Research Paper

GEOSPHERE The 2015 Ms 6.5 Pishan earthquake, Northwest Tibetan Plateau: A folding event in the western Kunlun piedmont

1,2 2 2 2 2 2 2 GEOSPHERE, v. 15, no. 3 Chuanyong Wu , Jianming Liu , Jin Li , Weihua Hu , Guodong Wu , Xiangde Chang , and Yuan Yao 1Guangdong Provincial Key Laboratory of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-sen University, Guangzhou, China 2Earthquake Agency of Xinjiang Uygur Autonomous Region, Urumqi, China https://doi.org/10.1130/GES02063.1

7 figures; 2 tables ■■ ABSTRACT investigate whether the Pishan earthquake generated a surface fault and what the deformation characteristics of this event are. The surface rupture caused CORRESPONDENCE: [email protected] Folding earthquakes are a popular area of research in convergent orogenic by an earthquake can provide a unique opportunity to investigate the impact of belts because they can cause destruction without an obvious surface offset. coseismic faulting on landscape evolution and to refine regional deformation CITATION: Wu, C.Y., Liu, J.M., Li, J., Hu, W.H., Wu, G.D., Chang, X.D., and Yao, Y., The 2015 Ms 6.5 Pishan The Ms 6.5 Pishan earthquake (Ms represents Richter magnitude scale), which models (Wallace, 1977; Yeats et al., 1997; Bull, 2009). The tectonic deformation earthquake, Northwest Tibetan Plateau: A folding event occurred in the western Kunlun piedmont, Northwest Tibetan Plateau, caused and uplift of the Tibetan Plateau have been popular areas of research. Two in the western Kunlun piedmont: Geosphere, v. 15, no. 3, significant property losses. Based on surface deformation data combined main models of upper crustal shortening and faulting (e.g., Tapponnier et al., p. 935–945, https://doi.org​ /10.1130​ /GES02063.1.​ with earthquake relocation results, structural geology, and seismic reflection 2001; Hubbard and Shaw, 2009; Xu et al., 2009; Jiang et al., 2013) and lower profiles, we determined that the Pishan blind thrust anticline is a seismogenic crustal viscous flow (e.g., Clark and Royden, 2000; Royden et al., 2008) have Science Editor: David Fastovsky Associate Editor: Huaiyu Yuan structure. The surface deformation caused by this earthquake was dominated been used to explain the growth of the plateau. The tectonic deformation and by layer folding and surface uplift, which generated tensional ground fissures growth pattern of western Kunlun, which is the northwestern margin of the Received 18 September 2018 at the surface. Therefore, we suggest that the Ms 6.5 Pishan earthquake was Tibetan Plateau, are not currently well understood. A thorough study of the Revision received 14 December 2018 a folding event. The Pishan earthquake only ruptured part of the Pishan an- Ms 6.5 Pishan event will allow us to understand the tectonic deformation and Accepted 14 March 2019 ticline, and the main locked part between the Tekilik fault and the Pishan seismotectonic model in this region. anticline did not rupture. This area of the western Kunlun range front may In this paper, we first report the surface deformation caused by the Pishan Published online 17 April 2019 have significant seismic risk. earthquake based on our field investigations. We then utilize geologic data, seismic reflection profiles and earthquake relocation results to study the seis- mogenic structure of the Pishan earthquake and the deformation characteristics ■■ INTRODUCTION of the Pishan blind thrust fold. Finally, we discuss the tectonic deformation along the western Kunlun range front and the seismic risk in this region. Folding earthquakes, which were observed in detail in the 1983 Coalinga earthquake, in Coalinga, California, USA (Stein and King, 1984), are a general type of rupture along foreland thrust systems. Although folding earthquakes ■■ ACTIVE TECTONIC SETTING OF THE WESTERN KUNLUN RANGE are generally characterized by surface uplift and layer folding (Stein and Yeats, 1989; Yeats et al., 1997) without obvious surface fault offset, they also can The western Kunlun orogenic belt is located on the northwestern margin of cause destruction. Over the past several decades, this kind of hazard has been the Tibetan Plateau (Fig. 1). In response to the Cenozoic India-Eurasia collision, exemplified by the 1906M s 7.7 Manasi earthquake (Ms represents Richter the western Kunlun Range was uplifted rapidly, and the overthrusting of Paleo- magnitude scale) in the northern piedmont of the Chinese Tian Shan (Zhang zoic bedrock onto Cenozoic strata can be widely observed along the range-front et al., 1994), the 1985 Ms 7.1 Wuqia earthquake, in Xinjiang, China, along the Tekilik fault (Cowgill, 2001; Yin et al., 2002). The crustal thickness in this area Pamir front (Feng, 1997), and the 2013 Ms 7.0 Lushan earthquake in the Long- can reach ~70 km (Negredo et al., 2007; Tseng et al., 2009), and the Cenozoic men mountain piedmont, in Sichuan, China (Xu et al., 2013). sediments in the foreland basin are more than 12 km thick (Matte et al., 1996). The 2015 Ms 6.5 Pishan earthquake, which occurred in the western Kunlun The strong uplift landforms, thick Cenozoic deposits in the foreland basin, and Range piedmont (Fig. 1), caused significant casualties and property losses. widespread active faults (Fig. 1) all attest to the intensive tectonic deformation Previous studies (e.g., Li et al., 2016; Lu et al., 2016; Zhang et al., 2016) have in this region (Sobel and Dumitru, 1997; Zheng et al., 2000; Chen et al., 2011). explored the seismogenic structure and rupture mechanism of this earthquake. Several strike-slip faults are present within the western Kunlun Range Based on the statistical results of seismic data, an earthquake with a magnitude (Fig. 1), which accommodate the different horizontal displacements of the This paper is published under the terms of the greater than 6.5 can generate an obvious surface rupture zone (Yeats et al., tectonic blocks. The Karakoram fault, which is 510 km long and has an aver- CC‑BY-NC license. 1997). However, surface deformation of this event has not been reported. We age slip rate of 6.9–10.8 mm/yr (Robinson, 2009), is a large dextral strike-slip

