Normal- and oblique-slip of the 2008 Yutian earthquake: Evidence for eastward block motion, northern Tibetan Plateau

Tectonophysics 584 (2013) 152–165

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Normal- and oblique-slip of the 2008 Yutian earthquake: Evidence for eastward block motion, northern Tibetan Plateau Xiwei Xu a,⁎, Xibin Tan a, Guihua Yu a, Guodong Wu b, Wei Fang b, Jianbo Chen b, Heping Song b, Jun Shen b a b

Key laboratory of Active Tectonics & Volcano, Institute of Geology, China Earthquake Administration, Beijing 100029, China Earthquake Administration of Xinjiang Uygur Autonomous Region, Urumqi 830011, Xinjiang, China

a r t i c l e

i n f o

Article history: Received 3 September 2011 Received in revised form 5 July 2012 Accepted 6 August 2012 Available online 18 August 2012 Keywords: Yutian earthquake Earthquake surface rupture zone Coseismic slip Altyn Tagh fault system Kunlun fault system Tibetan Plateau

a b s t r a c t The 2008 Yutian Mw 7.1 earthquake occurred in a junction area of the Altyn Tagh, Karakax and Kunlun fault systems and becomes a most northerly normal faulting event in the northern Tibetan Plateau. High resolution satellite image interpretation and field investigation indicate that the surface rupture zone produced by the Yutian earthquake is ~31 km long along the Yulong Kashgar fault, a NS-trending fault at the western piedmont of a snow-covered mountain at the headwater of the Yulong Kashgar River, about 20 km south of the Ashikule volcanoes. The surface rupture zone along the Yulong Kashgar fault consists of four different types of surface ruptures with both normal- and oblique-slip components. The maximum left-lateral slip and vertical offset, we measured in the field, are ~3.6 m and ~3.3 m, respectively. This NS-trending seismogenic fault belongs to one of the boundary faults between the Qaidam–Qilian and the western Kunlun blocks. Thus, the Yutian earthquake is as a result of abrupt normal- and oblique-slip faulting, which fits the eastward escape of the Qaidam– Qilian block, relative to the western Kunlun block. This surface rupture pattern supports the eastward block-like motion model whose deformation takes place mainly along the block boundaries delineated by mega-strike-slip faults in the northern Tibetan Plateau. More important may be that the Yutian earthquake has implication on future earthquake risk along the boundary faults of the Qaidam–Qilian block. © 2012 Elsevier B.V. All rights reserved.

1. Introduction At 06:33:00 LT on March 21 or 22:33:00 UTC on March 20, 2008, the Mw 7.1 Yutian earthquake struck the western piedmont of a snow-covered mountain with an elevation of more than 5000 m asl, between the Ashikule basin and the Guliya Ice Cap in the western Kunlun Mountains (Fig. 1), 150 km due south of Yutian. This event occurred in a junction area of the Altyn Tagh, Karakax and Kunlun fault systems and becomes a most northerly normal faulting event in the northern Tibetan Plateau (Elliott et al., 2010). Also this event is one of the recent earthquake ruptures in the Plateau: along the Manyi fault (Mw 7.6 Manyi earthquake, 1997) (Peltzer et al., 1999; Xu, 2000), the Kunlun fault (Mw 7.8 Kokoxili earthquake, 2001) (Lasserre et al., 2005; Lin et al., 2002; Xu et al., 2002, 2006, 2008a ), the Longmenshan thrust belt (Mw 7.9 Wenchuan earthquake, 2008) (Dong et al., 2008; Shen et al., 2009; Xu et al., 2008b, 2009;), the Yulong Kashgar fault (Mw7.1 Yutian earthquake) (Elliott et al., 2010; Shan et al., 2011) and the Ganzè-Yushu fault (Mw 6.9 Yushu earthquake, 2010) (Li et al., 2011; Lin et al., 2011; Sun et al., 2012). All of them exemplify a localized faulting behavior along the mega-strike-slip faults and mega-thrust-belt in the elastic upper crust of the northern Tibetan Plateau (Xu et al., 2008a,b, 2009; Yu et al., 2010; Zhou et al., 2010). ⁎ Corresponding author. E-mail address: [email protected] (X. Xu). 0040-1951/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2012.08.007

The surface ruptures of the 1997 Manyi, 2001 Kokoxili, and 2010 Yushu earthquakes are dominated by pure left-lateral faulting along the WNW-trending strike-slip faults (Fig. 1), representing an eastward motion of the Bayan Har and Qiangtang blocks bounded by those mega-strike-slip faults in the northern Tibetan Plateau (Klinger et al., 2005; Lin et al., 2002, 2011; Peltzer et al., 1999; Xu et al., 2002, 2006), while that of the 2008 Wenchuan earthquake by reverse- and oblique-faulting with right-lateral component demonstrates a crustal shortening along the NE-trending Longmenshan thrust belt at the eastern margin of the Tibetan Plateau, representing that the Bayan Har block did move southeastward and then collided with the South China block at its southeastern most margin (Diao et al., 2010; Xu et al., 2008b, 2009, 2010a). Because of occurrence of the Wenchuan earthquake on May 12, 2008, the most devastating earthquake in China in the past three decades, which has drawn great attention of scholars both at home and abroad, the 2008 Yutian earthquake has become a forgotten event. Only a few researches have been done into its tectonic setting (Chen et al., 2008; Yin et al., 2008), seismogenic fault both from relocated aftershocks (Tang et al., 2010) and preliminary field observation (Li H. et al., 2009), coseismic deformation from Ascending and Descending Orbit ASAR Data and satellite image interpretation (Elliott et al., 2010; Furuya and Yasuda, 2011; Hong et al., 2010; Shan et al., 2011; Wang et al., 2009), and Coulomb stress changes on the surrounding active faults (Wan et al., 2010). Its seismogenic

