spatial distribution analysis of landslides triggered by the...

Vol.15, No.1 EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION March, 2016

Earthq Eng & Eng Vib (2016) 15: 163-171 DOI: 10.1007/s11803-016-0313-5

Spatial distribution analysis of landslides triggered by the2013-04-20 Lushan earthquake, China

Chang Ming† , Tang Chuan‡, Xia Chenhao§ and Fang Qunsheng§

State Key Laboratory of Geo-hazard Prevention and Geo-environment Protection, Chengdu University of Technology, Chengdu

650023, China

Abstract: The 2013-04-20 Lushan earthquake (seismic magnitude Ms 7.0 according to the State Seismological Bureau) induced a large number of landslides. In this study, spatial characteristics of landslides are developed by interpreting digital aerial photography data. Seven towns near the epicenter, with an area of about 11.11 km2, were severely affected by the earthquake, and 703 landslides were identifi ed from April 24, 2013 aerial photography data over an area of 1.185 km2. About 55.56% of the landslide area was less than 1000 m2, whereas about 3.23 % was more than 10,000 m2. Rock falls and shallow landslides were the most commonly observed types in the study area, and were primarily located in the center of Lushan County. Most landslide areas were widely distributed near river channels and along roads.Five main factors were chosen to study the distribution characteristics of landslides: elevation, slope gradients, fault, geologic unit and river system. The spatial distribution of coseismal landslides is studied statistically using both landslide point density (LPD), defi ned as the number of landslides (LS Number) per square kilometer, and landslide area density (LAD), interpreted as the percentage of landslides area affected by earthquake. The results show that both LPD and LAD have strong positive correlations with fi ve main factors. Most landslides occurred in the gradient range of 40°−50° and an elevation range of 1.0−1.5 km above sea level. Statistical results also indicate that landslides were mainly formed in soft rocks such as mudstone and sandstone, and concentrated in IX intensity areas.

Keywords: Lushan earthquake; landslide; spatial distribution; impact factor

Correspondence to: Chang Ming, State Key Laboratory of Geo-hazard Prevention and Geo-environment Protection, Chengdu University of Technology, Chengdu 650023, China Tel: +86-28-84077505; Fax: +86-28-84078948E-mail: [email protected]†Lecture; ‡Professor; § Graduate Student

Supported by: Basic Work for the National Science and Technology Special Program (2011FY110100-3) and Special Research Fund for the Doctoral Program of Higher Education (20125122130001)

Received October 23, 2013; Accepted September 30, 2015

1 Introduction

A devastating Ms7.0 earthquake occurred in Lushan County, Sichuan Province in Southwest China at 8:02 am on 20 April 2013 according to the Chinese Earthquake Administration (Chang et al., 2013; Chen et al., 2013). The epicenter is situated at the south end of the Longmen Shan fault zone, which is about 100 km from the epicenter of the Wenchuan earthquake ( Fig. 1). The earthquake resulted in 196 fatalities, 11,470 injured, among which 995 were severely injured, and 21 people are still missing. It was estimated that losses totaled about 8.1 billion dollars. In order to fi gure out the distribution

Fig. 1 Map showing simplifi ed geological structures in the “4.20” Lushan and “5.12” Wenchuan earthquakes

Legend

Epicenter of Lushan earthquake

Epicenter of Wenchuan earthquake

Surface rupture

Active fault

Study area 0 10 20 40 km

164 EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION Vol.15

characteristics of landslides, the extent of the losses caused by the earthquake, and provide assistance for disaster relief work, an imagery interpretation study and emergency fi eld investigation was organized by the State Key Laboratory of Geohazard Prevention (SKLGP). The work focused on seven towns in the area of the epicenter of the earthquake in Lushan County: Taiping, Luyang, Baosheng, Longmen, Qingren, Shangli, and Siyan (Table 1).

A large number of landslides caused tremendous destruction to highways and bridges in the affected area. In addition, many houses and irrigation channels were under the threat of damage. Many researchers immediately began to study the on distribution characteristics of landslides. Using satellite images, 688 landslides were interpreted in area of 300 km2 (Xu and Xiao, 2013). Based on a fi eld investigation, 1,846 landslides were identifi ed after the Lushan earthquake (Pei and Huang, 2013). More recently, 1,129 landslides were mapped in a 2200 km2 area by interpreting aerial photos (Chen et al., 2013). Compared to the Wenchuan earthquake, more than 15,000 landslides were triggered ( Yin et al., 2009). In order to better understand the distribution characteristics of landslides caused by the Lushan earthquake, 703 landslides were identifi ed in key towns about 122.112 km2 near the epicenter. This research aims to contribute to the distribution characteristics of landslides induced by the Lushan earthquake by studying fi ve main factors: elevation, slope gradients, fault, geologic unit and river systems.

