**2.3 Characteristics of the landslides**

As many as 56,000 landslides have been identified by field investigations and using remote sensing technique with aerial photographs and satellite images. And the following distinctive characteristics can be summarized from these landslides:

1. Large scale

Many large scale landslides were induced by the 2008 Wenchuan earthquake. There are tens of landslides with a volume of 107 cubic meters (Wu et al., 2010), and 113 landslides with the area larger than 50,000 m2 as shown in Table 1 (data from Xu et al., 2009a). The largest one is the Daguangbao landslide in Anxian County with an area of 7,273,719 m2 and the volume of about 8.4×108m3 (Chigira et al., 2010).

2. The effect from the hanging and foot wall of the fault

It has been found that the majority of landslides are distributed in the range belonging to the hanging wall of the Yingxiu-Beichuan fault and Pengguan fault, northwest part of earthquake zone. All statistics seem to support that the landslide in hanging wall area is more active than in foot wall areas (Xu et al., 2009a; Yin et al., 2009a). For example, the distribution of large-scale landslides also shows the hanging /foot wall effect. It can be found from the Table 1 that 80 landslides occurred in the hanging wall, 70.8% of the total 113 large-scale landslides, and only 33 landslides occurred in the foot wall, 29.2% of the total number.


As many as 56,000 landslides have been identified by field investigations and using remote sensing technique with aerial photographs and satellite images. And the following

Many large scale landslides were induced by the 2008 Wenchuan earthquake. There are tens of landslides with a volume of 107 cubic meters (Wu et al., 2010), and 113 landslides with the area larger than 50,000 m2 as shown in Table 1 (data from Xu et al., 2009a). The largest one is the Daguangbao landslide in Anxian County with an area of 7,273,719 m2 and the volume of

It has been found that the majority of landslides are distributed in the range belonging to the hanging wall of the Yingxiu-Beichuan fault and Pengguan fault, northwest part of earthquake zone. All statistics seem to support that the landslide in hanging wall area is more active than in foot wall areas (Xu et al., 2009a; Yin et al., 2009a). For example, the distribution of large-scale landslides also shows the hanging /foot wall effect. It can be found from the Table 1 that 80 landslides occurred in the hanging wall, 70.8% of the total 113 large-scale landslides, and only 33 landslides occurred in the foot wall, 29.2% of the total

**No. Name Place Area /m2 Distance to fault /m Wall location**  1 Daguangbao Anxian 7,273,719 4,800 Hanging wall 2 Wenjiagou Mianzhu 2,945,520 3,900 Foot wall 3 Donghekou Qiangchuan 1,283,627 300 Hanging wall 4 Zhengjiashan Pingwu 1,014,987 2,400 Hanging wall 5 Shuimogou Shifang 915,608 700 Hanging wall 6 Dawuji Anxian 792,190 6,900 Hanging wall 7 Woqian Qiangchuan 695,672 200 Hanging wall 8 Dashanshu Mianzhu 693,687 6,900 Hanging wall 9 Hongshigou Anxian 687,520 2,240 Hanging wall 10 bingkoushi Pengzhou 575,556 12,600 Hanging wall 11 Tangjiashan Beichuan 572,009 2,780 Hanging wall 12 Huatizigou Pengzhou 541,193 4,980 Hanging wall 13 Wenjiaba Pingwu 537,101 380 Hanging wall 14 Niujuangou Wenchuan 527,700 300 Hanging wall 15 Haixingou Mianzhu 517,573 8,888 Hanging wall 16 Ma'anshi Pingwu 509,836 4,200 Hanging wall 17 Shibangou 1# Qiangchuan 496,983 2,300 Hanging wall 18 Guershan Beichuan 471,112 0 Hanging wall 19 Xiaojiashan Mianzhu 465,899 2,900 Hanging wall 20 Xinkaidong Pengzhou 449,685 6,800 Hanging wall 21 Boazangcun Anxian 418,744 4,030 Hanging wall 22 Mianjiaoping Beichuan 377,247 550 Foot wall 23 Weijiashan Beichuan 358,021 2,120 Hanging wall 24 Liqigou Jiangyou 355,113 10,000 Foot wall

distinctive characteristics can be summarized from these landslides:

**2.3 Characteristics of the landslides** 

about 8.4×108m3 (Chigira et al., 2010).

2. The effect from the hanging and foot wall of the fault

1. Large scale

number.


