliff 1Artificial 1Amphitheater 1concave 1Yes 2No 2Old slide 2Artificial slope 2Natural 2Quadrangle 2planar Facilities 3River head 3Valley slope 1Sabo dam 4Flank slope 4Others 2Gabion 3Other Hydrology Deposition type Volume of Deposition Deposition materials Water discharge 1Flow Type Width 1Rock 4Sand 1Yes 2No 2Slide type Thickness 2Gravel 5Wood Length 3Mud 6Others Damage 1House 2Road 3Tunnel 4Rice field 5Railway 6Railway Comment Recent Fig. 6 Research card for checking the heavy rainfall landslides in 2004 Fig. 7 Graph showing the length (red square) and depth (blue triangle) vs width of each landslide, resulted from the researching cards Fig. 8 Map showing the distribution of the July 13 rainfall-induced landslides (red dots) and deep-seated landslides (yellow spots) showing the Landslide database of NIED

Fig. 9 a July 13 landslide distribution on the outline geologic map, b graph showing the bars of the numbers of shallow landslides in the sandstone and mudstone area (modified from Kobayashi et al. 1993, using GIS) and areas of the sandstone and mudstone (mostly old deep-seated landslides) are occupied by rice fields, some of which are carp-breeding ponds. Geologically, the Yamakoshi area is composed of Miocene to Pliocene sedimentary rocks likely of the Izumozaki and Tochio area. GIS analyses have revealed that most of the landslides took place in the Yamakoshi areas compared to the southern areas including Kawaguchi town, which was documented to be having the highest number of earthshocks (Fig. 13). The map in Fig. 13 shows that most of the landslides geomorphologically distributed not only in the steep slopes but also in the gentle slopes. However, geologically, the large-scale landslides are concentrated in the gentle geological-dipping strata (derived from the geological maps of 1:50,000 in scale) areas rather than steep-dipping strata zone (Fig. 14). In terms of the geological lithologies, the landslide numbers by the October 23 earthquakes were found much more in the sandstone-rich zone rather than in the mudstone-rich zone (Fig. 15). the comparison between the two in the following: (1) The July 13 landslides do not necessarily coincide with the old deep-seated landslides derived from the website of NIED. Most of the landslides are small in scale and have shallow-seated characteristics. They are Comparison between the two triggered landslides GIS using analyses of the two triggered landslides revealed the similarity and differences as mentioned above. We are describing Fig. 10 Photograph showing landslide distribution in 1961 and 2004. Notice that the landslide concentration area in 2004 does not coincide with that in 1961 Fig. 11 Map showing the distribution of the epicenters of the October 23 earthquakes in 2004 and the tectonic faults and folds in the geologic maps (Active Fault Center 2004, http://unit.aist.go.jp/actfault/niigata/map.html

Recent Fig. 12 Landslide patterns of the earthquake-induced landslides in the Yamakoshi area. a Deep-seated slide by dip slipping, b long-run slide transforming from landslide to debris flows or mudflows, and c shallow-seated landslide distributed in steep slopes geomorphologically and in steepdipping areas of the strata of the sediments of a wing of an anticline (Kobayashi et al. 1993). Geological lithology and landslide distribution The landslides induced by the July 13 rainfalls are lithologically found in the mudstone-rich zone rather than the sandstone-rich zone (Fig. 9). In addition, the distribution is not consistent with the old deep-seated landslides by NIED (Fig. 8). Whereas, most of the October 23 deep-seated landslides are distributed in the gentle slope geomorphologically (Fig. 13) and gentle dipping strata geologically (Fig. 14), although most of the shallow-seated landslides occurred frequently on the steep slopes compared with the heavy rainfall landslides (Fig. 17). In particular, the recent earthquake-induced deep-seated slides are mostly distributed in Fig. 13 Landslide distribution of the all landslides of Damage map of Geographical Survey Institute of Japan (Suzuki et al. 2005) Fig. 14 Map showing the distribution of the three type landslides on the geologic strata dipping contour map derived from Yanagisawa et al. (1986) and Kobayashi et al. (1991). Green color: 2 24, orange color: 24 46, violet color: 46 68, white color: 68 90