2 Study area

The study area includes seven towns of Lushan County in the Sichuan Province. A large number of landslides were observed from the post-seismic aerial photography in this region. The area is located about 140 km southwest of Chengdu City, between the northern

latitudes of 30°05′ and 30°30′ and the eastern longitudes of 102°52′ and 103°04′.

The study area is located in the easternmost margin of the Tibetan plateau, which is characterized by middle-high mountains with elevations from 367 m to 3300 m. The maximum relative elevation difference is 2933 m. There were a series of predominantly north–northeast striking thrust faults that belong to the Longmen Shan thrust belt at the northwestern edge of the Sichuan Basin. The Shuangshi-Dachuan fault is part of the Longmen Shan front fault, which was a seismogenic fault with angles from 55° to 70° (Wang and Meng, 2009; Li et al., 2013).

Lushan River is the main river through the study area. It is about 34.3 km long and has a 1,067 km2 drainage area. The total annual runoff volume is about 1.5 billion m3 and average discharge can be 33m3/s. The study area has an annual average temperature of 15.2˚C, similar to a monsoon climate. The area is subjected to heavy rains, with an annual average rainfall of 1313 mm. The maximum recorded for 10 minute rainfall, hourly rainfall, and daily rainfall were 30.1 mm, 86 mm and 188 mm, respectively. The rain usually occurs in June to September every year, which accounts for about 80% of the year. The maximum intensity of the Lushan earthquake was IX, with as isoseismic line axis distributed from north to east. The area of earthquake intensity exceeded VI in a region of about 18628 km2 (Fig. 2).

3 Methodology

Ae rial photographs, ta ken on April 23, 2013 by the Sichuan Geomatics Center, were used to map the co-seismic landslides in the study region. Remote sensing images were taken on April 9, 2011 from a SPOT4 satellite prior to the earthquake. Combined with fi eld investigation, co-seismic landslides were interpreted by

Table 1 Distribution of landslides induced by the Lushan earthquake in the study area

ID Country name Area ( km2)Coseismic landslide

Number Area (km2)

1 Baosheng 19.13 270 0.595

2 Longmen 41.9 179 0.275

3 Taiping 16.85 140 0.2

4 Luyang 19.46 59 0.1

5 Qingren 16.89 34 0.01

6 Shangli 4.52 18 0.004

7 Siyan 3.36 3 0.001

In total 122.11 703 1.185

No.1 Chang Ming et al.: Spatial distribution analysis of landslides triggered by the 2013-04-20 Lushan earthquake, China 165

SKLGP. A 1:50,000 scale topographic map was provided by the Sichuan Center of Basic Geographic Information. A Digital Elevation Model (DEM) was produced from 1:50000 scale topographic map by a geographic information system (GIS) in order to determine four of the fi ve main factors: elevation, slope gradients, fault, and river system (Keefer, 2002; Dai et al., 2010; Tang et al., 2011a). The 1:50,000 scale geological maps supplied by the China Geologic Survey were able to provide the data for geologic units. Combined with th e other four factors, distribution cha racteristics of the landslide were studied using GIS tools.

In order to accurately analyze the distribution characteristics of a landslide, all landslides were mapped in the study area. The co-seismic landslides were identifi ed in different ways. One was the extent of destruction of the vegetation, which was dumped and grown (Fig. 3(a)). An other way was run-out tracks, which could be clearly recognized from 0.5 m resolution ratio of aerial photograp hs (Fig. 3(b)), es pecially in areas of bare land and zones without vegetation (Khazai and Sitar 2004; Lin et al., 2006; Khattak et al., 2010).

The landslide types and patterns were defi ned

through the fi eld work and interpretation of the aerial photographs and remote sensing imagery.

4 Results

4.1 Ge neral characte ristics of the landslides

According to the post-earthquake aerial photographs with a resolution ratio of 0.5 m, 703 co-seismic landslides with different sizes were recognized in the study area. An interpretation of the landsides map of Baosheng town center is shown in Fig. 4. Combined with fi eld survey data, the changes in the landslide area before and after the Lushan earthquake were observed (Fig. 5). The volume of co-seismic landslides ranged from several cubic meters to tens of thousands of cubic meters; these areas are typically from several square meters to million square meters. Comparing the changes, small, shallow, crack landslides were observed in the study area. These landslides rapidly dropped down from the steep slope mountain, which was located on an elevation about 1,500 m ASL. The landslide formed a long track, with an average length and width of about 300 m and 100 m, respectively.

