Preliminary investigation of some large landslides ... - Springer Link

Recent Landslides Landslides (2009) 6:47–54 DOI 10.1007/s10346-009-0141-z Received: 4 August 2008 Accepted: 7 January 2009 Published online: 4 February 2009 © Springer-Verlag 2009

Fawu Wang . Qiangong Cheng . Lynn Highland . Masakatsu Miyajima . Huabin Wang . Changgen Yan

Preliminary investigation of some large landslides triggered by the 2008 Wenchuan earthquake, Sichuan Province, China

Abstract The Ms 8.0 Wenchuan earthquake or “Great Sichuan Earthquake” occurred at 14:28 local time on 12 May 2008 in Sichuan Province, China. Damage by earthquake-induced landslides was an important part of the total earthquake damage. This report presents preliminary observations on the Hongyan Resort slide located southwest of the main epicenter, shallow mountain surface failures in Xuankou village of Yingxiu Town, the Jiufengchun slide near Longmenshan Town, the Hongsong Hydro-power Station slide near Hongbai Town, the Xiaojiaqiao slide in Chaping Town, two landslides in Beichuan County-town which destroyed a large part of the town, and the Donghekou and Shibangou slides in Qingchuan County which formed the second biggest landslide lake formed in this earthquake. The influences of seismic, topographic, geologic, and hydro-geologic conditions are discussed. Keywords Landslide . Earthquake . Sichuan earthquake . China . Disaster Introduction The 2008 Sichuan earthquake, or “Great Sichuan Earthquake” occurred at 14:28:01.42 CST (06:28:01.42 UTC) on 12 May 2008 in Sichuan Province, China. Its estimated magnitude according to the Chinese Earthquake Administration was Ms 8.0, Mw 8.3 (Mw 7.9 according to the US Geological Survey). It is also known as the Wenchuan earthquake, after the earthquake’s epicenter in Wenchuan County, Sichuan Province. The epicenter (30.986° N, 103.364° E) was 80 km west-northwest of Chengdu, the capital city of Sichuan, at a hypocenter depth of 19 km. Official statistics (as of 6 July 2008) list 69,197 confirmed dead, including 68,636 in Sichuan Province, 374,176 reported injured, and 18,340 listed as missing. It is the deadliest and strongest earthquake to hit China since the 1976 Tangshan earthquake, which killed at least 240,000 people. On 25 May, a major aftershock of Mw 6.0 occurred northeast of the original earthquake’s epicenter, in Qingchuan County, causing eight deaths, 1,000 injuries, and destroying thousands more buildings. On 27 May, two more major aftershocks, Mw 5.2 in Qingchuan County and Mw 5.7 in Ningqiang County in Shaanxi Province, collapsed more than 420,000 homes and injured 63 people. Aftershocks were distributed along the Longmenshan active fault system extending more than 300 km northeast from the main shock (Fig. 1). This fault system is within the mountain front between the Chengdu basin and the Tibetan plateau and because of this, earthquake-induced landslides severely hampering rescues and relief and were a significant contribution to the total earthquake damage, causing about 15% of the earthquake deaths.

