1

(Submitted to TECTONOPHYSICS on 30 Sep. 2008, 1 being processed to be accepted) 2

Co-seismic thrusting rupture and slip distribution 3

produced by the 2008 M7.9 Wenchuan earthquake, 4

China 5 Aiming Lin1*, Zhikun Ren1, Dong Jia2, Xiaojun Wu2 6

7 1Graduate School of Science and Technology, Shizuoka University, 8

Shizuoka 422-8529, Japan 9 2Department of Earth Sciences, Nanjing University, Nanjing 210093, China 10

11

12

* Corresponding author: 13

Institute of Geosciences, Faculty of Science 14

Shizuoka University 15

Shizuoka 422-8529, Japan 16

Tel & Fax: 81-54-238-4792 17

E-mail: slin@ipc.shizuoka.ac.jp 18

19

20

Abstract 21 Field investigations reveal that the Mw 7.9 Wenchuan (China) earthquake of 12 May 22

2008 produced a 285-km-long surface rupture zone, with dominantly thrusting slip 23

accompanied by a right-lateral component along the central-northern segments of the 24

zone, and left-lateral component along the southern segment, along the Longmen Shan 25

Thrust Belt, eastern margin of the Tibetan Plateau. The co-seismic ruptures mainly 26

occurred along the pre-existing Yingxiu-Beichuan, Guanxian-Anxian, and Qingchuan 27

faults, which are the main faults of the Longmen Shan Thrust Belt. The displacements 28

measured in the field are approximately 0.56.5 m in the vertical (typically 13 m), 29

accompanied by an average left-lateral component of < 2 m along the 60-km-long 30

2

southernmost segment of the rupture zone and an average right-lateral component of < 1 31

m along the 100-km-long centralnorthern segments. The maximum thrust slip amount 32

is estimated to be ~10 m, accompanied by 9 m of shortening across the rupture zone; 33

this finding is consistent with estimates based on seismic data. The rupture length and 34

maximum displacement and shortening amount are the largest among all 35

intracontinental thrust-type earthquakes reported to date. Our findings demonstrate that 36

i) the Wenchuan earthquake occurred upon pre-existing active faults of the Longmen 37

Shan Thrust Belt, thereby controlling the spatial distribution of co-seismic surface 38

rupture and displacement, and the rupture processes of the earthquake; ii) the long 39

rupture length and large thrusting slip resulted from compressive stress associated with 40

eastward extrusion of the Tibet Plateau as it accommodates the ongoing penetration of 41

the Indian Plate into the Eurasian Plate; and iii) present-day shortening strain upon the 42

eastern margin of the Tibetan Plateau is mostly released by seismic slip along thrust 43

faults within the Longmen Shan Thrust Belt. 44

45

Keywords: 2008 Mw 7.9 Wenchuan earthquake, Longmen Shan Thrust Belt, thrusting 46 slip, Tibetan Plateau, Sichuan Basin 47

48 1. Introduction 49

The magnitude (Mw) 7.9 Wenchuan earthquake occurred on 12 May 2008 in the 50

Longmen Shan region of China, the transition zone between the Tibetan Plateau and the 51

Sichuan Basin, resulting in extensive damage throughout central and western China (Fig. 52

1). Official estimates of casualties released by the Chinese Government as of 1 53

September 2008 include 69,197 confirmed deaths, 374,176 injured, and 18,209 missing. 54

To understand the seismic faulting mechanism and surface deformation features 55

associated with the earthquake, including rupture length, geometric characteristics, and 56

slip distribution of co-seismic surface rupture, our survey group traveled to the 57

epicentral area 2 days after the earthquake and undertook 10 days of fieldwork, during 58

which time we collected primary data related to rupture structures and the spatial 59

distribution of offset. Based on this preliminary fieldwork, we carried out additional 60

detailed fieldwork along the co-seismic surface rupture over the following 5 months. 61

Here we report the main results of our field investigations and discuss the co-seismic 62

rupturing mechanism and the implications of our findings for the tectonics of the eastern 63

