Nat. Hazards Earth Syst. Sci., 14, 1257–1267, 2014 www.nat-hazards-earth-syst-sci.net/14/1257/2014/ doi:10.5194/nhess-14-1257-2014 © Author(s) 2014. CC Attribution 3.0 License.

Brief Communication: Landslides triggered by the Ms = 7.0 Lushan earthquake,

X. L. Chen1, L. Yu1, M. M. Wang2, C. X. Lin3, C. G. Liu4, and J. Y. Li1 1Key Laboratory of Active Tectonics and Volcano, Institute of Geology, China Earthquake Administration, Beijing, 100029, China 2Earthquake Administration of Province, , 610041, China 3Beijing Institute of Geology, Beijing, 100120, China 4China Earthquake Networks Center, Beijing, 100045, China

Correspondence to: X. L. Chen ([email protected])

Received: 19 June 2013 – Published in Nat. Hazards Earth Syst. Sci. Discuss.: 7 August 2013 Revised: 10 April 2014 – Accepted: 12 April 2014 – Published: 23 May 2014

Abstract. Earthquake-triggered landslides have drawn much slide concentration area during the Lushan earthquake com- attention around the world because of the severe hazards they pared to other methods. pose. The 20 April 2013 Ms = 7.0 Lushan Earthquake, which occurred in the Longmen Shan region in Sichuan province, China, triggered more than 1000 landslides throughout an area of about 2200 km2, and completely blocked many roads 1 Introduction and exacerbated overall transportation problems in the moun- tainous terrain. Preliminary landslide inventory is complied At 08:02 LT (Beijing Time) on 20 April 2013, a strong immediately following the earthquake, mainly based on the earthquake with Ms = 7.0 in surface wave magnitude high-resolution remote sensing images. At the same time, the occurred at the eastern margin of the Tibetan Plateau distribution of these landslides is statistically investigated to in Sichuan province, China. Its focal depth was 13 km determine how the occurrence of landslides correlates with (http://www.cea.gov.cn/publish/dizhenj/468/553/100342/ distance from the earthquake epicenter, slope steepness, seis- 100343/20130422160312200248983/index.html). The mic intensity and rock type. Statistic analysis is conducted event is named the Lushan earthquake as its epicenter ◦ ◦ using landslide point density (LPD), which is defined as the (30.3 N and 103.0 E) was located in the administrative number of landslides per square kilometer. It is found that region of Lushan county (Fig. 1). According to a govern- LPD has a strong positive correlation with slope gradients ment report, there were 196 people killed in this event and a negative-exponential decline with the distance from (http://www.cea.gov.cn/publish/dizhenj/468/553/100342/ the epicenter. The higher LPD values occur in younger strata 100345/20130424151225623554842/index.html). systems like Quaternary and Tertiary sediments in the study The Ms = 7.0 Lushan earthquake occurred in the southern area. Spatially, the triggered landslides are controlled by the segment of Longmen Shan range, where a catastrophic earth- causative faults and mainly concentrated around the epicen- quake named the Wenchuan earthquake occurred five years ter. All the landslides are located within the area with seis- earlier (Xu et al., 2008; Yin et al., 2009; Huang et al., 2008; mic intensity ≥ VII and in line with seismic intensity. Gener- Qi et al., 2010). This recent earthquake occurred to the south ally, LPD value decreases with increasing distance from the of the Wenchuan earthquake, and the distance between these epicenter, and sometimes landslides are densely distributed two epicenters is around 90 km. along the roads in the mountainous region. Also, this study As it is known that strong earthquakes can trigger many reveals that the empirical relationship between distance and landslides in the mountainous terrain (Keefer, 1984; Bommer seismic magnitude is more suitable for estimating the land- et al., 2002; National Institute for Land and Infrastructure Management, Ministry of Land, Infrastructure, Transport