From 12 to 16 June, 1 month after the main shock, some of the authors investigated some of the damage in the area (Fig. 2). From 5 to 9 July, we investigated this area again. Only those landslides that could be safely and easily accessed were observed because road access to some areas was still blocked by landslides. Figure 2 is an index map of landslides observed and described in this report. The investigated landslides included the Hongyan Resort slide located southwest of the main epicenter, shallow mountain surface failures in Xuankou village of Yingxiu Town (the most damaged area), the Jiufengchun slide near Longmenshan Town, the Hongsong Hydropower Station (HPS) slide near Hongbai Town, the Xiaojiaqiao slide in Chaping Town, Beichuan County slides which destroyed a large part of the county-town, the Donghekou slide and Shibangou slide in Qingchuan County which formed the second biggest landslide lake from this earthquake. Comparing Fig. 2 to Fig. 1, it is notable that the investigated landslides are distributed nearly linearly along the line of epicenters of the mainshock and aftershocks. Jurassic and Cretaceous strata are overlain by Quaternary alluvium in the Chengdu basin to the southwest of the Longmenshan active fault system (Fig. 3). On the northwest or mountainous side of the Longmenshan active fault, granite is mapped near the epicenter of the main shock in Wenchuan County. Northeast of the granite on the northwest side of the Longmenshan active fault, a compressed and fragmented sequence of Silurian sedimentary rocks is distributed to beyond Guangyuan City. Devonian strata can be found near Jiangyou City, and Cambrian strata can be found in Qingchuan County. During both investigations, we observed that the landslide topography and morphology were being rapidly changed by road reconstruction, and water levels in landslide-dammed lakes were decreasing. This report gives our initial impressions (including topographic, geological, and runout characteristics) and some brief information to those who want to further investigate landslides triggered by this great earthquake. The following general observations were noted in the investigated area: (1) distance of landslides from the main-shock epicenter increases from southwest to northeast. This may provide information on seismic ground-motion variability; for example, at Wenchuan County near the epicenter, the vertical acceleration component may be greater than the horizontal one, while in Beichuan and Qingchuan County at the northeast part, the horizontal component may be greater than the vertical one. (2) Granite is mapped in Wenchuan County and the nearby area, while slate and mudstone, sandstone and limestone are distributed in Beichuan County and Qingchuan County. In this paper, we describe the landslides in sequence, from southwest to northeast, so that the influences of geology and seismic conditions on landslide distribution and characteristics can be identified.

Landslides 6 • (2009)

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Recent Landslides Fig. 1 Epicenter distribution of the main shock (star) and aftershocks (sizecoded circles) of the 2008 Sichuan earthquake. The map shows locations of aftershocks that occurred through 28 May 2008

Hongyan Resort slide Hongyan Resort is in the southwest of Wenchuan County. It is associated with Dujiangyan City, famous for the world-heritage Dujiangyan Irrigation System (DIS). The DIS lies on the Minjiang River in the northwest part of Dujiangyan City. It was built around 250 B.C. under the direction of Li Bing, the governor of the Shu Prefecture of the Qin State. Also, another beautiful world-heritage site, “Mt. Qingchengshan,” lies on the east side of the Hongyan Resort. Because the temperature is lower here in summer than in Chengdu City, Hongyan Resort is one of the more popular tourist destinations from Chengdu. Figure 4 shows a landslide at 103.480° E, 30.919° N (GPS coordinates) which destroyed a local road at Hongyan Resort. The sliding mass was dry debris that originated from weathered sandstone and conglomerate. The angle of repose of the landslide deposits was about 30°. It is reported that the landslide occurred almost simultaneously with the main shock. Forty-seven tourists from Chengdu Old Man Association and four local people were buried by the sliding mass as they walked along the road. The white dotted lines show the former location of the road connecting to a bridge, before the landslide buried it. Because the road was buried so deeply, it 48


was not slated for cleaning and rebuilding (at the time of our investigation), and a new road had been built for local transportation. From evidence seen around the gully where the landslides occurred, such as landslide topography on the nearby mountain and the area around the gully itself, it was recognized that all of the area is associated with an ancient landslide. Hongyan Resort is built on an ancient landslide. The long run-out debris of the old landslide provided gentle slopes that were easier to develop than the surrounding steeper hill slopes. The ancient landslide may have been triggered by a past great earthquake and formed a landslide dam. A gully with steep sides had formed through the former landslide dam by weathering and erosion over a long period of time. During the main shock, both sides of this gully collapsed, and the debris hit the tourist group. Widespread shallow failures in Xuankou village of Yingxiu Town Yingxiu Town was severely damaged in the main shock, with most of the old buildings damaged. Xuankou village at 103.478° E, 30.994° N, is located east of Yingxiu Town. Figure 5 illustrates the many mountain failures that were triggered by the earthquake. All of the newly exposed areas on the mountains are landslides; this

Fig. 2 Index image showing locations of landslides investigated by the authors

area had a very good vegetation cover before the earthquake. The blue roofs are rescue tents built after the earthquake. Some tents and buildings are located at the bottom of a gully (right in the photo). This location will be exposed to a high risk of damage in the upcoming rainy season when debris from the collapsed slopes may remobilize in debris flows.