Many water courses and roadways were seriously damaged by landslides (Fig. 5). Rock falls and some landslides originated from the mountain top, which caused them to fall with greater momentum (Zhang et al., 2013).

4.2 Analysis of relation with landslides distribution and impact factors

Seven key towns were chosen to research the spa tial distribution characteristics of landslides. Associated with lithology, topography, and hydrological conditions, fi ve main factors were chosen to study the distribution characteristics of landslides: elevation, slope gradients, fault, geologic unit and river system (Rodriguez et al.,

Fig. 2 Location of the study area and seismic intensity of the Lushan earthquake

Fig. 3 Photo showing typical features of shallow landslide in the study area (a) shallow landslide located in Baosheng country; (b) fractured rock located in Longmen country

(a) (b)

VIVIIVIIIIX

Epicenter

TownFaultRiver

Seismic intersity


1999; Wasowski et al., 2002; Kamp et al., 2008; Dai et al., 2010).

In order to accurately analyze the above mentioned impact factors, Two types of areal frequency of landslides were chosen (Koi et al., 2008; Tang et al., 2011b): (1) the number of landslides (LS Number) per square kilometer (LPD); and (2) the percentage of landslide areas affected by the earthquake (LAD).4.2.1 Elevation factor

The elevation is an important topographic factor that can help to determine the distribution characteristics of a seismic landslide (Forte et al., 2013). In order to understand the relationship of co-seismic landsides and elevation, landslide point density and landslide area density for different elevations was given (Fig. 6). The land slide point density showed that the landsides occurred within an elevation range from 500 m to 2,000 m above sea level, among which areal frequency of co-seismic landsides was the highest at elevations between 1000 m and 1500 m asl (Table 2). The landslide area density (LAD) at elevations above 1500 m asl were

signifi cantly reduced. It was the earthquake that directly led to the occurrence of co-seismic landsides, as it caused the rocks to release stress and transform the gravitational potential energy into kinetic energy in high elevations. This trend can be attributed to the effect of topographic amplifi cation on high elevation areas. Generally speaking, large deep landslides often appear in higher elevation locations, while small shallow landslides are usually found in lower altitude mountains. If a landslide is located in high elevations, it has great potential energy. However, the landslides triggered by the “4.20” Lushan earthquake are almost all the shallow type.4.2.2 Slope gradients fact or

The slope gradient is an other important factor that can be used to determine the distribution characteristics of seismic landslides. Slope gradient was subdivided into 10°segments by ARCGIS software. The landslide point density and landside area density for each class (LAD) is given in Table 3. It showed that co-sei smic landsides mainly occurred on locations in the gradient range from 30° to 50°. The LPD and LAD displayed an increase with the gradient, especially on gradients between 40°and 50°, with the highest landside density (Figs. 7−8). Co-seismic landslides had the same distribution characteristics in the Wenchuan earthquake (Chigira et al., 2010) and the same phenomenon of rainfal l landslides were found following the Chi-Chi earthquake (Chen et al., 2006).4.2.3 Distance to fault fac tor

The distance to the major seismogenic fault can refl ect the distribution characteristics of a landslide. In order t o avoid artifi cial divisions, the scope of the study area was defi ned by township borders. It can accurately prove that both distance to fault factors and distance to river system play an important role in earthquake

Fig. 4 Map showing the distribution pattern of landslides triggered by the Lushan earthquake in Baosheng town

Fig. 5 Imagery from two different data showing the landslide development in Baosheng country (a) SPOT4 image taken on April 9, 2011 before the Lushan earthquake; (b) aerial photograph taken on April 22, 2013 after the Lushan earthquake

0 0.5 1.0 km

Landsides

(a) (b)


landslides.In order to understand the relationship between

landsides and distance from the seimogenic fault, an analysis of seimogenic faults within a buffer zone was undertaken (Fig. 9). The LAD and LP D showed that co-seismic landsides mainly occurred in locations within a distance from 0−6000 m, and especially in the range from 0−3000 m (Fig. 10). However, they displayed a decrease in distances from 0−12,000 m and suddenly increased from 12,000 m to 15,000 m (Table 4). The analysis of the statistical data shows that landslide distribution is o bviously affected by the fault location. The nearer the fault, the greater the number of landslides.