Earthquake Induced a Chain Disasters 389

Among the large scale landslides, the two farthest landslides from the fault are about 12.6km in the side of hanging wall and 11.35km in the foot wall. The majority of landslides (about 70%) occured in the region of 3km from the fault. Fig. 3a and 3b show the accumulative percentage of landslide distribution as a function of the distance to the fault in hanging and foot wall respectively. An exponential decay has been found for the number of

> b. Distance to fault for percentage of landslides located on foot wall

b. Distance to fault for number of landslides

located on foot wall

landslides with the distance to the fault in both hanging and foot wall (Fig. 4).

Fig. 3. Relationship between percentage of landslides and large-scale distance to fault. a: Distance to fault for percentage of landslides located on hanging wall; b: Distance to fault

Fig. 4. Relationship between number of landslides and distance to fault. a: Distance to fault for number of landslides located on hanging wall; b: Distance to fault for number of

The two largest scale landslides: Daguangbao landslide with the area of 7,273,719m2 and Wenjiagou landslide with the area of 2,945,520m2 are found locating at a distance of more

3. The effect from the distance to the faults

a. Distance to fault for percentage of landslides located on hanging wall

for percentage of landslides located on foot wall.

a. Distance to fault for number of landslides

4. Effect from the locking segment of the fault zone

located on hanging wall

landslides located on foot wall.


Table 1. The large scale landslides with the area larger than 50,000 m2 (data from Xu et al., 2009a).

**No. Name Place Area /m2 Distance to fault /m Wall location**  71 Hongmagong Qiangchuan 144,683 350 Foot wall 72 Baiguocun Qiangchuan 139,800 300 Foot wall 73 Huangtuliang Beichuan 135,084 550 Hanging wall 74 Qinglongcun Qiangchuan 134,079 790 Foot wall 75 Pengjiashan Beichuan 127,156 2,900 Hanging wall 76 Wangjiayan Beichuan 125,381 400 Hanging wall 77 Yibadao Mianzhu 125,059 9,600 Foot wall 78 Laohuzui Wenchuan 125,039 2,700 Hanging wall 79 Beichuanzhongxuexinqu Beichuan 124,365 300 Foot wall 80 Xiaomeizilin Mianzhu 122,530 5,800 Foot wall 81 Xiangshuishi Pengzhou 119,194 4,600 Hanging wall 82 Gaojiamo Pingwu 115,301 1,600 Hanging wall 83 Jiadanwan 2# Dujaingyan 114,905 9,300 Hanging wall 84 Dahuashu Beichuan 113,111 0 Hanging wall 85 Wangjiabao Beichuan 112,418 0 Hanging wall 86 Jiankangcun Pingwu 111,106 340 Hanging wall 87 Xiaojiaqiao Anxian 110,085 3,000 Foot wall 88 Lingtou Qiangchuan 102,116 800 Hanging wall 89 Longwangou Beichuan 99,821 650 Hanging wall 90 Zhangzhengbo Qiangchuan 99,726 790 Foot wall 91 Nanyuecun Dujaingyan 99,350 0 Hanging wall 92 Hongkouxiangxiajiaping Dujaingyan 96,345 790 Hanging wall 93 Dujiayan Qiangchuan 94,769 960 Foot wall 94 Madiping Qiangchuan 94,633 2,600 Hanging wall 95 Maochongshan 1# Pingwu 92,355 1,200 Hanging wall 96 Yandiaowo Qiangchuan 92,128 340 Foot wall 97 Chuangzigou Mianzhu 91,718 2,200 Foot wall 98 Xiaoxishan Qiangchuan 90,298 1,000 Hanging wall 99 Xishanpo Beichuan 83,663 1,140 Hanging wall 100 Hejiayuan Qiangchuan 83,359 1,990 Foot wall 101 Zhaojiashan Qiangchuan 82,329 1,000 Foot wall 102 Liushuping 1# Qiangchuan 81,000 780 Hanging wall 103 Weiziping Qiangchuan 74,661 470 Hanging wall 104 Gongziba Qiangchuan 71,221 220 Hanging wall 105 Maerping Qiangchuan 70,982 7,500 Hanging wall 106 Maochongshan 2# Pingwu 70,252 1,200 Hanging wall 107 Muhongping Qiangchuan 68,288 2,600 Foot wall 108 Machigai Qiangchuan 66,602 500 Foot wall 109 Zixicun Pingwu 57,820 2,400 Hanging wall 110 Liushuping 2# Qiangchuan 54,810 1,000 Hanging wall 111 Dongjia Qiangchuan 54,353 1,000 Foot wall 112 Majiawo Qiangchuan 50,591 1,100 Hanging wall 113 Xiaowuji Qiangchuan 50,122 2,100 Foot wall Table 1. The large scale landslides with the area larger than 50,000 m2 (data from Xu et al.,

2009a).