Published by Copernicus Publications on behalf of the European Geosciences Union. 1258 X. L. Chen et al.: Landslides triggered by the Ms = 7.0 Lushan earthquake and Tourism, 2004; Yin et al., 2009; Chen et al., 2010), the Lushan earthquake is no exception. Preliminary inter- pretation of remote sensing images, which are provided by the Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences and field investigations, which was conducted by the Institute of Geology, China Earthquake Ad- ministration, show that more than a thousand of landslides were triggered during this earthquake. However, with respect to the Wenchuan earthquake, the landslides triggered by the Lushan earthquake are smaller both in the amount and the affected area. The phenomenon of two very strong earth- quakes occurring successively in the Longmen Shan fault zone (LSFZ) in five years is attractive to scientists, not only for exploring the relation between them, but also for studying the geohazards caused by the Lushan earthquake (Han et al., 2013; Xu et al., 2013). The occurrence of earthquake-triggered landslides is gen- erally related to the magnitude of ground motion, distance from an earthquake fault or epicenter, rock types, slope gra- dient and so on (Harp el al., 1981; Keefer, 1984; Wang et al., 2003, 2007; Jibson et al., 2004; Meunier et al., 2007; Qi et al., 2010; Dai et al., 2011; Catani et al., 2013). The Lushan earthquake-triggered landslides provide a good opportunity for statistical analysis of a landslide distribution known to have been produced by an earthquake. In this study, LPD, defined as the number of landslides per square kilometer and widely used in landslide statistic Figure 1. Geological map for the 20 April 2013 Ms = 7.0 Lushan analysis (Wang et al., 2007; Dai et al., 2011) is used to sta- Earthquake and the adjacent region (revised from Xu et al., 2009). tistically investigated how the occurrence of landslides cor- relates with distance from the earthquake epicenter, slope steepness, seismic intensity and rock type. Additionally, it rupture is on the Yingxiu–Beichuan strand (Xu et al., 2008; is concluded that the empirical relationship between distance Zhang et al., 2010). Except for these three main faults in the and seismic magnitude is more suitable for estimating the LSFZ, there are others faults and folds developed in this re- landslide concentrated area for the Lushan earthquake com- gion during the geological evolution (Fig. 1). pared to other methods. Also, in order to effectively evaluate The Longmen Shan range is deforming as a result of the the landslide hazard severity, interpretation standards for de- collision between the Indian and the Eurasian plates. This limitating landslides should be developed. has resulted in structural stress accumulation on the collision edges and the release of stress in the fault zone that brought on the catastrophic Wenchuan earthquake, China’s most dis- 2 Tectonic setting astrous event since the (Xu et al., 2008; Zhang et al., 2010). As with the Wenchuan earthquake, The Ms = 7.0 Lushan earthquake occurred at the southern the Lushan earthquake was also caused by the slipping of segment of the LSFZ in the eastern margin of the Tibetan slightly dipping abnormal faults in the deep southern part Plateau (Han et al., 2013). The LSFZ, which strikes ap- of Longmen Shan nappe structure zone. The epicenter of proximately 45◦ and dips 50–75◦ toward the northwest (Xu the Lushan earthquake was located in the area with increas- et al., 2008) lies along the middle segment of the Cen- ing Coulomb stress, so that the occurrence of the Wenchuan tral Longitudinal Seismic Belt (CLSB) of China, and sepa- earthquake may have triggered or accelerated the Lushan rates the seismically active Tibetan Plateau from the tecton- earthquake occurrence (Xu et al., 2013). ically stable Ordos block, Sichuan basin, and South China Unlike the Wenchuan earthquake, which generated long block (Zhang et al., 2010). It consists of three sub-parallel ground surface ruptures, the field investigation after the thrust faults, namely the Wenchuan–Maowen (F1), Yingxiu– Lushan earthquake indicated that except for some fracture Beichuan (F2) and Guanxian–Anxian (F3) faults, and a evidence at the surface, no obvious surface rupture zones frontal blind thrust fault (F4) (Xu et al., 2008; Zhang et were formed (Han et al., 2013). With the understanding of al., 2010). During the 2008 Wenchuan earthquake, the event the distribution of relocated aftershocks, focal mechanism ruptured several strands of this fault zone, and the primary solutions and surface structural geology, it is inferred that

Nat. Hazards Earth Syst. Sci., 14, 1257–1267, 2014 www.nat-hazards-earth-syst-sci.net/14/1257/2014/ X. L. Chen et al.: Landslides triggered by the Ms = 7.0137 Lushan Fig. 3. earthquake Slope map for the study region and the landslides triggered by the 20 April 2013 1259 Ms = 7.0 138 Lushan Earthquake.