3

marginal zone of the Tibetan Plateau. 64

65

2. Tectonic setting 66 The Longmen Shan region is characterized by high elevations of up to 7556 m 67

above sea level (Mt. Gongga) and topographic relief of more than 5 km over distances 68

of less than 50 km (Fig. 1), representing one of the steepest mountain fronts along any 69

margin of the Tibetan Plateau. The eastern margin of the Tibetan Plateau is bound by 70

the Longmen Shan Thrust Belt, which strikes northeastsouthwest for a distance of 71

~500 km (Jia et al., 2006). The basement of the Longmen Shan Thrust Belt is dominated 72

by strongly folded and deformed pre-Mesozoic rocks. 73

The Longmen Shan Thrust Belt is dominated by four major thrust faults: the 74

WenchuanMaowen, YingxiuBeichuan, GuanxianAnxian, and Qingchuan faults (Fig. 75

1; Deng et al., 1994; Li et al., 2006). Trenching surveys and field investigations reveal 76

that these faults have been active throughout the Late Quaternary, with slip rates of up 77

to 1.01.5 mm/yr (Li et al., 2006; Densmore et al., 2007). Seismicity is generally 78

restricted to small events in the area around the Longmen Shan Thrust Belt (Editorial 79

Board, State Seismological Bureau, 1989; Li et al., 2006). The historic record of 80

earthquakes in this region over the past 1400 yr reveals only three earthquakes of M > 6 81

(M 6.5 in 1657, M 6.2 in 1970, and M 6.2 in 1972) and documents a remarkable lack of 82

large earthquakes of M > 6.5 along the Longmen Shan Thrust Belt (Editorial Board, 83

State Seismological Bureau, 1989; Editorial Board, Annals of Sichuan Province, 1998). 84

However, recent trenching surveys and field investigations carried out after the 2008 85

Wenchuan earthquake reveal that a great Tang-Song Dynasty (~AD 8001000) 86

earthquake that ruptured a >200-km-long thrust fault within the Longmen Shan Thrust 87

Belt, China, duplicated on the co-seismic surface ruptures produced by the 2008 88

Wenchuan earthquake, and constrain a millennial recurrence interval for magnitude ~8 89

earthquakes (Lin et al., 2009). These studies demonstrate that the major pre-existing 90

thrusts of the Longmen Shan Thrust Belt are currently active as source faults of large 91

earthquakes in the late Holocene. 92

93

3. Deformation characteristics of co-seismic surface ruptures 94 Field investigations reveal that a 285-km-long surface rupture zone developed 95

during the 2008 Mw 7.9 Wenchuan earthquake (herein termed the Wenchuan rupture 96

4

zone) follows the GuanxianAnxian Fault, the YingxiuBeichuan Fault, and to a lesser 97

extent along the Qingchuan Fault (Fig. 1). Within the Longmen Shan Thrust Belt, these 98

faults define a left-stepping en echelon pattern with ~10 km clearance (Fig. 1). The 99

co-seismic surface ruptures are generally concentrated within a zone of up to 50 m in 100

width (generally < 20 m), largely following the strike of pre-existing fault traces within 101

the Longmen Shan Thrust Belt. 102

Based on geometric and spatial distribution features, the Wenchuan rupture zone can 103

be divided into three main rupture segments: northern, central, and southern. The 104

northern segment, restricted to the fault trace of the Qingchuan Fault (herein termed the 105

Qingchuan segment), extends for ~50 km, terminating at the town of Shazhou 106

(~105.50E, ~32.65N) in the northeast, near the border between Sichuan and Gansu 107

provinces (Fig. 1). The central segment (herein termed the Beichuan segment) is ~105 108

km in length, and occurs along the northeastern segment of the YingxiuBeichuan Fault. 109

The southern segment, ~130 km in length, branches into two parallel sub-rupture zones: 110

one along the southwestern segment of the YingxiuBeichuan Fault between the towns 111

of Yingxiu and Beichuan (herein termed the Yingxiu rupture zone), terminating to the 112

south of Yingxiu town; and another along the GuanxianAnxian Fault (herein termed 113

the Guanxian rupture zone), which forms the topographic boundary between the 114

Longmen Shan Range and the Sichuan Basin, terminating to the south of Dujiangyan 115