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Fig. 1. (a) Distribution of main active faults of the Tibetan Plateau, modified after Tapponnier et al. (2001), Deng et al. (2003), Taylor and Yin (2009), Xu et al. (2010a) and Xu X. et al. (2011). Black coarse lines represent the main active faults and red circles the historical earthquakes (M ≥5) recorded during 193 BC — 2010 AD; blue lines represent recent earthquake surface rupture zones; ATF: Altyn Tagh fault system; KKF: Karakax fault system; KLF: Kunlun fault system; KRKF: Karakoram fault system; XF: Xianshuihe–Ganzè–Yushu fault system; XJF: Xiaojiang fault system; JLF: Jiali fault system; LTB: Longmenshan thrust belt; HFT: Himalayan Frontal Thrust; QQB: Qaidam-Qilian block; WKB: Western Kunlun block; BHB: Bayan Har block; QTB: Qiangtang block; LSB: Lhasa block; Black arrows indicate block motion direction; 3D cartoon displays block pull-apart at the Yutian earthquake epicenter or convergence motion at the 2008 Wenchuan earthquake epicenter. (b) Seismogenic fault (red line) and focal mechanisms of the Yutian earthquake and adjacent main active faults. AB represents the Ashikule basin where the Neogene to late Quaternary volcanos have been erupted; White small circles are epicenters of the relocated aftershocks (Tang et al., 2010) and focal mechanisms of the Yutian earthquake come from USGS CMT, Global CMT and XEA.

fault was inferred to be a NNE- or nearly SN-trending normal fault with a left-lateral component (Chen et al., 2008; Elliott et al., 2010; Furuya and Yasuda, 2011; Hong et al., 2010; Li Z. et al., 2009; Tang et al., 2010; Wang et al., 2009; Yin et al., 2008). Although previous investigations mostly by remote sensing and seismological modeling allowed a first order mapping of the surface

ruptures and a broad picture of the coseismic slip distribution on the fault plane, they are subject to several simplifying assumptions and need a validation in the field. Additionally, together with other normal faulting events, e.g. the 2008 Gerze (Mw 6.4), Zhongba (Mw 6.7), Damxung (Mw 6.3) earthquakes (Elliott et al., 2010; Sun et al., 2008, 2011), the Yutian earthquake, one of the largest normal faulting

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events to have occurred recently, shows a local crustal extension in the western Plateau (Elliott et al., 2010). A question then arises whether or not this local crustal extension is related to the eastwards block-like motion in the eastern Plateau. Further detailed research and discussion is necessary. Our research group reached the epicentral area between 4900 and 5600 m asl, during the fieldwork campaign in the western Kunlun Mountains in May 2011 to survey the surface rupture zone produced by the 2008 Yutian earthquake. Here we present the surface rupture characteristics of the Yutian earthquake, e.g. width of the surface rupture zone, style of faulting and sense of slip along strike, for a potential comparison with the average slip distribution derived from published models, which will help us to understand normal faulting behavior along the boundaries between active blocks, to check whether or not the eastward motion of the Qaidam-Qilian block exists in the northern Tibetan Plateau owing to the convergence between the Indian and Eurasian Plates (e. g. England and Houseman, 1985; England and McKenzie, 1982; Molnar and Tapponnier, 1975; Searle et al., 2011; Tapponnier et al., 1982, 2001), and therefore to have implication on earthquake risk assessment of the boundary faults of the Qaidam-Qilian block. 2. Tectonic Setting The Yutian earthquake occurred on the Yulong Kashgar fault, an active normal- and oblique-slip fault along the tectonically complex boundary between the Tianshuihai terrane and the Qiangtang and Bayan Har terranes near the southwestern border of the Tarim Basin. The boundaries of those terranes correspond mainly to the Triassic Jinsha suture, the Permo-Triassic south Kunlun (Anyemaqen) and north Kunlun sutures, as well as the Altyn Tagh fault system and Karakoram fault system (Leloup et al., 2012; Li, 2003; Matte et al., 1996; Pan et al., 2002; Ren, 2002; Tapponnier et al., 2001). However, in Neotectonics many Quaternary mega-strike-slip faults, e. g. the Altyn Tagh fault system, Karakax fault system (Kegang fault and Kangxiwar fault) and Longmu-Gozha Co fault, a southwestern most segment of the Kunlun fault system, and numerous normal faults, have developed and sliced the Tianshuihai, Qiangtang and Bayan Har terranes and their boundary sutures (Fig. 1; Deng, 2007; Deng et al., 2003; Searle et al., 2011; Tapponnier and Molnar, 1977; Tapponnier et al., 2001; Taylor and Yin, 2009; Xu and Deng, 1996; Xu X. et al., 2011). By definition of active block which is a tectonic unit that has been separated from each other by Quaternary active faults and whose tectonic deformation occurs mainly on its boundary faults (Xu et al., 2003; Zhang et al., 2003), then several active blocks, for instance, the Qaidam-Qilian, Bayan Har, Qiangtang and western Kunlun blocks, have been divided by those mega active faults in the northern Tibetan Plateau (Fig. 1). Specifically, the western Kunlun block, also known as the Tianshuihai terrane (Wittlinger et al., 2004), is bounded by the Karakax fault system on the north, the Longmu–Gozha Co fault on the south, the Karakoram fault system on the west and the southwestern most segment of the Altyn Tagh fault system on the east (Deng et al., 2003; Meade, 2007; Raterman et al., 2007; Xu X. et al., 2011), while the Qaidam–Qilian block by the Kunlun fault system on the south, the Altyn Tagh fault system on the northwest and the Hexi Corridor on the east (Fig. 1a). The Yulong Kashgar fault, the seismogenic fault of the Yutian earthquake, which is located near an NE-trending normal- and oblique-slip fault, the southwestern most segment of the Altyn Tagh fault system, separates the western Kunlun block from the Qaidam–Qilian block (Fig. 1). This division of the active blocks in the northern Tibetan Plateau, however, is different from others in the extension of the Kunlun fault system to the west and specific subdivision of the active blocks between the Altyn Tagh fault system and the Kunlun fault system. One view is that the Kunlun fault system was extended along an inferred fault trace to the west (Deng et al., 2003), and another view along the