Fig. 3 Geological map of the epicenter areas of the main shock and aftershocks. Q Quaternary system, J1-2 Lower and Middle of Jurassic system, J3 Upper layer of Jurassic system, K1 Lower layer of Cretaceous system, S Silurian system, γ granites. The complicated strata between S and K1 in the figure are from Cambrian system to Tertiary system

Jiufengchun slide Jiufengchun village is located between Longmenshan Town (near the Chengdu basin) and Yinchanggou Resort, a large resort valley in the mountains. Figure 6 is a series of photos of a landslide composed of weathered granite at 103.848° E, 31.294° N, in Jiufengchun village. Figure 6a is the front part of the slide which cut local traffic from Longmenshan Town to Yinchanggou Resort. Before the earthquake, more than 17 families lived along the road. As soon as the earthquake occurred, the landslide ran out from the mountain (top and center, Fig. 6b), burying more than 60 local people and 30 tourists. A witness said that all of this occurred in about a minute. Fig. 6a shows debris (pillars and boards) from destroyed and displaced wooden houses. Figure 6b shows the slide travel path. Landslide debris filled the valley bottom between two

Fig. 4 The Hongyan Resort slide caused 51 casualties. The dotted lines indicate the former location of a road


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Recent Landslides

Fig. 5 Shallow surface failures on mountain slopes in Xuankou village of Yingxiu Town

mountain ridges and the debris surface was gently sloping, suggesting rapid movement. The lower valley walls were stripped of vegetation. The contact line between vegetation on the upper slopes and bare ground on the lower slopes may indicate the maximum height reached by the debris during movement. Figure 6c shows the central area of the debris from the toe of the landslide. The road (with cars passing by) was rebuilt after the earthquake. The width of the landslide at this part was about 300 m. The distance from the toe to the source area of the original failure (the mountain at the end in Fig. 6b) was estimated to be about 1.5 km (the distance exceeded the ∼1 km range of our laser measuring

Fig. 6 The Jiufengchun slide. (a) The front part of the slide cutting the local road; (b) view of the source area at the middle of the slide; (c) view of the rear part of the slide, looking from the toe; (d) the river was dammed by the slide, forming a

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tool). Figure 6d shows the toe of the landslide which partially dammed a river originating in Yinchanggou that flows into the Chengdu basin. It is interesting that the landslide reached the river but did not block it. Our questions at this landslide were: (1) why did the dry weathered granite (presumably it has a high permeability) move so rapidly and travel for such a long distance; (2) why did the landslide just reach the river and stop? To answer the first, we searched in the middle of the slide mass and found a groundwater spring (seen in Fig. 6e). It suggested to us that while the sliding mass may have been dry, the sliding zone was probably saturated. When the original failure triggered by the strong main shock occurred, the sliding mass perhaps loaded suddenly onto a saturated layer, and formed an undrained shear. It could not have traveled further because the weathered granite has high permeability and any excess pore pressure generated may have dissipated during runout. Such phenomenon can be explained by the undrained loading model proposed by Sassa et al. (1996) when landslides triggered by the Hyogoken-Nanbu earthquake were studied, but there may be other explanations. For the second question, we examined the prior topography of the area recorded on a map made before the recent earthquake. Figure 6f shows that the original road was straight at the site of the 2008 landslide, but the contours between the road and the river indicate that the river may have been deflected and/or dammed at this site in the past. Thus, based on the topography, it is estimated that the landslide may have recurred in the area of a past landslide, and the runout distance of the 2008 event corresponds well with that of the previous landslide. Also, in Fig. 6f, a water line was shown in the centre of the landslide area, indicating that at least the

lake; (e) groundwater found at the middle of the slide; (f) map of the landslide area before the earthquake