City (formerly known as Guanxian) near the epicentral area of the Wenchuan 116

earthquake (Fig. 1). 117

The total length of the Wenchuan co-seismic surface rupture zone, initiating south 118

of Yingxiu town (~103.47E, ~31.03N) and terminating near Shazhou town to the 119

northeast (~105.50E, ~32.65N), is estimated to be ~285 km, representing the longest 120

rupture reported worldwide for intracontinental thrust-type earthquakes, comparable 121

with the longest reported strike-slip rupture zone of 400450 km produced by the 2001 122

Mw 7.8 Kunlun earthquake upon the northern Tibetan Plateau (Lin et al., 2002, 2003; 123

Lin and Nishikawa, 2007). The co-seismic surface rupture length coincides with the 124

seismic rupture area constrained by aftershock data (Fig. 1). 125

The Wenchuan rupture zone is mainly defined by distinct thrust faults, fault scarps, 126

and fold structures, including mole track structures as that produced by the 2001 Mw 7.8 127

Kunlun earthquake occurred in the northern Tibetan Plateau (Lin et al., 2004), and 128

numerous extensional fractures. The thrust faults are generally expressed at the surface 129

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by fault planes, fault scarps, and fold structures developed widely along the co-seismic 130

surface rupture zone (Figs. 2-4). The fault planes on which the main slip occurred were 131

observed at several locations: they strike N1050E and dip to the northwest at an 132

average of ~30 (Fig. 2ad). At some locations where basement rocks were exposed, the 133

dip of the fault plane reaches 85 (Fig. 2eh). 134

A thrusting sense-of-slip is indicated by slickenside striations developed on 135

co-seismic fault planes (Fig. 2fh) and the geometry of fault scarps, upon which little or 136

no horizontal slip is recorded (Fig. 2d). Co-seismic fault scarps generally occur on 137

pre-existing active fault scarps upon which vertical offsets have accumulated (Fig. 3). 138

This accumulated vertical offset indicates that large thrusting events have occurred 139

repeatedly upon individual faults; however, co-seismic surface ruptures along the 140

Qingchuan segment occurred along the surface trace of the Qingchuan Fault, and the 141

relation between these two features is unclear because the co-seismic surface rupture 142

sprays into numerous extensional cracks and push-up structures (e.g., small-scale mole 143

tracks) distributed over a wide area with little or no offset (Fig. 4a). 144

The co-seismic fault scarps are generally characterized by complex surface 145

morphology and thrusting and folding structures (Fig. 5). Fault planes observed in 146

outcrops of basement rocks generally dip at high angles (> 50) and show a linear 147

surface morphology in geometry (Figs. 5a and 2fh). In contrast, fault scarps observed 148

upon alluvial terrace risers and fans, where unconsolidated deposits overlie basement 149

rocks, show a complex morphology, with folding structure (Fig. 5be). 150

Such typical structures were observed at Site 11, where a 2.8-m-high co-seismic 151

fault scarp is duplicated on a 2.6-m-high pre-existing fault scarp developed within the 152

sand-gravel and soil alluvial deposits (Fig. 6). The pre-existing fault scarp is identified 153

by the presence of a man-made stone wall that partially collapsed during the 2008 154

earthquake (Fig. 6). The ground surface of the corn field, in addition to alluvial deposits 155

and soil layers, is folded and offset, which protruded and overlapped the corn field in 156

the footwall of the thrust fault plane for a distance of 5.7 m (Fig. 6). This finding 157

indicates that the amounts of both thrusting slip and shortening produced by the 158

Wenchuan earthquake exceeded 5.7 m (Fig. 6; see Discussion for details). 159

This type of thrust fault scarp (Fig. 6) is termed a protruded scarp (Fig. 5e) 160