103°0'0"E 103.0°E ± 30.5°N

103°0'0"E F2 Baoxing

! ! F1 !! ! ! ! !! ! !! !! !! ! ! ! ! ! !! ! ! !!!! ! ! ! !! !!! !!!!! ! !! ! F3 !!!!!!! ! ! ! !!! !! ! ! ! ! !! ! ! 5 !!! ! ! !! !! ± ! ! !!!! ! !!!!!! !!! !! !! !! !!!! !! ! ! Baoxing !!!!!!! !!! !!! ! ! ! !! ! ! !!! ! ! !!! ! !!!!!!!! ! ! !!!! ! !!!!!!! ! !!!! !! !!!!!! !! !!! !!!!! ! !!!!! !!! ! ! !!! ! ! ! !!!!!!! ! !!! ! !!!!!!!! !!!!!!!! !! !!!! !!! !!!!!! !!!!!!!!!!!! ! ! !!!! !!!! !!!!! ! !! ! !! !!! !!!!!! ! !! ! ! !! !!!!! !!!! ! !!! !!! ! !! !! ! ! !!! ! R ! ! !! !! ! ! ! ! ! !!!!! ! ! !! !!! ! !!! F2 ! !! !! !!! !!!! !!!!! !! ! ! !!! ! !! !! !!!! ! ! ! !!! !! ! ! ! !! !! ! ! F1 !!! ! !! ! !! ! ! !!!!!!!!! ! ! !!!!!! !! !! !!! ! ! !!!! ! ! !! ! !!! ! !!! !! ! ! ! !! ! ! Q ! !!! ! ! ! ! ! !! !!!!! !! ! !!!!!! ! ! !! !! !! F3 !!!!!!!! !! ! !! ! !! !! ! ! ! !!! !!!! !!! ! ! !! !!! !! !!! ! ! faults !!!! ! !!! ! ! ! !! ! ! ! !!!! ! ! 5 !!!! ! ! ! ! ! !! !! !!! !! ! ! !!!! ! !! 5 ! ! !!!!!! !!! !! E ! ! !! !! ! !! ! !!!! ! !!!! ! ! !!! ! ! Baoxing ! !!!!!!! ! ! ! !!! !!! ! !!! !! ! ! ! ! ! !! ! ! ! !! ! !! !!!! ! ! !! !!! landslides ! !!!!!!!! !!!!! Lushan!! !!! ! ! !! !!!! ! !!!! !!! !!!!!!!! !! ! !! !!!!!! !!!!!!! ! ! ! !!!! ! ! !!! ! !!!!!!!!!! ! !!! !!!!!!!!!!!!!!! !! ! !!! !!! !!!!!!!!!!!!!!!!!!!! ! !!!! !!!!! !!!!!!! !!!!!!!!!! ! ! K ! !!! !!!!!!! ! ! ! epicenter !! !!! !! ! ! ! ! ! ! ! R !! !! ! ! ! ! ! !! !!! ! 5 Lushan !! ! ! ! ! !!! ! Mingshan R ! ! ! ! ! 5 !! ! ! Tianquan ! ! ! ! ! ! ! ! Elevation (m) J ! !!!!!! ! !! ! ! !! ! ! !! !! ! !!! !!!! Tianquan ! !!!!! ! ! ! ! !!! ! ! !!! !! ! High : 4801 T ! ! !!!! !!! ! !! !!!! !!! !!!! ! ! !!!!!!!!! ! !!!!!! !! ! ! !!!! ! 30°0'0"N !!! ! !!! !! ! !!! ! ! !! !!! ! ! Mings han Kilometers ! ! !! ! 5 P ! !! ! ! !!! !! !! ! ! ! !!! ! faults !!!! !!!!! ! !! ! ! !!! ! !! !! 04 8 16! ! 24!! !! 32 5 ! !Yaan ! ! !! ! ! ! ! ! ! !! ! ! ! ! ! !! ! ! ! Low : 474 ! !! ! !!! !! ! !!!!!!! landslides ! !!!! Lushan ! ! ! C !! ! ! ! ! ! ! ! R! epicenter ! ! 5 131 ! Mingshan D ! 5 !! Tianquan ! 30.0°N ! Elevation (m) !! Yaan 132 F1: Wenchuan! ! ‐Maowen fault, F2: Yingxiu‐Beichuan fault, F3: Guanxian‐Anxian fault ! S Figure 2. Elevation! ! map for the study region andHigh the : 4801 land- ! !! 133 Fig. 2. Elevation map for the! study region and the landslides triggered by the 20 April 2013 Ms = 30°0'0"N O Kilometers 5 M = slides triggered by the 20 AprilYaan 2013 s 7.0 Lushan Earth- 134 7.0 Lushan04 Earthquake 8 16 (from 24 SRTM,! 32 90m, http://datamirror.csdb.cn/admin/datademMain.jsp). Low : 474 Z quake (from SRTM,! 90 m, http://srtm.csi.cgiar.org/SELECTION/ 131 Pt 132 inputCoord.aspF1: Wenchuan).‐Maowen fault, F2: Yingxiu‐Beichuan fault, F3: Guanxian‐Anxian fault 0 10 km epicenter landslide 133 Fig. 2. Elevation map for the study region and the landslides triggered by the 20 April 2013 Ms 139= 134 7.0 Lushan Earthquake (from SRTM, 90m, http://datamirror.csdb.cn/admin/datademMain.jsp). 140 Fig.Figure 4. Geological 4. mapGeological for the region damaged map forby the the Lushan region earthquake damaged (revised after by CGS, the Regional Lushan geological 141 earthquake (revisedmap from of Sichuan China Province Geological (1:200, 000), 2001). Survey (CGS), Re- 142 gional geological map of Sichuan province (1 : 200000), 2001). 143 Table 1. Simplified strata system of the severely damaged zone by the Ms 7.0 Lushan earthquake (revised after Qi 144 et al, 2010)

Sequence Symbol Lithology TheQuaternary study region Q has exposures Alluvium, of Loose the deposit strata from the pre- PaleozoicTertiary to Quaternary E periods. Mudstone, Almost sometimes all intercalated bedrock with appear- mudstone ing inCretaceous this region areK weathered Conglomerate and deformed. Rocks found hereJurassic mainly include J sandstone, Sandy mudstone, slate, mudstone, and sandyshale. stone intercalated Allu- with mudstone