Fig. 7 The Hongsong Hydro-Power Station slide

central area of the travel path could have been saturated before and at the time of the earthquake. Hongsong Hydropower Station slide Figure 7 is an oblique view of the Hongsong HPS slide at 104.017° E, 31.401° N, in Songlin Village, Hongbai Town. A channel has been cut through the toe of this landslide to lower the water level in a landslide-dammed lake and allow the river to resume normal flow. A local railway to a phosphate mine located upstream of the landslide has been rebuilt over the landslide after the original track was buried by the debris. In addition, this landslide destroyed a diversion dam and buildings associated with a now damaged hydropower station. It was reported that the landslide killed six people who operated the station when the earthquake occurred. The material of the sliding mass transformed during failure from weathered slate to sandy and muddy debris. Shining surfaces of rock-mass defects can be seen in the head-scarp of the landslide. This slide occurred as a translational slide on the upper slopes, and it is suggested that it transformed to a rotational slide at the river side because of thicker colluvium there. Due to the long runout distance of this slide, we think that the steep slope angle at the source area and the thick colluvium might have played a part in controlling of motion. With the steep slope angle at the rear part, the sliding mass should have had a high gravity potential, facilitating rapid movement when the earthquake triggered the failure. The source area should have been in a relatively dry condition, while, at the lower part of the slope (consisting of thick colluvium), water content should have been higher and some layers in the colluvium may have been fully saturated due to the lower permeability of the deposits. Therefore, when the lower part was pushed by the upper part, undrained sliding may have occurred and resulted in long runout. Comparing this case with the Jiufengchun slide, the differences in soil properties between the material of the sliding mass (and sliding zone) should be emphasized. Generally, weathered granite has much smaller undrained shear strength than the weathered slate because the weathered granite can be classified as a sandy soil and the weathered slate classified as silt or clay. Grain crushing occurring in undrained shearing may cause high excess pore pressure and result in high mobility (Sassa et

al. 1996; Wang 1999, Wang et al. 2000). The different motion styles and their controlling factors should be studied further through more detailed investigation. Xiaojiaqiao slide in Chaping Town The Xiaojiaqiao slide (104.278° E, 31.647° N) is unique among the landslides triggered by the earthquake because no trace of groundwater was found in or around the slide. The sliding mass is weathered dolomite. Figure 8 is an annotated oblique view of the slide. At least 100 people were killed when the Xiaojiaqiao slide inundated the village. Immediately after failure, the landslide debris infilled the valley between the upstream and downstream side scarps of the landslide (Fig. 8) and a lake formed (Xiaojiaqiao Lake) was flooding local buildings and infrastructure. A channel was cut through the landslide to stop the filling of the lake, allowing the river to flow over the landslide debris at a lower elevation. A road has been constructed over the landslide as a temporary thoroughfare as the original road was obliterated by the landslide. There was a large gap evacuated between the main scarp and the head of the landslide debris. The distance between the top of the main scarp and the top of the landslide debris was approximately 50 m. The sliding mass moved away from the source area and reached to the opposite bank of the river. From observing the resting angle of the sliding mass combined with eye-witness accounts by local people, it is easy to visualize how rapidly the landslide moved. A vast

Fig. 8 An oblique view of the Xiaojiaqiao slide


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Recent Landslides parameters and geotechnical properties of the rock mass or rock mass defects are required to establish the initial cause of the failure. Fragmentation may explain the behavior of the debris once failure had been initiated.

Fig. 9 Comparison of Qushan Town before (a: from Google Earth) and after the earthquake (b)

sliding plane was exposed, and the secondary falling deposits, which may have been caused by aftershocks, covered part of the sliding plane. Also from the channel-cut surface along the river, the debris was very dry. Information on seismic Fig. 10 The Wangjiayan slide (left) and Jingjiashan slide (right) in Qushan Town, Beichuan County