south Kunlun (Anyemaqen) suture to the west (Deng et al., 2010). In both cases the Bayan Har block bounded by the Kunlun and Xianshuihe-Ganzè-Yushu fault systems is a very long but narrow block, and two other active blocks, eastern Kunlun-Qaidam and Qilian blocks, were divided between the Kunlun and Altyn Tagh fault systems (Deng et al., 2003, 2010). Because of uncertainty of the westward extension of the Kunlun fault system, existence of thrust-related folds in both the eastern Kunlun-Qaidam and Qilian blocks, as well as the boundary between those two blocks cutting NS-trending major active faults in the other divisions, we prefer our own block division, for the first order, into the Qaidam-Qilian, Bayan Har, Qiangtang and western Kunlun blocks in the northern Tibetan Plateau (Fig. 1a). The Altyn Tagh fault system, an ENE-trending fault more than 2000 km long, runs on the northern edge of the Tibetan plateau and consists of the Altyn Tagh strike-slip fault and Altyn Tagh thrust (Fig. 1a). Its left-lateral slip rate east of 85.0°E ranges from ~5 mm/year in the east to ~17 ± 2 mm/year in the west (Cowgill, 2007; Cowgill et al., 2009; Gan et al., 2007; Mériaux et al., 2004, 2005; Seong et al., 2011; Xu et al., 2005). This left-lateral slip rate from the offset landforms agrees with geodetic data from 89°E to 91°E to some extent. The latter gives a left-lateral shear strain rate of 9 ± 5 mm/year and a contraction rate of 3 ± 1 mm/year across the Altyn Tagh fault system (Bendick et al., 2000). The southwestern most segment of the Altyn Tagh fault system in the epicenter west of 84.0°E strikes NE with normal- and oblique-slip components and cuts into the Kunlun Mountains (Fig. 1b). The Longmu–Gozha Co fault to the south is an EW-trending left-lateral strike-slip fault with a slip rate of ~10 mm/year from the InSAR observation (Wright et al., 2004). It was structurally linked to the Altyn Tagh fault system, through a normal left-step east of the Yulong Kashgar fault (Elliott et al., 2010; Raterman et al., 2007 ), but from the distribution and strike of the mega-strike-slip faults in the Tibetan Plateau newly published (Elliott et al., 2010; Taylor and Yin, 2009; Xu X. et al., 2011), the Longmu-Gozha Co fault is more likely to belong to the westernmost segment of the Kunlun fault system (Fig. 1a), along which the 1997 Manyi and the 2001 Kokoxili earthquakes were ruptured (Klinger et al., 2005; Peltzer et al., 1999; Xu, 2000; Xu et al., 2002 ), and whose left-lateral slip rate reaches ~12.5 mm/year (Van der Woerd et al., 2002). Alternatively, since the Kunlun and Xianshuihe–Ganzè–Yushu fault systems are joined each other to the east of the Manyi fault, the Longmu-Gozha Co fault could be also said to be the westward extension of the Xianshuihe–Ganzè– Yushu fault system (Fig. 1a). To the north, the EW-trending Karakax fault system ~700 km long consists of the Kangxiwar and Kegang faults on the northwestern edge of the Plateau (Fig. 1b; Deng, 2007; Elliott et al., 2010; Raterman et al., 2007; Taylor and Yin, 2009), and is considered to be the westernmost part of the Altyn Tagh fault system (Elliott et al., 2008; Li et al., 2012; Wright et al., 2004). Specifically, the Kangxiwar fault is a left-lateral strike-slip fault, the Kegang fault a thrust fault, and between them is the Kunlun wedge (Wittlinger et al., 2004). This structural pattern shows a slip partitioning at the southwestern border of the Tarim basin (Deng, 2007; Elliott et al., 2010; Lacassin et al., 2004; Raterman et al., 2007; Taylor and Yin, 2009; Wittlinger et al., 2004). The left-lateral slip rate of the Kangxiwar fault reaches 8–12 mm/year (Fu et al., 2006), or about 6–7 mm/year inferred by using the offset landforms (Li et al., 2012), while the InSAR measurement gives a much lower rate of 5 ±5 mm/year (Elliott et al., 2008; Wright et al., 2004), and a present-day rate of left-lateral slip close to 7 ± 3 mm/year or only 2– 3 mm/year and convergence rate less than 4 mm/year deduced from GPS measurements (Meade, 2007; Shen et al., 2001b). The Yulong Kashgar fault, a nearly NS-trending fault that controls a NS-trending graben, is close to the southwestern most segment of the Altyn Tagh fault system, a boundary between the Qaidam–Qilian and western Kunlun blocks (Fig. 1) and was initially mapped from SPOT satellite images by Tapponnier and Molnar (1977), Tapponnier