vial depositsTriassic are developed T along Sandy stone, the limestone, river coursesslate (Fig. 4, TablePermian 1). P Thick limestone intercalated slate TheCarboniferous steepwestern C margin of Limestone, theSichuan marble and sandy basin stoneis known to beDevonian seismically active.D The earliest Quartzose sandstone earthquake documented 135 Silurian S Sandy stone, phyllite intercalated with limestone 136 F1: Wenchuan‐Maowen fault, F2: Yingxiu‐Beichuan fault, F3: Guanxian‐Anxian fault in this region can be traced back to the M = 6.0 earthquake Ordovician O Limestone, marble ands phyllite of Baota formation in September 1327 (Department of Earthquake Disaster Pre- 135 vention, State Seismological Bureau, 1995). In total, there 136 F1: Wenchuan‐Maowen fault, F2: Yingxiu‐Beichuan fault, F3: Guanxian‐Anxian fault Figure 3. Slope map for the study region and the landslides trig- have been 17 earthquakes with magnitudes ≥ 4.7 in the area gered by the 20 April 2013 Ms = 7.0 Lushan Earthquake. subjected to the Lushan earthquake (Xu et al., 2013b). Fre- quent tectonic activities have created a topography with high mountains intersected by deeply incised valleys and devel- oped a geologically weak region with fragile strata. There- the seismogenic fault of the Lushan earthquake can be clas- ◦ fore, this region is notoriously prone to landslides (Yang et sified as a typical blind reverse-fault which strikes 212 and al., 2002; Wang et al., 2008). dips toward northwest with a dip angle of 38◦ ± 2◦ (Xu et al., 2013). Topographically, as the transitional zone from the Tibetan 3 Landslides generated by the Lushan earthquake Plateau to the Sichuan Basin plain the relief in the Longmen Shan range gradually decreases eastward. To its west, eleva- The Lushan earthquake triggered more than 1000 land- tions reach more than 4000 m above sea level, while at east slides throughout an area of about 2200 km2, and completely of this belt, elevations of the Sichuan Basin lie only 600 m blocked many roads and exacerbated overall transportation above sea level. On the whole, the elevations in the north- problems in the mountainous terrain. As a whole, the land- western region are higher than that in the southeastern, while slides triggered by the Lushan earthquake spread in the NE– the reliefs in northwestern side are steeper than in southeast- SW direction, the same as the LSFZ, showing strong influ- ern side. In this study region, the elevations are usually lower ence from the tectonics. Because the southwestern region of than 3000 m (Figs. 2, 3). the epicenter is occupied by an intermountain basin, land-

www.nat-hazards-earth-syst-sci.net/14/1257/2014/ Nat. Hazards Earth Syst. Sci., 14, 1257–1267, 2014 1260 X. L. Chen et al.: Landslides triggered by the Ms = 7.0 Lushan earthquake

Table 1. Simplified strata system of the severely damaged zone by the Ms = 7.0 Lushan earthquake (revised after Qi et al., 2010).

Sequence Symbol Lithology Quaternary Q Alluvium, Loose deposit Tertiary E Mudstone, sometimes intercalated with mudstone Cretaceous K Conglomerate Jurassic J Sandy slate, mudstone, sandy stone intercalated with mudstone Triassic T Sandy stone, limestone, slate Permian P Thick limestone intercalated slate Carboniferous C Limestone, marble and sandy stone Devonian D Quartzose sandstone Silurian S Sandy stone, phyllite intercalated with limestone Ordovician O Limestone, marble and phyllite of Baota formation Sinian Z Metamorphic sandy stone, metamorphic limestone Archean Pt Granite, diorite, gabbro slides are absent in the inner basin while continue distribut- of landslides can always produce severe damage because of ing along the basin sides (Fig. 3). The landslide inventory their large volumes (Highland and Bobrowsky, 2008; Chen map as well as the field investigation show that the landslide et al., 2012b; Xu et al., 2013b). concentration area is limited to the epicentral region (Figs. 2, In this paper, we will only discuss the general distribution 3). Away from the epicenter, landslides were scattered and features of earthquake-induced landslides and will not dis- occurred mainly in the form of rock falls. tinguish between different types of landslides or discuss the differences. 3.1 Types of landslides 3.2 Landslide inventory Landslides are a type of slope movement that can be classi- fied by a combination of material compositions of the slid- Landslide inventory is an essential part of seismic landslide ing mass and the type of movement (Varnes, 1978). Dur- hazard analysis (Harp et al., 2011; Guzzetti et al., 2012). ing our field investigation, we found that landslides triggered Compared with traditional landslide inventory compilation, by the Lushan earthquake were of various types. Although landslide detection and mapping now benefit from both op- sometimes landslides are of more than two failure types, tical and radar imagery. Many studies and much research on they can be primarily classified into three types, namely landslides have proved that remote sensing can be considered rock falls (Fig. 5a), shallow slope failures (Fig. 5b) and a powerful instrument for landslide mapping, monitoring and rock or soil slides (Fig. 5c), referencing Highland and Bo- hazard analysis (Qi et al., 2010; Dai et al., 2011; Guzzetti browsky (2008). et al., 2012; Tofani et al., 2013; Xu et al., 2013a). For the The area damaged by the Lushan earthquake is cov- Lushan earthquake, remote sensing technology plays a vital ered with fast-growing vegetation (Xu et al., 2013b), so the role in accessing the information in quake disaster areas. ground surface failures always have clear scarps and are eas- In our study, compilation of landslide inventories is mainly ily found and delineated during the field investigation, con- performed by means of interpretation of aerial photography. ducted by the Institute of Geology, China Earthquake Ad- Because most landslides triggered by the Lushan earthquake ministration. Remote sensing images, which are provided by had small sizes both in plane area and volume, it is better to the Institute of Remote Sensing and Digital Earth, Chinese map landslides as points which represent the failure sources Academy of Sciences were also interpreted. Rock falls are near the top scarps. Similar to Dai et al. (2011), landslides the most common phenomena that can be found on steep were identified in the remote sensing images by the follow- slopes along the roads and rivers, with volumes usually less ing characteristics: (1) landslides scarps showed newly de- than several tens of cubic meters (Xu et al., 2013b). Shal- nuded vegetation on the slopes; (2) landslides scarps showed low slope failures are usually composed of weathered and distinct white or brown coloration as compared to the sur- fractured materials, appearing on gentle-to-moderate slopes roundings and (3) landslide debris movement paths could be where surface slides are likely to occur even without strong clearly observed. Individual boulders rock falls are not ac- shaking. The depth of this kind of landslide is usually less counted in this study. than 1 m and the volumes are in the range from several to sev- The remote sensing images with resolution of 0.6 m, which eral tens cubic meters (Xu et al., 2013b). Rock or soil slides are provided by the Institute of Remote Sensing and Dig- generally have a slide plane which is on surface ruptures or ital Earth, Chinese Academy of Sciences, are used in this on relatively thin zones of intense shear strain. These kind study. Except for some areas where are covered by clouds