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Two slides in Beichuan County-town Qushan Town is the capital of Beichuan County. It was one of the towns most severely damaged by the earthquake. However, unlike most other badly affected towns, most of the damage in Qushan was caused by landslides. Figure 9 shows two views, one before the earthquake and linked in Google Earth, the other taken by the first author 1 month after the earthquake on 15 June 2008. The two photos were taken at approximately the same position (104.451° E, 31.819° N). Through comparison of the two photos, it can be seen that nearly half of Qushan Town was destroyed by two landslides. Figure 10 gives more details of the two landslides. The landslide at the left side of the photo is Wangjiayan slide, which slid onto and buried the old town area, and the one at the right side is Jingjiashan slide, which inundated the new town area. The two slides have different resting angles, and different runout distances. The source material for the slide debris of the Wangjiayan slide was altered sandstone and shale. The thickness was estimated at about 20 to 30 m. It moved for about 300 m, and the resting angle of the sliding mass was only about 10°. Before the earthquake, the slope angle of the Wangjiayan slope was quite steep. It is estimated from viewing the previous photo and topographic map that the slope angle may have reached 35°. In the path of the Wangjiayan slide, there were many public facilities, including Beichuan County’s Education Bureau, the Self-Defense Bureau, a public hospital, a kindergarten, and a jail. Almost all of the people in this area were buried by the slide, with few people escaping. The casualties attributed to this slide were estimated to be 1,600 persons. It can be imagined how rapidly the landslide moved.

Fig. 11 View of the Donghekou slide from the slide toe

In the path of the Jingjiashan slide, there was a high school. When the earthquake occurred, all of the 400 students and teachers were in classrooms at the school, and were killed by the slide. Because of a prohibition on outsiders entering the Qushan Town area, the exact resting slope angle could not be measured. From the photos, it is estimated that the slope angle of the slide after it buried the high school is about 25°, and the runout distance was much shorter than that of the Wangjiayan slide. The slope angle before the earthquake was about 35–40°; the slide-mass material is dolomite. The difference in runout distance between the Wangjiayan and the Jingjiashan slides may be due to a difference in the material content. Generally, dolomite has large joints inside and has high permeability. The slope failure style was likely like that of a rockfall, showing a highly brittle nature, rather than sliding. In comparison, the surficial material of the weathered sandstone and shale of the Wangjiayan slide may have a low permeability layer beneath the weathered sandstone and shale. Even in a relatively dry season, the lower permeability layer may retain water. When triggered by an earthquake, the mechanism of undrained shearing behavior may occur in such a slope. This phenomenon is very similar to that occurred in the Hongsong HPS slide, as described in the previous section.

Donghekou slide and Shibangou slide in Qingchuan County The Donghekou slide (105.109° E, 32.432° N) is a rapid, long runout slide which blocked a river forming a landslide lake at Donghe village, Qingchuan County. This slide occurred at the confluence of the Jinzhujiang River and the smaller Hongshi-he River. Two buses and a car were buried by the landslide, and at least 300 villagers were killed. Around this landslide are effects that may have been caused by an air blast at the time of the failure, indicating that movement was extremely rapid. Figure 11 is a view from the toe of the slide, towards the source area. The 11° travel angle (measured from the toe) indicates the high mobility of this slide. The source area was Cambrian-age metamorphic rock comprising weathered brittle sandy slate with inter-bedded coal seams. In the main scarp of the landslide, dolomite can be seen overlying slate. The source area was steep and the travel area flat. A river in the travel path forms an important hydrogeological condition for the slide, and may have induced undrained behavior in the sliding mass during movement, resulting in rapid and long runout landsliding. Figure 12 shows the Shibangou slide at 105.121° E, 32.413° N, another large landslide along the Jinzhujiang River, about 2 km upstream from the Donghekou slide. It is reported that the

Fig. 12 Side view of the Shibangou slide


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Recent Landslides Shibangou landslide buried four villages: Chefutan, Majialiang, Zhoujia, and Shibangou, with estimated casualties of 300–400 people. It formed the second biggest landslide lake from the earthquake. Field observations suggest that the landslide may have cut across bedding planes and moved as a rotational slide with one body. The geological structure is similar to that of Donghekou slide, with dolomite overlying slate of Cambrian age. Trees on the slide mass are tilted, but remain standing in groups. The slide debris ran up the opposite bank of the river, and the travel angle of the slide was measured at about 12°, indicating rapid movement.

