co-seismic thrusting rupture and slip distribution produced by the 2008 mw4 7.9 wenchuan...
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(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
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* 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
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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
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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
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marginal zone of the Tibetan Plateau. 64
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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
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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
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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
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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
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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
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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
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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
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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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Philip, H., Rogozhin, E., Cisternas, A., Bousquest, J.C., Borisov, B., Karakhanian, A., 399 1992. The Armenian earthquake of 1988 December 7: Faulting and folding, 400 neotectonics and paleoseismicity. Geophysical Journal of International 110, 401 141-158. 402
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
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