3. The effect from the distance to the faults

Among the large scale landslides, the two farthest landslides from the fault are about 12.6km in the side of hanging wall and 11.35km in the foot wall. The majority of landslides (about 70%) occured in the region of 3km from the fault. Fig. 3a and 3b show the accumulative percentage of landslide distribution as a function of the distance to the fault in hanging and foot wall respectively. An exponential decay has been found for the number of landslides with the distance to the fault in both hanging and foot wall (Fig. 4).

Fig. 3. Relationship between percentage of landslides and large-scale distance to fault. a: Distance to fault for percentage of landslides located on hanging wall; b: Distance to fault for percentage of landslides located on foot wall.

a. Distance to fault for number of landslides located on hanging wall b. Distance to fault for number of landslides located on foot wall

Fig. 4. Relationship between number of landslides and distance to fault. a: Distance to fault for number of landslides located on hanging wall; b: Distance to fault for number of landslides located on foot wall.

4. Effect from the locking segment of the fault zone

The two largest scale landslides: Daguangbao landslide with the area of 7,273,719m2 and Wenjiagou landslide with the area of 2,945,520m2 are found locating at a distance of more

Earthquake Induced a Chain Disasters 391

For instance, the Wangjiayan landslide (No. 76 in Table 1 and No. 21 in Table 2), occurred at the old town area of Beichuan city, had a run-out distance of 550m. It destroyed hundreds of buildings and resulted in more than 1600 fatalities (Yin et al., 2009a). The Daguangbao landslide (No. 1 in both Table 1 and 2 ) is another long run-out example. The affected area is estimated more than 7.2 km2. Its run-out distance is estimated as 4,500m. The most complex in dynamic mechanism is the Donghekou landslide (No. 3 in Table 1 and No. 4 in Table 2 ) which has the run-out distance of over 2.4km. It blocked two rivers and formed two

It has been found that the run-out distance is proportional to the area and volume of landslide. The regression formulas of *D/H*=lg*S*-3.12 has been obtained with a coefficient of determination of *R*2=0.7681, where *D* is the run-out distance, *H* is the slope height and *S* is the area of landslide. and *D/H*=0.54lg*V*-1.26 with a coefficient of determination of *R*2=0.6290,

Since the mechanism of long run-out landslide is very important in landslide disaster

D/H

Fig. 6. The relationship between normalized run-out distance and (a) the area of landslide, *S*

There are 34 landslide dams formed by the earthquake induced landslides. These landslide dams blocked the major large rivers. The water impounded by landslide dams created dam

The largest scale landslide dam was formed by the Tangjiashan landslide. It blocked the upper portion of the Jianjiang River at a location of about 5 km from Beichuan County Town. The dam crest extended approximately 600 m across and 800 m along the valley (Xu et al., 2009b). The maximum height of the dam is about 124 m. The maximum capacity of the

Because of its rather loose nature and absence of controlled spillway, it is feared that the landslide dam may fail catastrophically and lead to downstream flooding with high casualties. Hundreds of thousands of residents in downstream Mianyang City were evacuated to the higher locations out of the town before a temporary drainage channel was

landslide lake was 2.4×108m3, with the length of 20 km (Yin et al., 2009a).

1.5 2 2.5 3 3.5 4

D/H=0.54lgV-1.26 R2=0.6290

<sup>105</sup> <sup>10</sup><sup>6</sup> <sup>10</sup><sup>7</sup> <sup>10</sup><sup>8</sup> <sup>10</sup><sup>9</sup> <sup>1</sup>

Estimated volume, V /m3

landslide lakes at Donghe village of Qingchuan County.

where *V* is the volume of landslide (see Fig. 6).

<sup>10</sup><sup>5</sup> <sup>10</sup><sup>6</sup> <sup>10</sup><sup>7</sup> <sup>1</sup>

Area of landlside, S / m2

(m2), and (b) the volume of landslide, *V* (m3).

digged in the dam by Chinese government.

7. Large number of landslide dams

1.5 2 2.5 3 3.5 4

lakes.

D/H

mitigation, it will be discussed in Section 4.

D/H=lgS-3.12

R2 =0.7681

than 3.9km from the fault from Table 1, although most of large-scale landslides, which many researchers have been studying on, are located in the region of less than 1km from the fault. For example, the Donghekou landslide (No.3 in Table 1) has a distance of 0.3km, the Woqian landslide (No.7 in Table 1) has a distance of 0.2km and Niujuangou landslide (No.14 in Table 1) has a distance of 0.3km from the fault. By examining the positions of the two landslides with the fault zone, it has be found that the two landslides are just located at the locking segment of the fault zone where high stress is believed to be concentrated and a lot of energy was absorbed by the locking of the rupture fault. Therefore, it should be notice that large scale landslides may occur at such kind of locking segment of the fault zone.