(Gorden and Lewis, 1980). The observed structural features demonstrate that the 161

near-surface unconsolidated deposits and surface soils were entrained in the thrust, 162

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overriding the ground surface of the corn field in the footwall. Similar structures have 163

also been observed along the Senya Thrust, which formed during the1896 M7.5 Rikuu 164

earthquake, northeast Japan (Research Group for the Senya Fault, 1986), and the Spitak 165

Thrust, which formed during the 1988 Ms 6.9 Armenian earthquake (Philip et al., 1992). 166

Mole track structures are widely observed along the surface rupture zone, generally 167

developed within alluvial deposits and cemented ground such as roads, forming a linked 168

array of contractional structures along the rupture zone (Fig. 4ac) similar to those 169

produced by the 2001 Mw 7.8 Kunlun earthquake (Lin et al., 2004; Lin and Nishikawa, 170

2007). The mole tracks are typically 0.31 m in height, 15 m in width, and 110 m in 171

length, and generally trend sub-parallel to the overall trend of the rupture zone. The 172

mole track structures that formed within the co-seismic surface rupture zone are 173

considered to represent a contractional structure associated with thrusting and folding. 174

Extensional cracks, sand boils, and landslides are widely distributed along the 175

surface rupture zone (Fig. 4de). Numerous sand boils produced by seismic-related 176

liquefaction were observed in streams and low terrace risers, with series of vents 177

generally being aligned parallel to the surface rupture zone (Fig. 4ef). 178

179

4. Co-seismic displacement 180 Co-seismic displacements were measured at 74 locations along the surface rupture 181

zone based on offset linear surface-markers such as roads, stream channels, and terrace 182

risers oriented perpendicular-subperpendicular to the surface rupture zone (Fig. 7). The 183

amount of vertical offset at each site was measured and calculated from profiles 184

measured across faults, fault scarps, and fold structures using a tape measure and an 185

Advantage Laser Rangefinder (Lasser Atlanta Optics Inc., 2000) with an error of 15 186 cm. 187

The measured displacements reveal the dominance of vertical displacements along 188

the entire surface rupture zone (Fig. 8). The vertical displacements range from several 189

centimeters to 6.5 m, generally in the range 13 m. Strike-slip offset was also 190

observed at some locations along the surface rupture zone (Fig. 8). The maximum offset 191

was measured at a location near Site 3, along the Yinxiu rupture zone, where the terrace 192

riser is offset 6.5 m in the vertical (Fig. 2e). Right-lateral slip was mainly observed at 12 193

locations along the 50-km-long southwestern-most segment of the surface rupture zone 194

(Figs. 7ab and 8). In contrast, left-lateral slip was observed at 10 locations along the 195

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100-km-long centralnortheastern segment (Figs. 7cd and 8). The maximum 196

left-lateral slip of 4.2 m was observed at Site 4 where the co-seismic surface rupture 197

strikes northwest almost perpendicular to the general trend of the rupture zone, recorded 198

by the displacement of a small path through a field (Fig. 7d). The landowner at this site 199

(Ms. Huang) reported that both the 4.2-m-right-lateral slip and 2.2-m-high fault scarp 200

formed during the earthquake. The maximum amount of right-lateral offset was found at 201

Site 9 near the town of Beichuan. Although a strike-slip component was locally 202

observed along the co-seismic surface rupture zone, it was only identified at 22 of the 203

91 measurement locations, with an average strike-slip displacement of ~12 m (Fig. 8). 204

This finding reveals that displacement was dominated by vertical offset, accompanied 205

by a minor strike-slip component, representing a dominantly thrusting mechanism for 206

the earthquake. 207

208

5. Discussion and conclusions 209 5.1. Co-seismic thrusting slip and shortening amount 210

The amount of co-seismic slip is a key factor in assessing the seismic moment, 211

rupture mechanism, and degree of seismic hazard. The slip amount recorded by 212

co-seismic surface ruptures along strike-slip faults can generally be measured in the 213

field, as with the strike-slip Kunlun Fault in the northern Tibetan Plateau, along which 214

the 2001 Mw 7.9 earthquake produced a 400450-km-long surface rupture zone (Lin et 215

al., 2002, 2003); however, it is generally difficult to measure the amount of co-seismic 216

thrusting slip along thrust faults in the field due to the complex geomorphic expression 217

of the co-seismic surface rupture, particularly in rupture areas with thick deposits of 218

unconsolidated material and where the fault plane is not exposed at the surface. 219