© 2019 The Authors

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76° 78° 80° 82° A 70° 80° 90° 100° B N ▲ Tian Shan 40° Fig. 1B ian Shan Tagh fault T Altyn ▲KaKalppin Karakoram fault Tibetan Plateau ▲ 30° 40° 40° ▲ India ▲ ▲ ▲ ▲ ▲ Artux ▲ Kashgar ▲ Tarim Basin 75˚ 76˚ 77˚ 78˚ 79˚ 80˚ 81˚ 39˚ M Z F ▲ ▲

38° Fig. 2 ↓ 38˚ 38° Yecheng ▲T K F ▲Pishan Moyu ▲Fig. 3 ↓ ▲ Hotan Luopu YHTF ▲ CeC le 37˚

K K F Western Kunlun range ▲ ▲ K X F

36° 36˚ 36°

0 50 100km

76° 78° 80° 82°

Focal mechanism Focal mechanism Ms = 4.0 - 4.9 Thrust Strike slip Normal Seismic solution of the Pishan solution of the Ms = 3.0 - 3.9 ▲ Ms = 2.0 - 2.9 fault fault fault station Ms 6.5 earthquake other earthquakes Ms = 1.0 - 1.9

Figure 1. Topographic relief map and distribution of major active faults in the western Kunlun and the adjacent region of China. The focal mechanism solutions of earthquakes with magnitudes greater than Ms 5 and the spatial distribution of the aftershocks of the Ms 6.5 Pishan earthquake are all shown in section B. Sixteen focal mechanism solutions (data from the U.S. Geological Survey, https://earthquake.usgs.gov​/earthquakes/) of recent events with magnitudes greater than Ms 5 are shown with blue. KKF—Karakoram fault; TKF—Taxkorgan normal fault system; KXF—Kangxiwar fault; YHTF—Yech- eng-Hotan thrust fault-fold zone; MZF—Mazhatagh thrust fault-fold. Ms represents Richter magnitude scale.