#### 5. Direction effect

By examining the sliding directions of large-scale landslides along Hongshihe valley, it has been found that the directions parallel to or perpendicular to the fault are dominated as shown by the rose diagram in Fig. 5. It is implied that the landslides are controlled by the earthquake wave propagation and the fault movement. The slopes parallel to or perpendicular to the fault are easy collapsed.

Fig. 5. Rose diagram showing the motion direction of large-scale landslides along the Hongshihe (after Xu et al., 2009a)

#### 6. The long run-out characteristic

For rainfall induced landslides, the run-out distances are mostly less than 2 times of the slope height (2H). For example, 95% of the 19,035 landslides induced by rainfall are less than 50m based on the records from 1972 to 2008 in Japan. However, numerous rapid and long run-out landslides occurred during the 2008 Wenchuan Earthquake. Table 2 lists 21 long-run-out landslides with the horizontal distance larger than 500m. These landslides traveled over extraordinarily large distances with extremely high speeds and produced catastrophic results.

than 3.9km from the fault from Table 1, although most of large-scale landslides, which many researchers have been studying on, are located in the region of less than 1km from the fault. For example, the Donghekou landslide (No.3 in Table 1) has a distance of 0.3km, the Woqian landslide (No.7 in Table 1) has a distance of 0.2km and Niujuangou landslide (No.14 in Table 1) has a distance of 0.3km from the fault. By examining the positions of the two landslides with the fault zone, it has be found that the two landslides are just located at the locking segment of the fault zone where high stress is believed to be concentrated and a lot of energy was absorbed by the locking of the rupture fault. Therefore, it should be notice that large scale landslides may occur at such kind of locking segment of the fault zone.

By examining the sliding directions of large-scale landslides along Hongshihe valley, it has been found that the directions parallel to or perpendicular to the fault are dominated as shown by the rose diagram in Fig. 5. It is implied that the landslides are controlled by the earthquake wave propagation and the fault movement. The slopes parallel to or

Fig. 5. Rose diagram showing the motion direction of large-scale landslides along the

For rainfall induced landslides, the run-out distances are mostly less than 2 times of the slope height (2H). For example, 95% of the 19,035 landslides induced by rainfall are less than 50m based on the records from 1972 to 2008 in Japan. However, numerous rapid and long run-out landslides occurred during the 2008 Wenchuan Earthquake. Table 2 lists 21 long-run-out landslides with the horizontal distance larger than 500m. These landslides traveled over extraordinarily large distances with extremely high speeds and produced

5. Direction effect

perpendicular to the fault are easy collapsed.

Hongshihe (after Xu et al., 2009a) 6. The long run-out characteristic

catastrophic results.

For instance, the Wangjiayan landslide (No. 76 in Table 1 and No. 21 in Table 2), occurred at the old town area of Beichuan city, had a run-out distance of 550m. It destroyed hundreds of buildings and resulted in more than 1600 fatalities (Yin et al., 2009a). The Daguangbao landslide (No. 1 in both Table 1 and 2 ) is another long run-out example. The affected area is estimated more than 7.2 km2. Its run-out distance is estimated as 4,500m. The most complex in dynamic mechanism is the Donghekou landslide (No. 3 in Table 1 and No. 4 in Table 2 ) which has the run-out distance of over 2.4km. It blocked two rivers and formed two landslide lakes at Donghe village of Qingchuan County.

It has been found that the run-out distance is proportional to the area and volume of landslide. The regression formulas of *D/H*=lg*S*-3.12 has been obtained with a coefficient of determination of *R*2=0.7681, where *D* is the run-out distance, *H* is the slope height and *S* is the area of landslide. and *D/H*=0.54lg*V*-1.26 with a coefficient of determination of *R*2=0.6290, where *V* is the volume of landslide (see Fig. 6).

Since the mechanism of long run-out landslide is very important in landslide disaster mitigation, it will be discussed in Section 4.

Fig. 6. The relationship between normalized run-out distance and (a) the area of landslide, *S* (m2), and (b) the volume of landslide, *V* (m3).

7. Large number of landslide dams

There are 34 landslide dams formed by the earthquake induced landslides. These landslide dams blocked the major large rivers. The water impounded by landslide dams created dam lakes.

The largest scale landslide dam was formed by the Tangjiashan landslide. It blocked the upper portion of the Jianjiang River at a location of about 5 km from Beichuan County Town. The dam crest extended approximately 600 m across and 800 m along the valley (Xu et al., 2009b). The maximum height of the dam is about 124 m. The maximum capacity of the landslide lake was 2.4×108m3, with the length of 20 km (Yin et al., 2009a).