4.2.4 Geologic unit factor The geologic units play an impo rtant role in the

occurrence of landslides. Five major geologic units were interpreted and drawn on a simplifi ed geologic map (Fig. 11). Over 95% of t he landslides occurred in three geologic units: Paleogene mudstones, Triassic dolomites, and Cretaceous sandstones. The frequencies of slope failures were shown in different geological units in Fig. 12.

The lithology of cretaceous units was primarily composed of mudstones and sandstones, which showed the landslide area as a percentage of the total landside area was 56.29%. In addition, LAD and LPD showed that

Table 2 Relationship between the distribution of landslides and the elevations

Elevation (m) Area (km2) Area (%)Coseismic landslide

LS Area(km2) LS Number LS Area (%) LPD( LS/km2) LAD (%)

500−1000 80.07 65.57 0.44 462 36.71 5.77 0.54

1000−1500 40.44 33.12 0.74 238 62.62 5.89 1.83

>1500 1.61 1.31 0.01 3 0.68 1.87 0.50

In total 122.11 100.00 1.19 703 100.00 5.76 0.97

Table 3 Relationship between the distribution of landslides and slope gradients

Slope (°) Area (km2) Area (%)Coseismic landslide

LS Area (km2) LS Number LS Area (%) LPD( LS / km2) LAD (%)

0−10 56.01 45.87 0.34 96 28.61 1.71 0.61 10−20 23.51 19.25 0.12 182 10.21 7.74 0.51 20−30 25.33 20.74 0.28 202 23.29 7.98 1.09 30−40 12.60 10.32 0.24 131 20.42 10.40 1.92 40−50 3.43 2.81 0.20 68 16.46 19.81 5.68 50−60 1.23 1.01 0.01 24 1.01 19.48 0.97 In total 122.11 100.00 1.19 703 100.00 5.76 0.97

Landslide point density (LPD)Landslide area density (LAD)

Land

slid

e po

int d

ensi

ty (L

S/km

2 )

7

6

5

4

3

2

1

0

3

2

1

0

Land

slid

e ar

ea d

ensi

ty (%

)

500−1000 1000−1500 >1500 Elevation (m)

Fig. 6 Plots showing landslide point density and landslide area density for different elevation

Landslide point density (LPD)Landslide area density (LAD)

6

5

4

3

2

1

0

Land

slid

e ar

ea d

ensi

ty (%

)

20

16

12

8

4

0

Land

slid

e po

int d

ensi

ty (L

S/km

2 )

0−10 10−20 20−30 30−40 40−50 50−60 Slope (◦)

Fig. 7 Plots showing landslide point density and landslide area density for different slope gradients


co-seismic landsides mainly focus on Triassic dolomites (Table 5). The mudstones were mainly distributed in the study area, which were likely to lead to the occurrence of small landsides. 4.2.5 Distance to river system

The distance f rom the river system was one of the most important factors used to evaluate distribution characteristics of the landslides. This analysis can help local residents choose appropriate locations for post-earthquake reconstruction work. Field surveys and interpretation of aerial photographs indicated that

Fig. 8 Map showing the distribution of landslides at different slope gradients

Fig. 9 Map showing the distribution of landslides at different distances to Shuangshi-Dachuan fault

2

1

0

12

10

8

6

4

2

0

Land

slid

e ar

ea d

ensi

ty (%

)

Landslide point density (LPD)

Landslide area density (LAD)

Land

slid

e po

int d

ensi

ty (L

S/km

2 )

0−3 3−6 6−9 9−12 12−15 15−18 Distance of Shuangshi-Dachuan fault (km)

Fig. 10 Plots showing landslide point density and landslide area density for different distances to Shuangshi-Dachuan fault

Fig. 11 Map showing the distribution of landslides at different geologic units

0°−10°10°−20°20°−30°30°−40°a40°−50°50°−60°60°−90°

EpicenterFaultTown boundaryLandslides

0 2 4 6 8 km

0−3000 m3000−6000 m6000−9000 m9000−12000 m12000−15000 m15000−19000 m

EpicenterFaultTown boundaryLandslides0 2 4 6 8 km

Triassic dolomitesPermian carbonateJurassic sandstonesPaleogene mudstonesDevonian carbonateCretaceous sandstonesQuaternary alluviumSinian dolomite



Table 4 Relationship between the distribution of landslides and t he distance to fault

Distance to fault (km) Area (km2) Area (%)Coseismic landslide

LS Area(km2)

LS Number

LS Are(%)

LPD( LS / km2)

LAD (%)