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et al. (2001) and Armijo et al. (1986). This fault is characterized by obvious offset landforms, including ~ 1000 m high bedrock fault triangular facets and cumulative offset glacial moraines in left-lateral sense (Fig. 2a). It is worth to point out that the Yulong Kashgar fault and its associated graben forms one of the most northerly regions of

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the normal faulting in the western Tibetan Plateau (Fig. 1) and shows a local crustal extension, which can be also witnessed by the Neogene to late Quaternary potassium-rich shoshonitic volcanic eruptions in the western Kunlun Mountains (Searle et al., 2011; Xu J. et al., 2011). That is to say, the Yutian earthquake occurs along the

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Fig. 2. Map of the Yutian earthquake surface rupture zone. (a) Post-earthquake SPOT 5 satellite image showing the Yutian rupture zone and topography of this region. Red arrows indicate the sites where cumulative left-lateral offset of the glacial tongues and moraines exist across the Yulong Kashgar fault, while small black arrows the Yutian earthquake surface rupture zone and white box the figure location. (b) Interpretation map showing distribution of the surface breaks generated by the Yutian earthquake and Quaternary geology. Horizontal black arrows indicate the section boundary, and the black dots the figure locations.

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Yulong Kashgar fault, one of the boundary faults between the Qaidam–Qilian and western Kunlun blocks (Fig. 1b). 3. Surface Rupture Features 3.1. Rupture Types and Faulting Segmentation Our field investigations and detailed mappings from postearthquake SPOT 5 and Quickbird images show that the 2008 Yutian earthquake ruptured the NS-trending Yulong Kashgar fault along the western piedmont or snowline of a snow-covered mountain, west of the NE-trending southwestern most segment of the Altyn Tagh fault system (Figs. 1 and 2). This surface rupture zone, hereafter called the Yutian rupture zone, starts at 35.58083°N, 81.53147°E, in the north, extends southwards along the piedmont of the snowcovered mountain, east of the headwater of the Yulong Kashgar River, and ends at 35.32514°N, 81.48867°E, in the south. It cut the pre-Cenozoic bedrocks, late Pleistocene to Holocene colluviums, moraines and glaciers to form a 31-km-long surface rupture zone along the piedmont (Fig. 2). Here, it should be noted that the southernmost part of the Yutian rupture zone in the snow-covered remote area, south of 35.41856°N, is identified from SPOT 5 image acquired on 18th, November 2008 with a resolution of ~2.5 m (Fig. 2a). The northernmost

a

end of the rupture zone we saw in the field and identified from Quickbird image acquired on 30 July 2008 with a resolution of ~0.61 m is located in a flat dry riverbed (Fig. 2), and is expressed by en echelon or right-stepping transtensional breaks (Fig. 3a,b). Those transtensional breaks strike N330°± 10°W and dip to the west with a vertical offset of ~25 cm and a left-lateral slip of ~8 cm (Fig. 3c). The location of the northernmost end of the surface rupture zone we observed is a little different from that the other research group obtained only several months after the earthquake, which was located at 35.59137°N, 81.54609°E (Li H. et al., 2009). This difference might be caused by either flood erosion in the past 3 years or the other research group mistook the secondary surface cracks that no obvious vertical offset or left-lateral slip is observed at its northernmost end for the Yutian rupture zone (Fig. 3d). From the field observation and satellite image interpretation, it is known that the newly formed surface ruptures are localized in a narrow zone. Their widths range from 10 m up to 50 m in general, but the maximum width of the Yutian rupture zone we measured at stepovers from the Quickbird images reaches 135 m (Fig. 4). In addition, the surface ruptures can be classified into four different configurations (Fig. 5): (1) oblique normal fault scarp, (2) oblique graben-like scarp, (3) shear break and associated mole track, and (4) transtensional break.

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Fig. 3. Map of the surface breaks at the northernmost end of the Yutian rupture zone. (a) Original Quickbird image (see Fig. 2a for location). (b) Interpretation map showing distribution of the surface breaks mapped from field observation and Quickbird images taken after the earthquake. (c) Photograph showing one of the surface breaks along which the southwestern side falling down about 25 cm with a 8 cm left-lateral slip (view to NE). (d) Photograph showing one of the surface cracks along which no obvious offset occurred (view to SE).