The Wenchuan rupture zone shows the complex morphology typical of thrust faults, 220

and this made it difficult to directly measure the amount of thrusting slip in the field. At 221

certain sites where the fault plane is exposed, it is possible to estimate the amount of 222

thrust slip. For example, at one location, the ground surface of a corn field was observed 223

in both the hanging wall and footwall of the thrust fault, and a soil-sandgravel layers 224

are folded in the hanging wall (Fig. 6). These features suggest that the ground surface of 225

the corn field in the hanging wall was offset by the fault and slid forward along the fault 226

plane before collapsing onto the ground surface in the footwall of the fault (Fig. 9). 227

Considering the dip of the fault plane observed in the trench exposure at this site (30; 228

8

Fig. 6), the amount of thrust displacement (D) upon the fault plane is calculated as 229

follows: 230

D = H/sin = 2.8 m/sin 30 = 5.6 m, 231

where H is the vertical offset (height of the co-seismic fault scarp) and is the dip of 232 the fault plane (Fig. 9). This coincides with the width of buried ground surface of 5.7 m 233

observed in the exposure (Fig. 6). The vertical offsets measured in the field along the 234

Wenchuan rupture zone generally range from 1 to 3 m (Fig. 8); therefore, thrusting slip 235

is estimated to be 26 m, with an average of 4 m accompanying an average shortening 236

amount of 3 m (assuming an average fault-plane dip of 30). 237

These thrust slip amounts and the distribution of displacement along the Wenchuan 238

rupture zone are comparable with estimates based on seismic inversion analyses (e.g., 239

Chen et al., 2008; Ji, 2008). The maximum amount of thrusting slip (up to 10.3 m, with 240

a shortening amount of ~9.0 m) is obtained at Site 11, where the vertical offset of 5.15 241

m was observed on a lower terrace riser of unconsolidated alluvial deposits (Fig. 7a), 242

assuming an average dip of 30 for the main fault plane as observed at Sites 67 (Fig. 243

2ab) and 11 (Fig. 6). This maximum amount of thrusting slip is also comparable with 244

previous estimates (based on seismic data) of ~9 m (Ji, 2008), ~12 m (California 245

Institute of Technology, 2008), and 12.5 m (Chen et al., 2008). The large vertical offsets 246

of > 4 m are distributed throughout two surface-rupture areas around Sites 35 and Site 247

11 in the southern and northern segments (Fig. 8), respectively, approximately 248

corresponding to the locations for which the maximum slips were estimated based on 249

seismic inversion analyses undertaken by California Institute of Technology (2008) and 250

Chen et al. (2008). The large left-lateral slip components of 2.0-4.2 m observed at the 251

locations around Site 4 are probably caused by the northwest-southeast trend of the 252

co-seismic surface rupture which is perpendicular to the northeast-southwest-trending 253

rupture zone. The northwest-southeast-trending co-seismic fault conjugates with the 254

northeast-southwest-trending fault, oblique to the east-west compressive tectonic stress 255

which caused the left-lateral slipping. The horizontal slip components observed on the 256

co-seismic surface rupture would be effected by the rupture geometry and topography 257

where the rupture occurred. 258

Based on field observations and seismic inversion results, we conclude that the 259

maximum thrusting slip amount associated with the Wenchuan earthquake was 10.3 m, 260

accompanied by 9.0 m of shortening. These values probably reflect the thrust slip of a 261