Nat. Hazards Earth Syst. Sci., 14, 1257–1267, 2014 www.nat-hazards-earth-syst-sci.net/14/1257/2014/ 223 area. 224 225 3.3 Variation of landslide distribution with slope gradient 226 227 Many researches show that topographic features can affect landslide distribution (Qi, 2006; 228 Keefer, 1984, 2006; Dai et al, 2011; Catani et al., 2013). In general, steeper and higher slopes are 229 more prone to landslide activity than gentle slopes (Keefer, 1984; Wang et al., 2007; Dai et al, 230 2011; Chen et al., 2012). In this study, DEM (SRTM, 90m) was used to generate slope angles (Fig. 231 3), and for each 5° interval of slope gradient, a LPD value was calculated. Because the areas with 232 slope gradient greater than 45° together cover less than 2% of the total surface area, they are 233 classified as one category. Fig. 6 shows the LPD value within each 5° interval of slope gradient. It X. L. Chen et al.: Landslides triggered by the Ms = 7.0 Lushan234 reveals earthquake that LPD values steadily increase with slope gradient. 1261 235

0.8 0.7 0.6

Density 0.5

0.4

Point 0.3

0.2 0.1 0 Landslide < 10.0 10.0 - 14.9 15.0 - 19.9 20.0 - 24.9 25.0 - 29.9 30.0 - 34.9 35.0 - 39.9 40.0 - 45.0 > 45.0 Slope gradient ( °) 236 237 Figure 6. LandslideFig. 6. Landslide point density density (LPD) vs. slope vs. gradient slope in the gradient study area in the study 238 239 3.4area. Landslide concentration with geological formations 240 241 The study area is mainly composed of Mesozoic and Cenozonic strata, which crop out in the 242 easternand shadows,side of the region landslides (Fig. 4). Paleozoic can and be pre detected‐Paleozoic sediments easily are from limited the to the im- 243 northwestern side of the region with higher and steep relief (Fig. 2, Fig. 3, Fig. 4). 244 agesOur study in shows the majority that most of the of landslides the study are presented region in becausethe Cretaceous of strata, the which clear 245 consistscarps of siltstone, they left. mudstone, or mudstone intercalated with shale. These rocks are heavily 246 fracturedIn total,and always our have studyweak shear show strength. that So it is there reasonable are that 1129most of the landslides landslides 247 are concentrated on the slopes consisting of such rocks. Tertiary and Quaternary2 sediments 248 presentmapped near the and epicenter the in affected a comparatively area flat plain, is aroundtherefore, landslides 2200 frequently km (Figs. occurred 2, 249 here3). despite The the following relief is generally analysis gentle. on correlating LPD value with dis- 250 tanceAn LPD fromvalue is determinate the earthquake for each geological epicenter, unit for this slope study gradient,area. Except for seismic Triassic 251 sediments, LPD value decreases with the strata turning old (Fig. 7). The highest LPD value is for 252 theintensity Quaternary andsystem. rock Though type there are is not based many landslides on this (< affected30) occurred in area. the Quaternary 253 sediments, the LPD value is high because of this stratum’s smaller area of 20 km2. Without 254 regarding3.3 Variationto the study area of with landslide slope lower than distribution 10° in different strata with systems, the bigger LPD 255 values appear in the Quaternary and Tertiary sediments. Considering the epicenter location 256 within theslope geological gradient setting (Fig.3, Fig.4), it is reasonable that the landslides are prone to

Much research has shown that topographic features can affect landslide distribution (Qi, 2006; Keefer, 1984, 2006; Dai et al, 2011; Catani et al., 2013). In general, steeper and higher slopes are more prone to landslide activity than gentle slopes (Keefer, 1984; Wang et al., 2007; Dai et al, 2011; Chen et al., 2012b). In this study the Shuttle Radar Topography Mission’s digital elevation model (90 m) was used to generate slope an- gles (Fig. 3), and for each 5◦ interval of slope gradient, a LPD value was calculated. Because the areas with slope gradient greater than 45◦ together cover less than 2 % of the total sur- face area, they are classified in one category. Figure 6 shows the LPD value within each 5◦ interval of slope gradient. It re- veals that LPD values steadily increase with slope gradient.