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The oblique normal fault scarps (Fig. 5a) are well developed along the Yutian rupture zone (Fig. 6a,b,c,g). Their westward-facing fresh scarps strike NW, NNW and ENE, and are dominated by normal-slip

showing hanging wall subsidence with deposited colluviums or moraines (Fig. 2). The oblique graben-like scarp describes a shallow groove that is controlled by two sub-parallel faults, a main planar

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a

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Oblique normal fault scarp

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Graben-like scarp

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Fig. 5. Schematic diagrams of the oblique normal-faulting scarp features observed along the Yutian rupture zone.

fault and a secondary planar fault (Fig. 5b). Both of the two fault planes are angled downward toward each other and the secondary one disappears where it reaches to the main, and then a downdropped block is formed on the steep hill slope (Fig. 6d–f). The shear breaks are discontinuous in en echelon and mole tracks exist in their right-steps (Figs. 5c and 6h), while small pull-aparts or tension gashes are rarely seen along the Yutian rupture zone in the field, which are common for the 2001 Kokoxili or Kunlunshan earthquake and 2010 Yushu earthquake (Lin et al., 2011; Sun et al., 2012; Xu et al., 2006). The transtensional breaks and its related fault scarps are similar to the oblique normal fault scarps in tectonic morphology. Here we specifically refer to the breaks that cut the glaciers along the southern section of the Yutian rupture zone (Figs. 2 and 5d). The Yutian rupture zone tracks the above four types of the surface ruptures along the Yulong Kashgar fault to form a jagged surface rupture zone with both normal-slip and left-lateral strike-slip components (Figs. 2 and 4). This rupture zone can be divided into the northern, middle and southern sections which are different in strike and slip vector (Fig. 2). The northern section ~ 6 km long strikes to the NW (340° ± 10°) on average with poor linearity, and cuts the Holocene alluviums along the headwater of the Yulong Kashgar River, the glacial moraines and pre-Cenozoic bedrocks to the south (Figs. 2 and 3). It consists mainly of several NW-trending (325° ± 10°) left-lateral shear breaks in en echelon and mole tracks in the right-steps (Figs. 2 and 6g,h), and NWN-trending (350° ± 10°) oblique normal fault scarps (Figs. 2

and 6a–c). The left-lateral slip we measured ranges from ~ 8 cm to 13 cm and the uplift is ~25 cm on its northern end (Figs. 3c and 6g), while the maximum left-lateral slip identified by the offset gully and debris-flow on its NE-trending part from the Quickbird images reaches 1.2 m up to 1.8 m (Fig. 4b). This is in keeping with the location of the instrumentally determined earthquake epicenter (XEA & Harvard), which is near the northernmost end of the surface rupture zone to the west and northwest (Fig. 1b). The middle section ~11 km long between 35.53028°N and 35.43333°N veers westwards to the mountain front and then runs southwards along the western piedmont of the snow-covered mountain, east of the Yulong Kashgar River and strikes NWN (350°± 10°) on average. This section is characterized by fairly linear NW-trending (320°±10°) and NS-trending oblique graben-like scarps (Fig. 2). Those fresh graben-like scarps, as a steeper scarp and fresh groove, are overprinted on the halfway up the fault triangular facets, ~1000 m high escarpment with steep slopes separated by V-shaped gullies (Fig. 6d–f; Li H. et al., 2009). They cut the glacial moraines, late Pleistocene to Holocene alluviums, diluvium, debris flows and pre-Cenozoic bedrocks to form a localized surface rupture zone along the snowline (Fig. 6d). The offset landforms demonstrate that this section is dominated by normal faulting with left-lateral component. The left-lateral slip identified by the offset gullies and debris-flows from the Quickbird images reaches 2.1 m up to 3.0 m (Fig. 4c,f). A big strike-bend at 35.43333°N separates the southern section from the middle section (Fig. 2). The southern section strikes NNE

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b Oblique normal fault scarp Oblique normal fault scarp H0.8m

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d Oblique normal fault scarp Earthquake surface rupture zone

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f Graben-like scarp

Graben-like scarp valley valley

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h Shear break

th wid m 1.0

ip

l sl era t a l tLef 0 .13m

mole tra c k

Shear break

Fig. 6. Representative photographs of co-seismic surface rupture features of the Yutian earthquake (see Fig. 2b for locations). Red arrows indicate positions of the co-seismic ruptures. (a) View of the surface rupture zone along the northern section; (b) oblique normal fault scarp with 0.8 m vertical offset; (c) oblique normal fault scarp offsetting modern glacial moraine with ~1 m vertical offset; (d) remote view of the surface rupture zone along the middle section; (e) less than 10-m-wide graben-like scarp along the southern section; (e) 10-m-wide graben-like scarp along the middle section; (g) coseismic oblique-slip breaks with a width from 30 cm to 100 cm at the northernmost end of the Yutian rupture zone; (h) En echelon shear breaks and mole track in the right step.

(25°± 5°) and is dominated by transtensional breaks with normal and left-lateral slip components to form the oblique normal fault scarps. In the field we were able to trace the surface breaks southward to 35.41856°N, 81.53994°E, where it consists of only a narrow grabenlike scarp. From that point southward en echelon transtensional breaks that cut into a snow- and glacier-covered area are identified only from SPOT 5 images (Fig. 2).