9

deep-level seismogenic fault zone. 262

263

5.2. Tectonic implications of co-seismic thrusting and shortening 264 The geometric characteristics of co-seismic surface ruptures not only reflects the 265

surface morphology, but also the structure at depth and the pre-existing tectonic 266

environment (Yeats et al., 1997; Lin et al., 2001, 2003). The geometric features and slip 267

distribution of co-seismic surface ruptures associated with the Wenchuan event, in 268

combination with the orientation of the fault plane and plunge of striations developed 269

upon the main fault plane, indicate that co-seismic surface displacement is dominated 270

by thrust slip, with a lesser lateral-slip component. This finding is consistent with the 271

focal mechanisms determined by China Earthquake Networks Center (2008), Harvard 272

University (2008), and USGS (2008). 273

Our fieldwork results demonstrate that the co-seismic surface rupture of the 274

Wenchuan earthquake occurred along pre-existing active faults of the Longmen Shan 275

Thrust Belt along a distance of ~285 km. The aftershocks of magnitude > 4 that 276

occurred during the first month after the Wenchuan earthquake are concentrated along a 277

~300 km section of the co-seismic surface rupture zone (Fig. 1). InSar data also reveal 278

that surface deformation is restricted to a 285-km-long zone (Geographical Survey 279

Institute, 2008), coincident with the co-seismic surface rupture zone identified in this 280

study. These geological and seismic data indicate that the distribution of the co-seismic 281

surface rupture was mainly constrained by the orientation of pre-existing thrust faults of 282

the Longmen Shan Thrust Belt, as reported in previous studies (Jia et al., 2006; Li et al., 283

2006). 284

The 2008 Wenchuan earthquake occurred in response to compressive tectonic stress 285

oriented perpendicular to the Longmen Shan Thrust Belt, resulting from relative motion 286

between the Tibetan Plateau and the Sichuan Basin. Geological data from in and around 287

the Longmen Shan region suggest a low shortening rate during the late Quaternary 288

(Burchfiel et al., 1995, 2008; Kirby et al., 2000), and GPS data indicate an average 289

active shortening rate of < 3 mm/yr within the Longmen Shan region along the 290

Longmen Shan Thrust Belt, without a significant strike-slip component (Chen et al., 291

2000; Zhang et al., 2004; Shen et al., 2005; Meade, 2007). 292

The historic record of earthquakes in this region over the past 2000 yr reveals only 293

three identifiable earthquakes of M > 6 (M 6.5 in 1657, M 6.2 in 1970, and M 6.2 in 294

10

1972) and documents a remarkable lack of large earthquakes of M > 6.5 along the 295

Longmen Shan Thrust Belt (Editorial Board, State Seismological Bureau, 1989; 296

Editorial Board, Annals of Sichuan Province, 1998). If all large historical earthquakes 297

have been perfectly recorded in the study area, the recurrence interval of 298

large-magnitude earthquakes within the thrust belt, therefore, would be greater than 299

1,400-2,000 years. Recent trenching surveys and field investigations carried out after 300

the 2008 Wenchuan earthquake reveal that a large-magnitude (M 8.0) earthquake 301

occurred in the Tang-Song Dynasty (~AD 8001000) with an average vertical offset of 302

2-3 m (Lin et al., 2009). This new finding indicates that the recurrence interval of 303

large-magnitude earthquakes in the late Holocene is ~1,000-1,200 yr. Considering the 304

maximum and average shortening amounts of 9 m and 3 m produced by the 2008 305

Wenchuan earthquake, as estimated in this study, and a long-term shortening rate of < 3 306

mm/yr revealed by GPS data within the Longmen Shan region, a recurrence interval of 307

~10003000 years is estimated for the release of shortening strain energy stored within 308

the Longmen Shan Thrust Belt, comparable with the estimate proposed by Lin et al. 309

(2009). For understanding the recurrence interval of large earthquake, additional 310

paleoseismic studies are required. Our results confirm that present-day shortening 311

strain upon the eastern margin of the Tibetan Plateau, resulting from eastward extrusion 312

of the Tibetan Plateau as it accommodates the ongoing penetration of the Indian Plate 313

into the Eurasian Plate, is released by seismic slip along thrust faults within the 314