Because of its rather loose nature and absence of controlled spillway, it is feared that the landslide dam may fail catastrophically and lead to downstream flooding with high casualties. Hundreds of thousands of residents in downstream Mianyang City were evacuated to the higher locations out of the town before a temporary drainage channel was digged in the dam by Chinese government.


Note: ※ data from Huang et al. ( 2011a ); ♥ data from Wu et al. ( 2010 ); ♠ data from Qi et al. ( 2011 ); data from Chigira et al.( 2010 ); and ☼ data from Yin et al.( 2009a ); ♣ Estimated indirectly from geological sections or description. notes the value is mean one if the number of data more than one. 

*♀* Table 2. Some long run-out landslides triggered by the Wenchuan earthquake (arranged from the Horizontal distance) (modified from Xu et al., 2009a)  Earthquake Induced a Chain Disasters 393

Landslide susceptibility analysis (LSA) is necessary and important for land use planning and disaster mitigation. Many researchers have made great effort to identify the relationships of landslide characteristics such as distribution pattern, type, area coverage and volume with the triggering factors such as the magnitude, intensity and peak ground acceleration (PGA) of the earthquake, coseismic fault rupture (e.g. Lee et al., 2008; Rodriguez et al., 1999; Miles and Keefer, 2009; Keefer, 1984, 2000, 2002; Papadopoulos and Plessa, 2000). Some researchers have studied the relationships of landslide distribution with geoenvironmental factors such as lithology, morphology, presence of secondary active or inactive faults (e.g. Chigira and Yagi, 2006; Jibson et al., 2000; Khazai and Sitar, 2003; Keefer,

The Wenchuan earthquake induced landslides has been carried out by several researchers. For example, Huang et al. (2011a) studied the characteristics and failure mechanism of Daguangbao Landslide, the largest scale landslide, and suggested a classification system. Tang et al. (2011b) studied the effect of the quake on the landslides induced by the subsequent strong rainfall after earthquake by a case study in the Beichuan area. Qi et al. (2010) built a spatial database of landslides by using the remote sensing (RS) results which cover 11 counties seriously damaged by the earthquake. Yin et al (2009a, b) analyzed the landslide distribution, the mechanisms of some typical landslides, and evaluated the potential hazards of the landslide dams. Gorum et al. (2011) presented the preliminary results of an extensive study of the mapping the distribution of landslides by using a large set of optical high resolution satellite images. Yin et al. (2010) presented a quantitative result of the number and area of the landslides from Anxian to Beichuan. Dai et al. (2011) mapped over 56,000 landslides using aerial photographs and satellite images and characterized the spatial distribution of landslides by correlating landslide-point density and landslide-area

density with the physical parameters that control the seismic stability of slopes.

In this chapter, we show some results from landslide susceptibility analysis carried out in Qingchuan County. Our analysis was based on slope units rather than the traditional grid units. At first, the relationship of landslide distribution with an individual causative factor is analyzed. And then, landslide susceptibility is analyzed by using artificial neural network (ANN) method. Finally, a landslide susceptibility map is made based on the ANN results.

Qingchuan County is located at the north-western part of the earthquake zone as shown in

Up to now, most of such studies were carried out based on the grid units. There is a problem in grid-based study that a grid may contain different slopes and a large slope may contain several grids with different slope grades. Despite the problem, the grid units were still used

Nowadays, it becomes possible and easy to indentify slope units by using GIS-based hydrologic analysis tool (David, 2002), which is based on the watershed divide and drainage lines. The slope unit size should be determined when the tool is used. We suggest that the appropriate slope unit size should match the average size of the landslide bodies in the

just because the slope units are difficult to be indentified for a wide range in the past.

**3. Susceptibility analysis of earthquake induced Landslides** 

2000; Yagi et al., 2009).

**3.1 Study area and data source** 

**3.2 Slope unit** 

study area.

Fig. 7. The landslides in the area of 3,271km2 are studied.

Note: ※ from Yin et

*♀* from Xu et al., 2009a)

al.( 2009a ); ♣

notes the value is mean one if the number of data more than one.

data from Huang et al. ( 2011a ); ♥

data from Wu et al. ( 2010 ); ♠

Estimated indirectly from geological sections or description.

Table 2. Some long run-out landslides triggered by the Wenchuan earthquake (arranged from the Horizontal distance) (modified

data from Qi et al. ( 2011 );

◎

data from Chigira et al.( 2010 ); and

☼ data