No matter how different in fault geometry and faulting style, the northern, middle and southern sections of the Yutian rupture zone are essentially of normal faulting with left-lateral slip components. That is, the crust at the epicenter of the Yutian earthquake and its adjacent areas is undergoing local crustal extension in EW direction. The valley on the western side of the rupture zone, where the late Quaternary sediments have been settled down, may be a half graben basin,

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and the snow-covered hill a horst as a result of the normal faults (Fig. 1). 3.2. Coseismic Slips and Surface Rupture Widths Although many piercing lines across the Yutian rupture zone are identified on the Quickbird images to be offset in left-lateral sense (Fig. 4), coarse-grained deposits, such as moraine pebbles, near-source diluvial gravels on the slope, prevented us systematically measuring the coseismic left-lateral slips in the field. Only at several sites along the Yutian rupture zone we measured, with a 3D scanner, clearly offset geomorphic features, including piercing lines (rills, risers, etc) for strike slips and surfaces (terraces, fans, etc) for vertical offsets. Such measurements are accurate to within a few centimeters. The measured data show that the vertical offset for the northern section is in the range of 0.8 m and 1.0 m (Fig. 6b,c), while the visible left-lateral slip is ~0.17 m, and the width of a single surface break ranges from less than 1 m to 10 m. In the case where several rupture branches occur its width becomes wider up to ~100 m (Fig. 4a,b). The ratio of the vertical offset and the sinistral slip is larger than 4.5, indicating the northern section is dominated by normal-slip faulting. Here it is worth to point out that the left-lateral slip reaches 1.8 m ± 0.6 m on the NE-trending branch or 1.2 m ± 0.6 m on the NNE-trending branch in the right step between the northern and middle sections (Figs. 2 and 4b). The middle section is the main part of the Yutian rupture zone. The width of a single rupture zone is less than 40 m in general, but it becomes wider up to 135 m at sites where two graben-like scarps

adjacent to each other in en echelon to form a stepover and secondary surface breaks occurred (Fig. 4c,e,f). At the site (35.52856°N, 81.52844°E) there exist two branches of the surface rupture zone ~ 35 m apart and its rupture width reaches 40 m (Fig. 7a,b), while its sinistral slip is 1.2 ± 0.2 m and vertical offset 1.3 ± 0.3 m (Fig. 7b, c) that we measured from an offset gully and a hillside, respectively. The width for a single rupture zone is only 12 m (Fig. 7c). Those values increase southwards. At the site (35.46600°N, 81.54006°E) we observe that the surface rupture zone strikes to the NW (~ 327°) and dips toward the SE with a dip angle of ~ 65°. Its rupture width, sinistral slip and vertical offset are 20 m up to 40 m, 3.6 ± 0.5 m and 3.3 ± 0.3 m (Fig. 8), respectively, which yields the maximum sinistral slip and the maximum vertical offset obtained along the Yutian rupture zone. According to the fault geometry we observed here, this maximum vertical offset corresponds to a N60°E-trending crustal extension of ~1.5 m or an EW-trending crustal extension of ~1.3 m. The ratio of the vertical offset and the sinistral slip ranges from ~1 to 1.1, indicating the middle section is typically of oblique-slip faulting. From the measured data of the offset gully and its riser at a site (35.41856°N, 81.53994°E), where only one N20°E-trending grabenlike scarp exists on an alluvial fan, we know that the rupture width, vertical offset and sinistral slip on the southern section is ~ 18 m, 0.9 ± 0.1 m, and 1.5 ±0.2 m (Fig. 9), respectively. The ratio of the vertical offset and the sinistral slip is only 0.6, indicating the southern section is dominated by left-lateral strike-slip faulting with normal component. Together with systematical long-term cumulative offsets of the glacial tongues and moraines in left-lateral sense between 35.40°N

a Main surface breaks

Secondary surface breaks Main rupture branc

Surface break location

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Rupture width 12m

5382 5378 5374

Vertical offset 1.25 ± 0.25 m

5370

Secondary rupture branch 5366 5362

Horizontal distance (m) Fig. 7. Offset geomorphic features measured at the site (35.52856°N, 81.52844°E) along the middle section. (a) Photograph showing distribution of the surface breaks (red arrow sites) along two rupture branches ~35 apart; (b) measured topographic map with contour interval 2 m and height decreases from blue area to yellow area, showing a total surface rupture width of 40 m and a sinistral slip 1.2 ± 0.2 m from an offset gully and its western riser; (c) A–A′ topographic profile showing a vertical offset of 1.3 ± 0.3 m (see Fig. 7b for location).

200 m

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a

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Fig.8a

Flat land Flat land

Fig. 8. Offset geomorphic features measured at the site (35.46600°N, 81.54006°E) along the middle section. (a) Measured topographic map with contour interval 2 m and height decreases from blue area to yellow area; (b) A–A′ topographic profile showing a maximum vertical offset of 3.3 ± 0.3 m, profile location is marked in Fig. 8a; (c) photograph showing oblique normal fault scarp (red arrow sites); (d) Photograph showing the left-lateral slips of the landforms. Captions are as for Fig. 7.

and 35.46°N (Fig. 2), this kinematic feature is similar to that of the NE-trending southwestern most segment of the Altyn Tagh fault system (Fig. 1). Thus, the seismogenic fault of the Yutian earthquake is an eastward-bulged arc fault and its strike changes from 340° ± 10° (NW) along its northern section, to 350° ±10° (NNW) along its middle section and finally to 25° ± 5° (NNE) along its southern section. Although the offset landforms are measured and the coseismic slips are obtained at only several sites, those data further demonstrate that the Yutian rupture zone is characterized by normal- and oblique-slip faulting with southward decreasing in normal-slip component, while southward increasing in left-lateral strike-slip component. In addition, the width of the Yutian rupture zone less than 40 m in general and up to 135 m at the stepovers shows a localized surfacerupturing feature, and this localized feature is similar to other earthquakes in the northern Tibetan Plateau (Klinger et al., 2005; Sun et al., 2012; Xu et al., 2002, 2006, 2009; Yu et al., 2010; Zhou et al., 2010), and a maximum EW-trending crustal extension of ~1.3 m occurs along the Yulong Kashgar fault. This, at least, shows that most of the elastic strain accumulated during the interseismic period is released during earthquakes on a few, localized seismogenic faults across the Tibetan Plateau.