0−3 31.24 25.58 0.64 324 54.09 10.37 2.05

3−6 41.47 33.96 0.35 204 29.87 4.92 0.85

6−9 22.77 18.65 0.10 102 8.69 4.48 0.45

9−12 15.86 12.99 0.03 36 2.87 2.27 0.21

12−15 9.08 7.44 0.03 35 2.70 3.85 0.35

15−19 1.68 1.38 0.02 2 1.77 1.19 1.25

In total 122.11 100.00 1.19 703 100.00 5.76 0.97

Landslide point density (LS/km2)0 2 4 6 8 10 12

Landslide point density (LPD)

Landslide area density (LAD)

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6Landslide area density (%)

Quaternary alluvium

Paleogene mudstones

Triassic dolomites

Cretaceous sandstones

Jurassic sandstones

Fig. 12 Plots showing landslide point density and landslide area density for different geologic units

Fig. 13 Map showing the distribution of landslides at different distances to river system

Table 5 Relationship between the distribution of landslides and geologic units

Geologic units Area (km2) Area (%)Coseismic landslide

LS Area (km2)

LS Number

LS Area (%)

LPD ( LS / km2)

LAD (%)

Quaternary alluvium 7.32 6.00 0.02 13 1.52 1.78 0.25

Paleogene mudstones 37.16 30.43 0.23 178 19.75 4.79 0.63

Triassic dolomites 16.85 13.80 0.25 187 21.35 11.10 1.50

Cretaceous sandstones 56.42 46.21 0.67 321 56.29 5.69 1.18

Jurassic sandstones 4.35 3.56 0.01 4 1.10 0.92 0.30

In total 122.11 100.00 1.19 703 100.00 5.76 0.97


High: 3400Low: 0


co-seismic landsides mainly occurred about 600 m from the drainage system in the study area (Fig. 13). In consideration of the many landsides distributed near a river system (Table 6), the post-earthquake reconstruction work must take the safety distance to the river into account and strengthen disaster monitoring.

5 Discussion and conclusions

The “4.20” magnitude Ms 7.0 Lushan earthquake occurred in the Sichuan province, and was the second earthquake to strike the area in fi ve years. Much data were collected in order to study the distribution characteristics of these landslides. The Lushan and Wenchuan earthquakes induced different types of landslides. The Wenchuan earthquake produced many large deep-seated landslides, with volumes th at can be achieved in tens of million cubic meters. This phenomenon may indicate that the Wenchuan earthquake was directly triggered by a thrusting seism genic fault (Chang et al., 2014). The earthquake may have triggered many co-seismic landslides, which supply the volume of sediment material. If a sei smic event occurs during the rainy season, sediment material may be mobilized, leading to debris fl ows. Future studies should focus on the debris fl ow basin, which has a large distribution of landslides. The authors will continue to collect landslide data in order to fi nd more characteristics in future research. Early warning systems should be used in potentially high-hazard areas.

The epicenter zone with a total area of 122.11 km2 was chosen to study the distribution characteristics of the landslides. As a result of a combination of fi eld investigations and interpretations of aerial photographs, 703 landslides were identifi ed in seven towns. These landslides show some common characteristics in the study area, such as small area, shallow slide, and surface fracture. These landslides were concentrated in locations with slopes between 0°and 50o, especially in areas that

exceeded 20°. Landslides were mainly distributed in elevations between 1,000 and 1,500 m asl and focused on Paleogene mudstones, Triassi c dolomites, and Cretaceous sandstones. The concentration of landslides displayed close contact with the distance from the Shaungshi-Dachuan fault. The values of the landslide distribution appeared in the range from 0 to 0.4 km away from the river system.

Acknowledgment

This research is supported by Basic Work for the National Science and Technology Special Program (2011FY110100-3) and Special Research Fund for the Doctoral Program of Higher Education (20125122130001).

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Table 6 Relationship between the distribution of landslides and the distance to river system

Distance to river system (km) Area (m2) Area (%)

Coseismic landslideLS Area

(km2) LS Number LS Area (%)

LPD( LS / km2)

LAD (%)

0-0.2 74.63 61.12 0.31 205 25.91 2.75 0.41

0.2-0.4 20.98 17.18 0.37 128 31.56 6.10 1.78

0.4-0.6 12.65 10.36 0.18 112 15.11 8.85 1.41

0.6-0.8 7.38 6.05 0.13 92 11.05 12.46 1.77

0.8-1.0 4.23 3.47 0.14 79 11.73 18.66 3.28

1.0-1.2 1.53 1.26 0.04 46 3.46 30.01 2.67

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