Impressions and conclusions We draw the following conclusions from our observations of the landslides occurring as a result of the 2008 Wenchuan earthquake: 1. All of the slides, both those that moved for long distances, and for short distances, were extremely rapid. This caused many casualties because evacuation was nearly impossible during the sudden, rapid, and massive movement of the sliding masses. 2. Among the landslides, long-runout mass movements occurred in slate, mudstone and shale, as illustrated by the Donghekou slide, Shibangou slide, Hongsong HPS slide, and Wangjiayan slide in Beichuan County. Large rockfalls occurred in dolomite, such as in the Jingjiashan slide in Beichuan County. 3. The hydrogeological conditions, especially, where there was a saturated loose layer formed by a river or gully in the travel path, may have contributed to the rapid movement and long runout distance. Deeply weathered granite moved as a debrisflow-like slide, possibly because of the saturated and undrained behavior at the sliding zone. From the view point of geomorphology, we have the following impressions and conclusions: 1. Site accessibility is slowly improving as landslides are cleared from the roads, and damaged roads are rebuilt. There were thousands of smaller landslides, rockfalls, and subsequent debris flows that blocked and damaged roads, adding to the difficulty of access. These smaller events are still occurring sporadically, in unstable areas, and road maintenance will be ongoing for many months or years to come. 2. The landslides are changing in character as intensive construction takes place to mitigate their impact, for example, extensive grading, removing of landslide material and debris, and as roads are constructed on and around the landslides. 3. The geographical extent and patterns of distribution of landslides eventually will be mapped, adding to information about magnitude and intensity thresholds for earthquakeinduced landslides.

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Acknowledgements The first author acknowledges financial support from MEXT of Japan (No. 18403003, representative: F.W. Wang) for the first two investigations in the disaster area caused by the 2008 Sichuan earthquake. On 26 July to 3 August 2008, he made a third investigation with financial support from a scientific research fund for the Sudden-disaster investigation (Representative: K. Konagai) from MEXT of Japan. The financial support from National Fundamental Research Program of China (973 Program) (NoF2008CB425801) was appreciated. The authors deeply appreciated discussions with Prof. K. Konagai of University of Tokyo, Prof. S. Tsuchiya of Shizuoka University, Japan, and Prof. X.Y. Wu of Southwest Jiaotong University, China. The information from the local people who experienced the great disasters is extremely important for the report. Comments by Mauri McSaveney and Grant Dellow were deeply appreciated. References Sassa K, Fukuoka H, Scarascia-Mugnozza G, Evans S (1996) Earthquake-induced-landslides: Distribution, motion and mechanisms. Soils and Foundations JAN:53–64 Special Issue Wang FW (1999) An Experimental Study on Grain Crushing and Excess Pore Pressure Generation of Sandy Soils during Shearing –A Key Factor for Rapid Landslide Motion. Thesis submitted to the Graduate School of Science, Kyoto University in partial fulfilment of the requirements for the degree of Doctor of Science in the Division of Earth and Planetary Science, Japan. 114 pp. Wang FW, Sassa K, Fukuoka H (2000) Geotechnical simulation test for the Nikawa landslide induced by 1995.1.17 Hyogoken-Nambu earthquake. Soils and Foundations 40(1):35–46

F. Wang ()) Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan e-mail: [email protected] Q. Cheng School of Civil Engineering, Southwest Jiaotong University, No. 111, North Part 1, Second Circle Road, Chengdu 610031, China e-mail: [email protected] L. Highland National Landslide Information Center, US Geological Survey, MS 966, Box 25046, Denver Federal Center, Denver, CO 80225, USA e-mail: [email protected] M. Miyajima . C. Yan Department of Civil Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan M. Miyajima e-mail: [email protected] C. Yan e-mail: [email protected] H. Wang School of Civil Engineering and Mechanics, Huazhong University of Science and Technology, No. 1037, Luoyu Road, Hongshan District, Wuhan 430074, China e-mail: [email protected]