4. Discussion From InSAR, ASTER and Quickbird satellite images and teleseismic body wave modeling, it was known that the surface rupture zone is about 20 km up to 55 km long and could be divided into three segments: the northern segment that strikes to the NNE (~N194°E) and dips to the WNW with an average slip of 0.5 m–1.5 m, the middle segment that strike to the NW (~N168°E) and dips to the SW with an average slip of 3.2 m–4.1 m, and the southern segment that strikes to the NNE (~N20°E) and dips to the WNW with average slip of 2.9 m–3.9 m (Elliott et al., 2010; Furuya and Yasuda, 2011; Shan et al., 2011). Thus, the geometry, rupture segmentation and associated parameters of the Yutian rupture zone we obtained both from the field observations and satellite image interpretations agree well with the results from InSAR observations in faulting style, slip sense along the strike and average coseismic slips, but the complexity of the surface rupture zone was ignored in InSAR data modeling. The difference in strike of the northern section of the surface rupture zone is also obvious. The InSAR data give a strike to 194° for the northern section (Elliott et al., 2010), while our field survey gives a strike to 340°±10°. The reason for this error estimate by InSAR may be resulted from the discontinuous surface rupture pattern and many surface cracks near the surface rupture zone (Fig. 2).

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a

b

' 5588

Elevation (m)

5586 5584 5582 5580 5578 5576 5574

Rupture width ~ 18 m

Vertical offset 0.9 ± 0.1 m

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Gully Sinistral slip 1.5 ± 0.2 m

Rupture

5570

Collapse

Cracks '

Gully

Sinistral slip 1.5 ± 0.2 m

Gully

View to northwest

Fig. 9. Offset geomorphic features measured at the site (35.41856°N, 81.53994°E) along the southern section. (a) Measured topographic map with contour interval 2 m and height decreases from blue area to yellow area showing a 1.5 ± 0.2 m sinistral slip from offset gully and its riser; (b) A–A′ topographic profile showing a vertical offset of 0.9 ± 0.1 m, profile location is marked in Fig. 9a; (c) Photograph showing oblique graben-like scarp (red arrow sites). Captions are as for Fig. 7.

Primarily, the normal- and oblique-slip faulting of the Yutian earthquake suggests that there exists an EW-trending local crustal extension besides the dominated strike-slip faulting in the northern Tibetan Plateau (Armijo et al., 1986; Copley et al., 2011; Elliott et al., 2010; Tapponnier and Molnar, 1977; Tapponnier et al., 2001; Taylor and Yin, 2009). Since the Yutian earthquake and other normal events occurred at elevation above ~ 5000 m in the Tibetan Plateau, Elliott et al. (2010) concluded that the occurrence of such earthquakes is in keeping with the theory that the east-west extension is mostly driven by gravitational spreading forces, as first proposed by Molnar and Tapponnier (1978). While Furuya and Yasuda (2011) considered that the Yutian rupture zone might be resulted from the local extension in the releasing stepover between the ENE-trending Altyn Tagh fault system and the Longmu–Gozha Co Fault (Fig. 1). Both of them might be the reasonable explanations of the nucleation of the local crustal extension and then the occurrence of the Yutian earthquake. Tectonically, the northern Tibetan Plateau, where the Yutian earthquake occurred, is characterized by the conjugate mega-strike-slip faults with minor normal faults, whereas the southern Tibetan Plateau is dominated by EW-trending extension across NS-trending rifts (Armijo et al., 1986; Tapponnier and Molnar, 1977; Taylor and Yin, 2009; Taylor et al., 2003). The mega-strike-slip faults include the Altyn Tagh, Kunlun, Karakax, Karakoram, Xianshuihe-Ganzè-Yushu, Xiaojiang and Jiali fault systems. Those mega-strike-slip fault systems divide the Tibetan Plateau into, from south to north, the Lhasa, Qiangtang, Bayan Har, Qaidam-Qilian and western Kunlun blocks (Fig. 1). In this case, we prefer explaining the occurrence of the Yutian rupture zone as a result of abrupt normal- and oblique-slip faulting along the tectonically complex boundary between the Qaidam-Qilian

and western Kunlun blocks in the northwestern Tibetan Plateau (Fig. 1). The Yutian earthquake ruptured a nearly NS-trending fault that is close to the southwestern most segment of the Altyn Tagh fault system, a boundary between the Qaidam-Qilian and western Kunlun blocks (Fig. 1). Its normal- and oblique-slip faulting feature, especially the EW-trending crustal extension of ~1.3 m demonstrates the western Kunlun and the Qaidam-Qilian blocks are pulled apart at the epicenter of the Yutian earthquake and its adjacent areas (Fig. 1). This local crustal extension can be also witnessed by the Neogene to late Quaternary potassium-rich shoshonitic volcanic eruptions in the western Kunlun Mountains (Searle et al., 2011; Xu J. et al., 2011). Since the left-slip rates along the active faults west of E82° is less than those on the Altyn Tagh fault system and Kunlun fault system, it seems that the 2008 Yutian earthquake, a normal- and oblique-slip faulting rupture, is generated by the EW-trending local extension owing to the faster eastward motion of the Qaidam-Qilian block in the east than the western Kunlun block in the west. That is, the eastwards block-like motion model predicts that the slip rates of the active faults west of the block boundary should be lower than those in the east. This eastward block-like motion is also consistent with Quaternary compressive deformation along the Hexi Corridor at the easternmost margin of the Qaidam-Qilian block, where the Qaidam-Qilian block has being overridden the Gobi-Alashan Platform and Ordos block in the North China (Fig. 1; Xu and Deng, 1996). The reverse-slip dominated surface ruptures of the A. D. 180 Gaotai earthquake (M 7.5), the 1609 Hongyazi earthquake (M 7.25) and the 1927 Gulang earthquake (M 8.0) along the Hexi Corridor further demonstrate that the crustal shortening is still undergoing at the forward margin of the block-like motion (Xu et al., 2010b). Similarly, the obvious crustal shortening accomplished by