Longmen Shan Thrust Belt. 315

316

Acknowledgements 317 We thank JAXA (Japan Aerospace Exploration Agency) for kindly providing the 318

ALOS imagery data, Asia Air Survey Co. Ltd. for providing the Red Relief image map, 319

and PASCO Co. Ltd. for providing the Terra SAR-X image map. This work was 320

supported by the Tokyo Marine Kagami Memorial Foundation, Shizuoka University, a 321

Science Project (No. 18340158) of the Ministry of Education, Culture, Sports, Science 322

and Technology of Japan, and a Science Project (No. 40672132) of the National Natural 323

Science Foundation of China. 324

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Research Group for the Senya Fault, 1986. Holocene activities and near-surface features 403 of the Senya fault, Akita Prefecture, Japan- excavation study at Komori 404 Senhata-cho. Earthquake Research Institute, University of Tokyo Bulletin 61, 405 339-402 (in Japanese with English abstract). 406

Shen, Z., Lu, L., Wang, M., Burgmann, R., 2005. Contemporary crustal deformation 407 around the southeast borderland of the Tibetan Plateau. Journal of Geophysical 408 Research 110, 117. 409

United States Geological Survey, 2008. Magnitude 7.9 Eastern Sichuan, China 410 (http://earthquake.usgs.gov/eqcenter/eqinthenews/2008/us2008ryan/) (last accessed, 411 26 June 2008). 412

Yeats, R. S., Sieh, K., Allen, C.R. 1997. The Geology of Earthquakes. Oxford Univ. 413 Press, Oxford, 568p. 414

Zhang, P., Shen, Z., Wang, M., Gan, W., Burgmann, R., Molnar, P., Wang, Q., Niu, Z., 415 Sun, J., Wu, J., Sun, H., You. X., 2004. Continuous deformation of the Tibetan 416 Plateau from global positioning system data. Geology 32, 98099812. 417

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419

Figure 1. Location maps of the study area, showing topographic features and 420

distribution of the co-seismic surface rupture. (a) Landsat image of the Tibetan Plateau 421

and north India, showing the location of study area. Yellow arrows indicate the 422

eastward movement direction of the Tibetan Plateau. Red arrow indicates the movement 423

direction of Indian plate. (b) SRTM (Shuttle Radar Topography Mission; 90 m 424

resolution) color shaded-relief map showing the tectonic landforms of the Longmen 425

Shan region [fault data from Jia et al. (2006), Li et al. (2006), and Densmore et al. 426

(2006)] and aftershock distribution (black circles) following the 2008 Wenchuan 427

earthquake (seismic data from China Earthquake Networks Center, 2008). White solid 428

circles indicate the locations of main cities and towns. (c) Distribution map of 429

co-seismic surface ruptures, showing the locations of sites referred to in this study. Red 430

stars indicate the epicenter of the 2008 Wenchuan earthquake, as determined by China 431

Earthquake Networks Center (CENC, 2008), Harvard University (Harvard, 2008), and 432

the United States Geological Survey (USGS, 2008). 433

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434 435

Figure 2. Representative photographs of co-seismic surface ruptures. (ab) Terrace 436

risers offset vertically by 2 m [(a: Site 7), (b: Site 6); see Figure 1 for location details] at 437

sites where the fault dips to the northwest at ~30. (c) Thrust-related vertical offset of 438 the river channel shown in (b) (Site 6). (d) The road shown in (b) was offset vertically 439

by 2.0 m. Note the lack of horizontal displacement at this site. (e) The terrace riser was 440

offset vertically by 6.5 m, which is the largest offset observed in this study (a location 441

near Site 3). (f) The fault scarp observed in the basement rock (mudstone), where the 442

terrace was offset vertically by 4.8 m (Site 3). (gh) Slickenside striations (yellow 443

arrows) developed on the main thrusting fault plane at Site 3, which strikes N18E and 444

dips to the northwest at ~85. The striations indicate a thrusting-dominated slip sense. 445