3.4 Landslide concentration with geological formations

The study area is mainly composed of Mesozoic and Ceno- zoic strata, which crop out in the eastern side of the region (Fig. 4). Paleozoic and pre-Paleozoic sediments are limited to the northwestern side of the region with higher and steeper relief (Figs. 2, 3, 4). Our study shows that most of the landslides occurred in the Cretaceous strata, which consist of siltstone, mudstone or mudstone intercalated with shale. These rocks are heavily fractured and always have weak shear strength. So it is rea- sonable that most of the landslides were concentrated on the slopes consisting of such rocks. Tertiary and Quaternary sed-

Figure 5. Various landslide types during the 20 April 2013 Ms = 7.0 iments are present near the epicenter in a comparatively flat Lushan Earthquake. Photos by M. M. Wang. plain, therefore, landslides frequently occurred here despite the fact that the relief is generally gentle. An LPD value is determinate for each geological unit for this study area. Except for Triassic sediments, LPD value www.nat-hazards-earth-syst-sci.net/14/1257/2014/ Nat. Hazards Earth Syst. Sci., 14, 1257–1267, 2014 257 concentrated in the near epicenter area with steeper relief and softer materials. 258 1262 X. L. Chen et al.: Landslides triggered by the Ms = 7.0 Lushan earthquake 259

1.4 3.5 slope < 10º not analyzed 1.2 3 all slopes LPD = 4.923e‐0.69D

1 Density 2.5 R² = 0.971 Density 0.8 2 Point

Point 0.6 1.5 0.4 1 Landslide 0.2

Landslide 0.5 0 LPD, 0 Q EKJTpre‐T Strata systems 5 101520253035 260 D, Distance to the epicenter (km) 261 Fig.7. Landslide point density vs. strata systems for the study area Figure 7. Landslide point density vs. strata systems for282 the study 262 Figure 8. Landslide distribution with the distance from the epicen- area. 283 Fig. 8. Landslides distribution with the distance from the epicenter during the 20 April 2013 Ms = 7.0 Lushan 263 3.5 Distribution of landslides with the distance to epicenter and seismic intensity = 284 ter during the 20 April 2013 MEarthquakes 7.0 Lushan Earthquake. 264 265 Causative fault is an important factor that can influence the distribution of landslides285 during a 266 strongdecreases shaking event with(Khazai older and Sitar, strata 2003; Wen (Fig. et al., 7). 2004; The Wang highest et al., 2008; LPD286 Catani value et al.,Based on the Lushan earthquake seismic intensity distribution map issued by China 267 2013).is With for the the increasing Quaternary of distance system. from the causative Though fault not or epicenter, many landslides the287 number Earthquake of Administration ( http : //www.cea.gov.cn/publish/dizhenj/464), it is found that all 268 triggered(< landslides30) occurred presents ina negative the Quaternary‐exponential decline sediments, (Simonett, the 1967; LPD 288Keefer, value 2000;the landslides are located within the area with seismic intensity ≥ VII, and the landslides 269 Wang iset al., high 2007; becauseChen et al., 2012). of this stratum’s smaller area of289 20 kmnumber2. is in scale with seismic intensity (Fig. 9). In the epicentral region where it has seismic 270 The Ms7.0 Lushan earthquake is caused by a blind thrust fault, and there is no obvious290 surfaceintensity◦ of IX, the LPD value reaches 1.72/km2, which is many times greater than that in the 271 ruptureWithout found during regarding the field investigation the study which area was with conducted slopes by the lower Institute than of Geology, 10 291 region with seismic intensity of VII (Fig. 10). 272 China in differentEarthquake strataAdministration systems, thesooner larger LPDafter valuesthe appearearthquake in 292 273 (http://www.eqthe Quaternary‐igl.ac.cn/wwwroot/c_000000090002/d_0976.html and Tertiary sediments. Considering). Therefore, in this the study, epi- the 274 distancecenter to the locationepicenter is withinused to analyze the geological the landslides variation. setting An (Figs. LPD value 3, is 4), calculated it is 275 for each 5 km interval centered at the epicenter. The analysis result shows that LPD value 276 decreasesreasonable with the increasing that the distance landslides to the epicenter are prone (Fig. 8). toMo concentratedre than 90% of the inlandslides the 277 are locatednear-epicenter within 30 km from area the epicenter. with steeper relief and softer materials. 278 279 3.5 Distribution of landslides with the distance to 280 epicenter and seismic intensity 281 Causative fault is an important factor that can influence the distribution of landslides during a strong shaking event (Khazai and Sitar, 2003; Wen et al., 2004; Wang et al., 2008; Catani et al., 2013). With increasing distance from the causative fault or epicenter, the number of triggered land- slides shows a negative-exponential decline (Simonett, 1967; Keefer, 2000; Wang et al., 2007; Chen et al., 2012b). The Ms = 7.0 Lushan earthquake is caused by a blind thrust fault, and there is no obvious surface rupture found during the field investigation which was conducted by the Institute of Geology, China Earthquake Administration soon after the earthquake (http://www.eq-igl.ac.cn/wwwroot/c_ 000000090002/d_0976.html). Therefore, in this study, the distance to the epicenter is used to analyze the landslide vari- ation. An LPD value is calculated for each 5 km interval cen- tered at the epicenter. The analysis result shows that LPD value decreases with the increasing distance to the epicen- ter (Fig. 8). More than 90 % of the landslides were located Figure 9. Seismic intensity distribution and the landslide limitation within 30 km of the epicenter. line for the 20 April 2013 Ms = 7.0 Lushan Earthquake. Based on the Lushan earthquake seismic intensity distribution map issued by China Earthquake Adminis- tration (http://www.cea.gov.cn/UploadFile/dizhenj/2013/04/ 3.6 Distribution of landslides along the roads 1366973148533.png), it is found that all the landslides are located within the area with seismic intensity ≥ VII, and the Roads or distance to roads have been successfully used landslide number is in line with seismic intensity (Fig. 9). In in landslide susceptibility assessments because man-made the epicentral region where it has seismic intensity of IX, the roads can modify the hill slope profile and become an impor- LPD value reaches 1.72 km2, which is many times greater tant factor in influencing landslide occurrence (Devkota et than that in the region with seismic intensity of VII (Fig. 10). al., 2013; Ramakrishnan et al., 2013; Catani et al., 2013). In southwest China, it is a normal phenomenon that landslides