density by the earthquakes is higher at more acute ridges and deeper-cut valleys. While the landslide density by the heavy rainfalls is higher at deeper-cut valleys, however, on the ridges, the density is low. On the other hand, landslide density at each profile curvature (Fig. 16b; number/km 2 : omitting categories less than 0.5 km 2 ) shows that the landslide density by the earthquakes depends on concavity of slopes and is higher at more uneven slopes. The landslide density by the rainfalls is almost constant on concaved slopes; however, it is high with the more convex slopes (Fig. 16b). On the whole, the landslide density of the earthquakes is higher than that of the heavy rainfalls in both of the plan and profile curvature. We have compared the landslide density against slope gradients between the two triggers (Fig. 17). As the results, in case of the numbers of the landslides within the grids cells of 100 to 1,000, the density (numbers of the landslides per km 2 ) of the October 23 earthquake-induced landslides increases exponentially with the slope gradient, while the density of the July 13 heavy rainfallinduced landslides keeps constant with the slope gradient in steep slopes. Namely, the difference between the two graphs is increasing with slope gradients. However, at more than 40, both of the landslide density shows large variation because the numbers of the steep slopes are very few. Fig. 15 Landslide distribution in the sandstone-rich zone and mudstone-rich zone in the Yamakoshi area. a Total landslides in the both of the sandstone and mudstone zone (modified from Yanagisawa et al. 1986; Kobayashi et al. 1991). b Graph showing the landslide area percent in the sandstone (yellow) and mudstone (mudstone) the sandstone-rich zone rather than the mudstone-rich zone lithologically (Fig. 15). In addition, they are in places coincided with the old deep-seated landslides shown by NIED. Shallow landslide density for curvatures and slope gradient In this chapter, we focus on shallow landslides using the two detailed data: one is the data of 1:25,000-scale damage maps prepared by GIS (for shallow landslides by the earthquake), and the other is interrelated by Asia Air Survey (for shallow landslides by the rainfalls). Areas of analysis are shown in Fig. 2. In terms of slope shapes, the landslides by rainfalls were much more affected than those by earthquakes. The landslide densities because of the earthquakes are higher at nick points, whereas those by the rainfalls are high on the upper slopes of valleys and in the valleys and are low on the ridges (Iwahashi et al. 2006). Landslide density at each plan curvature (Fig. 16a; number/km 2 : omitting categories less than 0.5 km 2 ) shows that the landslide Summary and conclusion In 2004, moderate mountain areas in Mid-Niigata region, Japan, were damaged by the two triggering landslides; on July 13, heavy rainfalls up to 430 mm within 2 days induced more than 3,600 landslides, and on October 23, intensive earthquakes triggered more than 4,400 landslides (Sekiguchi and Sato 2006). Both of the areas have similar topography and geology namely, the hilly mountains up to 600 m in elevation, and Neogene sedimentary rocks such as sandstone and mudstones are characteristic compositions. So far, the landslides caused by heavy rainfalls and intensive earthquakes were checked through air photographs and research in the field. As the results, the patterns of landslides by the heavy rainfalls were classified into shallow- and deep-seated slides with mudflows, and the landslides by the earthquakes were classified into three types: deep-seated slides and slumps mostly by dip slipping and flow-type slides transforming into debris flows or mudflows. To compare the characteristics of landslides between the two triggers, the two events are valuable for the research of landslide distribution and densities and relationships to slopes and geological lithologies because both of the two areas are similar in geomorphological and geological settings. We have done first of all field work for the July 13 landslides using researching cards (Yamagishi and Ayalew 2005), and then we have researched the landslides caused by the October 23 landslides. Later, we have interpreted both of the landslides through the air photographs of both events and analyzed these data using GIS techniques. In particular, we compared the landslide distribution in 2004 with that in 1961 by air photographs of the Nishiyama Hills (Izumozaki area). Finally, the comparison between the two triggering landslides has revealed that the landslides by the heavy rainfalls tended to occur in the other slopes than the 1961 landslide areas, and the July 13 landslides were geomorphologically found with a lower density (number/km 2 ) than those by the earthquakes, in relation-

Fig. 16 Figures showing the shallow landslide density to plan and profile density. a Landslide density at plan curvature (m), b landslide density at profile curvature (m) Recent ship to slope gradients and curvatures using GIS. In particular, the landslide density by the rainfalls keeps constant with the slope gradients. Geologically, the landslides by the heavy rainfalls were mostly concentrated into the mudstone-rich zones rather than the sandstone-rich zones. Whereas, those by the intensive earthquakes were more concentrated in the sandstone-rich zones rather than mudstone-rich zones. Acknowledgment We are grateful to Niigata Prefecture and Asia Air Survey, Asahi Koyo, and Naka-Nihon Air for providing air photographs and interpretation data, and thanks to Niigata University for providing funds for researching and getting much information. We thank also Dr. T. Komatsubara of the Geological Survey of Japan (GSJ) in Advanced Industrial Science and Technology (AIST) for providing us with digital data of the geological maps and useful comments on geological correlation. We are also grateful to Dr. K. Kawashima of the Research Center for Natural Hazards and Disaster Recovery, Niigata University, for providing useful precipitation data of many stations. Finally, we appreciate Dr. H. Ochiai for valuable reviewing. References Fig. 17 Graph showing the landslide density comparison between the earthquake- and rainfall-induced landslides Active Fault Research Center, National Institute of Advanced Industrial Science and Technology (2004). Available at: http://www.gsj.jp/jishin/chuetsu1023/index.html Hirata N, Sato H, Sakai, Sakai S, Kato A, Kurashimo E (2005) Fault system of the 2004 Mid Niigata Prefecture Earthquake and its aftershocks. 2:153 157 Iwahashi J, Sato HP, Yamagishi H (2006) The spatial distribution of shallow landslides caused by the Mid Niigata Prefecture Earthquake in 2004. J Geogr Survey Inst 110:81 89 (in Japanese; http://www.gsi.go.jp/report/jiho/vol110/9.pdf) Kobayashi I, Tateishi M, Yoshioka T, Shimazu M (1991) Geology of the Nagaoka district. With Geological Sheet Map at 1:50,000. Geol. Surv. Japan, 132p (in Japanese with English abstract 6p) Kobayashi I, Tateishi M, Uemura T (1993) Geology of the Izumozaki district. With Geological Sheet Map at 1:50,000. Geol. Surv. Japan, 91p, (in Japanese with English abstract 4p) Sassa K (2005) Landslide triggered by the 2004 Mid-Niigata Prefecture earthquake in Japan. 2:135 142 Sato H, Sekiguchi T, Kojiroi R, Suzuki Y, Iida M (2005) Overlaying landslides distribution and topographical data:the Mid Niigata prefecture earthquake in 2004, Japan. 2:143 152 Sekiguchi H, Sato H (2006) Feature and distribution of landslides induced by the Mid Niigata Prefecture Earthqake in 2004, Japan. 43:142 154, (in Japanese with English abstract)

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