The pen shown for scale is 15 cm long. H: vertical offset.446

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447

448 Figure 3. Representative photographs of co-seismic surface ruptures developed along 449

pre-existing fault scarps. (a) Terrace risers and the river channel were vertically offset 450

by 23 m (Site 5) at a site where the pre-existing fault scarp was higher than 5 m. (b) 451

Co-seismic surface ruptures developed along a pre-existing fault scarp at Site 1, 452

northeast of Yingxiu town. (c) Co-seismic surface rupture developed along a 453

pre-existing fault scarp at Site 9. The lower terrace and river channel were vertically 454

offset by 23 m, with a right-lateral slip component of 2.3 m. 455

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456 457 Figure 4. Representative photographs of mole track structure (ac), extensional cracks 458

(d), and sand boils caused by liquefaction (ef). (c) Mole track structure developed on a 459

folded section of road at the northeastern termination of the Wenchuan rupture zone 460

(near Site 6). (b) Mole track structure developed along a road at Site 9. Prior to the 461

earthquake, the road surface to the left in the photograph was the same height as that to 462

the right, which now records a vertical offset of ~2.5 m. (c) Mole track structure 463

developed within cemented ground at the end of the Beichuan segment (Site 12 (d) 464

Typical extensional cracks (Site 13). (e) Extensional cracks and sand boils observed at 465

Site 10. (f) Sand boil caused by liquefaction (Site 11). 466

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467

468

Figure 5. Schematic diagrams of the scarp features of thrust faults observed along the 469

Wenchuan rupture zone. (a) Simple thrust scarp developed in basement rocks; (b) 470

collapsed fault scarp developed in thin unconsolidated deposits overlain upon basement 471

rocks; (c) flexural-fold fault scarp developed in thick unconsolidated deposits; (d) mole 472

track structure developed on the flexural-fold scarp; (e) protruded fault scarp developed 473

within thick viscid soilclay layers that overlie thick unconsolidated sandgravel layers. 474

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475 476

Figure 6. Representative morphology of the co-seismic fault scarp duplicated on a 477

pre-existing fault scarp of the YingxiuBeichuan Fault (Site 9). The pre-existing fault 478

scarp is identified by the presence of a man-made stone wall of 2.6 m in height. Note 479

that the ground surface of the corn field is buried across a horizontal distance of ~5.7 m. 480

See text for details. 481

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482 483

Figure 7. Representative photographs of vertical offsets accompanied by right-lateral 484

(ab) and left-lateral slip components (cd). (a) The lowest terrace in the photograph is 485

vertically offset by 5.15 m, with ~1.0 m of right-lateral offset (R1.0 m) recorded by the 486

displaced path (Site 11). This site records the largest vertical and horizontal offsets 487

observed during our fieldwork. (b) The road was vertically offset by ~1.1 m, 488

accompanied by a right-lateral slip component of 0.5 m (Site 8). (c) The road was 489

vertically offset by ~1.1 m, accompanied by a left-lateral slip component of 1.0 m (Site 490

4). (d) The fault scarp at this site records a vertical offset of 2.2 m, with a left-lateral slip 491

component of 4.2 m (Site 4).492

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493

494

Figure 8. Slip distribution of the co-seismic displacements measured along the 495

Wenchuan rupture zone. (a) Each slip amount was measured at an individual co-seismic 496

surface rupture. (b) Co-seismic ruptures along the pre-existing GuanxianAnxian Fault 497

(blue), YingxiuBeichuan Fault (black), and Qingchuan Fault (pink). Colors in (a) 498

correspond to those in (b). 499

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500

501

Figure 9. Schematic diagram showing the formation of a protruded fault scarp, based on 502

a real example (Site 9) observed in the present study. The surface soil layer (a) was cut 503

and offset by thrusting (b). The fault scarp protruded and then collapsed during the 504

earthquake. (c) The dear surface soil layer was protruded for a horizontal distance of 505

~5.7 m as observed in the field (Fig. 6). The thrusting slip amount was calculated to be 506

5.6 m. See text for details. 507

508