Nat. Hazards Earth Syst. Sci., 14, 1257–1267, 2014 www.nat-hazards-earth-syst-sci.net/14/1257/2014/ 293 294 F1: Wenchuan‐Maowen fault, F2: Yingxiu‐Beichuan fault, F3: Guanxian‐Anxian fault 295 Fig. 9. Seismic intensity distribution and the landslides limitation line for the 20 April 2013 Ms = 7.0 Lushan 296 Earthquake 297 X. L. Chen et al.: Landslides triggered by the Ms = 7.0 Lushan earthquake 1263

2 though the maximum distance between a triggered land- 1.8 slide and the epicenter was around 45 km, related anal- 1.6 Density 1.4 ysis shows that more than 90 % of the landslides were 1.2 Point

located within 30 km from the epicenter. 1 0.8 0.6 2. In the study by Keefer, the relationship between the seis- Landslide 0.4 mic magnitude and the affected area can be expressed as 0.2 LPD, 0 VII VIII IX seismic intensity log10A = M − 3.46(±0.47), (2) 298 299 Fig. 10. LPD valueFigure variation 10. LPD with valueseismic variation intensity for with the seismic 20 April 2013 intensity Ms = for7.0 Lushan the 20 Earthquake where A is the affected area in unit of square kilometers, = 300 April 2013 Ms 7.0 Lushan Earthquake. M is seismic magnitude for 5.5 < M ≤ 9.2 (Keefer, 2002). Using this equation, the affected area during the 301 3.6 Distribution of landslides along the roads Lushan earthquake is calculated to be in the range of 302 are likely to occur along roads or river banks (Yang et al., 1175–10 232 km2. 303 Roads or distance2002; to roads Wang have et al.,been 2008; successfully Chen et used al., in 2012a), landslide and susceptibility a strong assessments seismic event will definitely aggravate this kind of geohaz- As for the Lushan earthquake, it is located in southwestern ard and usually generates landslide dams along river systems China, where landslides triggered by strong earthquakes are (Yin et al., 2009). mainly located within the area with seismic intensity ≥ VII In this study region, the relief is typical of high mountains (Earthquake-resistant and damage prevention department of intersected by deeply incised valleys, and roads are usually State Seismic Bureau, 1995; Yang et al., 2002; Chen et al., constructed near rivers. Therefore, after the earthquake, it is 2012a). Also, for earthquakes with magnitude 7.0 that oc- found that many landslides were densely developed along curred in southwestern China, the landslide-affected areas roads and river banks, especially in the mountainous areas, range from 2600 to 8843 km2 (Chen et al., 2012a). Accord- which exacerbated overall transportation problems (Fig. 11). ing to the Lushan earthquake seismic intensity map issued For example, there were 79 landslides along 5 km length by the China Earthquake Administration, the area within the of Road S210 near (Fig. 11). Our statistics seismic intensity line of VII is around 5655 km2. show that more than 20 % of the landslides occurred in a In this study, the landslide inventory obtained from inter- 100 m buffer of the roads during this event. pretation of remote images indicates that the landslides are mainly distributed in the area with seismic intensity ≥ VII. They are bounded by the semi-minor axis of intensity VII, 4 Discussions but not extended to the southern boundary of the prolate axis of intensity VII (Fig. 9). The affected area is calculated to be 4.1 Estimation of the area affected by landslides 2200 km2. The area affected by earthquake-triggered landslides shows Both the empirical relationships between the distance and strong correlation with earthquake magnitude and earth- seismic magnitude (Eq. 1) and seismic magnitude and area quakes with larger magnitudes can have larger impact areas (Eq. 2) are used to estimate how widely a given magnitude (Keefer, 1984; Bommer et al, 2002; Rodríguez et al, 1999). earthquake can influence a region. However, from this study, Therefore, seismic magnitude and distance from the epicen- it seems that the empirical relationship between the distance ter or the causative seismic fault are usually used to judge the and seismic magnitude is a good method to estimate the com- limitation boundary of landslide affected area: paratively concentrated landslide affected area. 1. Kawabe et al. (2000) suggested that the maximum dis- 4.2 Criteria of landslide size tance (D) from epicenter to landslide can be calculated using As mentioned above, both the 2008 Ms = 8.0 Wenchuan earthquake and the 2013 Ms = 7.0 Lushan earthquake oc- logD = 0.5M − 2.0, (1) curred in the LSFZ in succession (Figs. 1, 12). The Wenchuan earthquake occurred to the north of the Lushan where M is the seismic magnitude, D is the distance in earthquake, and the distance between these two epicenters is unit of km. around 90 km. Unlike the Wenchuan earthquake, which gen- For the Lushan earthquake, after substituting 7.0 for erated 240 km and 90 km long ground surface ruptures (Xu M in Eq. (1), we obtain D = 31.6 km, which is very et al., 2008; Yu et al., 2010), the Lushan earthquake did not close to the distance calculated from the landslide in- produce obvious ground surface ruptures (Han et al., 2013). ventory in this study, and it is also within the semi- Landslide distribution during these two earthquakes was minor axis (33 km) of seismic intensity VII (Fig. 9). Al- extended in the NE–SW direction, the same as the LSFZ