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the 2008 Mw 7.9 Wenchuan earthquake along the Longmenshan thrust belt indicates that the east-southeastward extrusion of the Bayan Har block of the Tibetan Plateau and collision with the South China block does exist and is responsible for the growth of high topography in the Plateau (Xu et al., 2009, 2010a). Similarly, there exist a lot of NS-aligned normal faults in the western parts of the Bayan Har, Qiangtang and Lhasa blocks (Fig. 1). All of them may have been resulted from the pull apart owing to faster eastwards block motion in the eastern than in the western (Fig. 1a). That is to say, block-like motion prevails in the eastern and northern Tibetan plateau, while the EW-trending crustal extension is dominated where active NS-trending grabens and associated normal faults have developed at least in the upper crust (Armijo et al., 1986; Elliott et al., 2010). Thus, this kind of surface rupture pattern, in fact, supports the eastward block-like motion model whose deformation takes place mainly along the block boundaries delineated by mega-strike-slip fault systems in the northern Tibetan Plateau (Avouac and Tapponnier, 1993; Peltzer and Tapponnier, 1988; Replumaz and Tapponnier, 2003; Tapponnier et al., 1982, 2001; Xu et al., 2009), other than the continuous deformation model whose deformation is dominated by broadly distributed shortening in the Plateau (Burchfiel et al., 2008; England and McKenzie, 1982; England and Molnar, 1997; Flesch et al., 2001; Houseman and England, 1996; Royden et al., 1997; Shen et al., 2001a; Vilotte et al., 1986). If the eastwards block-like motion model is correct, the occurrence of the Yutian earthquake shows that the Qaidam-Qilian block did move eastwards during the earthquake. This eastwards motion did increase accumulated left-lateral elastic strain along the Altyn Tagh fault system on the northern margin of the Qaidam-Qilian block and accumulated reverse elastic strain along the Hexi Corridor on the easternmost margin, and decrease the accumulated left-lateral elastic strain along the Kunlun fault system on the southern margin, which implies that the Altyn Tagh fault system has a greater seismic risk potential than the Kunlun fault system. We need to pay more attention to occurrence of great earthquakes along the Altyn Tagh fault system and the Hexi Corridor in the future. 5. Conclusions The Mw 7.1 Yutian earthquake ruptured a normal fault close to the NE-trending southwestern most segment of the Altyn Tagh fault system. The surface rupture zone consists of the northern, middle and southern sections and its strike changes from NW along the northern section, to NNW along the middle section and finally to NNE along the southern section to form a 31-km-long eastward-bulged arc surface rupture zone. The geomorphic expression of the ruptures consist of oblique normal fault scarp, oblique graben-like scarp, shear break and associated mole track, and transtensional break cutting into the area covered by snow and glacier. All of them are characterized by dominantly normal- and oblique-slip faulting to show a southward decreasing in normal-slip component but increasing in left-lateral strike-slip component. The measured maximum vertical offset and left-lateral slip are ~ 3.3 m and ~ 3.6 m, respectively, and this maximum vertical offset corresponds to a maximum EW-trending crustal extension of ~ 1.3 m. Then this extension occurs as a result of oblique pull apart along the boundary between the Qaidam–Qilian and western Kunlun blocks, owing to the faster eastward motion of the Qaidam–Qilian block in the east than the western Kunlun block in the west. This eastward motion is also consistent with compressive deformation along the eastern margin of the Tibetan Plateau, and then the block-like motion prevails in the eastern and northern Tibetan Plateau. Besides, the earthquake surface rupture width less than 40 m in general demonstrates a localized surface rupturing feature other than distributed deformation and is similar to the other earthquakes (Manyi Mw 7.6 earthquake, Mw 7.8 Kokoxili earthquake, Mw 7.9

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Wenchuan earthquake, Mw 6.9 Yushu earthquake) in the northern Tibetan Plateau. This, at least, implies that most of the elastic strain accumulated during the interseismic period is released during earthquakes on a few, localized seismogenic faults across the Tibetan Plateau. Besides, if the eastwards block-like motion model is correct, the Altyn Tagh fault system and the Hexi Corrdor have the greater seismic risk potential than the Kunlun fault system. We need to pay more attention to occurrence of great earthquakes along the Altyn Tagh fault system and Hexi Corridor in the future. Acknowledgements This research is supported by Natural Science Foundation of China (grant numbers 40974057 and 40821160550) and International Scientific joint project of China (grant number 2009DFA21280). We are grateful to the two anonymous reviewers for their detail suggestions and comments significantly improved the manuscript. References Armijo, R., Tapponnier, P., Mercier, J.L., Han, T.L., 1986. 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