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Figure 11. Landslide distribution with the roads and streams for the 20 April 2013 Ms = 7.0 Lushan Earthquake. striking, indicating the influence of causative faults (Wang criteria used to evaluate landslide hazard severity (Keefer, et al., 2008; Huang et al., 2008; Chen et al., 2010; Qi et al., 1984; Bommer et al., 2002; Wang et al., 2007). Landslide 2010; Dai et al., 2011). Although there may be many differ- inventory maps record the location of all landslides that have ences between the landslides triggered by these two earth- left discernible features in an area, but sometimes landslide quakes, the most distinct differences, which can be instantly features may not be recognized in the field or through the found from the landslide inventory maps, would be the af- interpretation of aerial photographs, as they are often ob- fected area and the amount of landslides (Fig. 12). scured by erosion, vegetation, urbanization and anthropic ac- As an essential part of seismic landslide hazard analysis, tion (Malamud et al., 2004). Although there are many rea- both the landslide-affected area and landslide quantity are sons influencing the level of completeness of a landslide

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Figure 12. Landslide distribution during the 2008 Wenchuan earthquake (revised from Dai et al., 2011) and the . inventory, the lack of a uniform definition of landslide size is 5 Conclusions also an important one. The research of Qi et al. (2012) shows that landslide size has little influence on the distribution char- The Ms = 7.0 Lushan earthquake triggered more than 1000 acteristics, but sometimes a landslide inventory without uni- individual landslides throughout an area of 2200 km2. Al- form size standards can seriously influence the statistics re- though landslide damages from the Lushan earthquake were sults, especially when calculating LPD values. Exemplified not as serious as from the Wenchuan earthquake, landslides by the Wenchuan earthquake, landslide quantities from dif- did completely block many roads and exacerbated overall ferent researchers show obvious discrepancies: the quantity transportation problems in the mountainous region. ranges from 15 000 (Yin et al., 2009), to 35 000 (Huang et al., There was no ground surface rupture generated during the 2009) and to 56 000 (Dai et al., 2011). The latest study shows Lushan earthquake, nevertheless, landslide spatial distribu- that the number of the landslides triggered by the Wenchuan tion still shows domination from the causative faults. Land- earthquake is up to 197 481 and distributed over an area of slides were mainly concentrated around the epicenter and in about 110 000 km2 (Xu et al., 2013a). It is obvious that using line with seismic intensity. LPD value decreases with the in- the smallest amount of 15 000 individual landslides (Yin et creasing distance from the epicenter, and more than 90 % of al., 2009) and the largest 197 481 will produce different re- the landslides were located within 30 km distance of the epi- sults. Thus, it is necessary to urgently propose the creation center. of standards for delimitating landslides in order to improve Landslide distribution has a positive correlation with slope landslide inventory quality. gradient. Higher LPD values occur in the young strata www.nat-hazards-earth-syst-sci.net/14/1257/2014/ Nat. Hazards Earth Syst. Sci., 14, 1257–1267, 2014 1266 X. L. Chen et al.: Landslides triggered by the Ms = 7.0 Lushan earthquake systems composed of soft rocks like mudstone and sand- Department of Earthquake Disaster Prevention?State Seismologi- stone. Numerous landslides occurred along the roads and cal Bureau: Catalogue of historical strong earthquakes in China river banks in the mountainous area. Man-made roads in (23rd century B. C. to 1911), Beijing: Seismological Press, 1995 mountainous areas become important factors which must be (in Chinese). accounted for when undertaking earthquake-triggered land- Guzzetti, F., Mondini, A. C., Cardinali, M., Fiorucci, F., Santangelo, slide assessments. M., and Chang, K. T.: Landslide inventory maps: new tools for an old problem, Earth Sci. Rev., 112, 42–66, 2012 Also, it is concluded that the relationship between the dis- Haeussler, P. J., Schwartz, D. P., Dawson, T. E., Stenner, H. D., tance and seismic magnitude was more precise for estimating Lienkaemper, J. J., Sherrod, B., Cinti, F. R., Montone, P., Craw, the boundary of landslide-affected areas during the Lushan P. A., Crone, A. J., and Personius, S. F.: Surface Rupture and earthquake, and standards for delimitating landslides when Slip Distribution of the Denali and Totschunda Faults in the 3 doing remote image interpretation should be developed. November 2002 M 7.9 Earthquake, Bulletin of the Seismological Society of America, 94, 23–52, 2004. Han, Z. J., Ren, Z. K., Wang, H., and Wang, M. M.: The surface rup- Acknowledgements. The authors are grateful for the support ture signs of the Lushan “4.20” Ms = 7.0 earthquake at Longmen from the National Key Technology R & D Program (Grant township, Lushan county and its discussion. Seismology and ge- No. 2012BAK15B0103) and the National Key Basic Research ology, 35, 388–397. 2013. Program of China (Grant No. 2013CB733205). Thanks are given Harp, E. L., Keefer, D. K., Sato, H. 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