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fault that has accommodated ~300 km of northward translation of the Pamir considered to be the frontal active belt of the western Kunlun nappe structure (Hamburger et al., 1992; Burtman and Molnar, 1993; Robinson et al., 2007; (Pan et al., 2010). The active foreland folds mainly formed during the Quaternary Strecker et al., 1995). The Kangxiwar fault is a large crustal-scale sinistral (Chen et al., 2001; Liu et al., 2004; Si et al., 2007) and are the main structures strike-slip fault (Tapponnier and Molnar, 1977; Peltzer et al., 1989; Fu et al., accommodating the N-S crustal shortening deformation. The total crustal 2006) that represents part of the northwestern boundary of the Tibetan Pla- shortening calculated based on a balanced section is 24.6–54 km (Jiang et al., teau. The sinistral slip rate determined by geological (e.g., Fu et al., 2006; Li 2013), and the crustal shortening rate in this region determined by GPS data et al., 2008) and geodetic (e.g., Shen et al., 2001; Wright et al., 2004; Elliott et is ~2 mm/yr (Shen et al., 2001; Li et al., 2016). al., 2008) methods is ~10 mm/yr. The Taxkorgan fault is an extensional dextral fault system (Brunel et al., 1994; Robinson et al., 2004; Cowgill, 2010; Zubovich et al., 2010) that is composed of several secondary faults (Li et al., 2011). The ■■ TECTONIC DEFORMATION FEATURES OF THE PISHAN ANTICLINE rate of extension of this fault decreases gradually from north to south (Rob- inson et al., 2007; Li, 2013). A petroleum industry seismic profile (Fig. 3; Liang et al., 2012) shows two- Several rows of folds and thrust faults have developed along the western fold belts between the range front and Pishan city. The southern fold is called Kunlun range front (Matte et al., 1996; Si et al., 2007; Du et al., 2013; Li et al., the Kekeya anticline (Du et al., 2013). Near the range-front, the strata dip at 2016). The late Cenozoic tectonic deformation is characterized by foreland high angles, and the dips of Cretaceous strata exposed at the surface can folds and thrust faults propagating from the range front to the Tarim Basin reach ~80° (Si et al., 2007), which indicates that the Kekeya structural belt in the piedmont, forming thin-skinned nappe structures (Li and Wang, 2002), contains intense tectonic deformation and a high-angle thrust fault. North of such as the Pishan and Kekeya anticlines (Fig. 2). The Mazhatagh thrust fault the Pishan anticline, the seismic profile reveals shallowly dipping Cenozoic and related folds, which are located ~200 km north of the western Kunlun, are units and gentle folds. The maximum dip of the Neogene mudstone in the

Figure 2. Geological structure map in the Pishan earthquake region of the Northwest Tibetan Plateau. Ms represents Richter magnitude scale.

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Kekeya anticline 0 Pishan anticline N 2 Figure 3. Petroleum industry seismic reflection

km profile near Pishan, Northwest Tibetan Plateau 4 (modified after Liang et al., 2012; for profile loca- tion, see Fig. 1). Several aftershocks with mag- Pz nitudes greater than Ms 3.0 and the main focal

Depth / Pz 6 mechanism solutions are shown in the profile. Pz Q—Quaternary; N2a—Pliocene Artux formation; N1a—Miocene Anjuan formation; N1wq—Mio- 8 cene Wuqia formation; E—Paleogene; K—Creta­ Pz ceous; J—Jurassic; Pz—Paleozoic. Ms represents 0 5km Richter magnitude scale. 10

Pishan anticline is less than 20° (Si et al., 2007), indicating the presence of a bending moment faults may form because of local tensile stresses at the crest shallowly dipping fault ramp under the anticline (Li et al., 2016; Lu et al., 2016). of the anticline (Fig. 4E; Li et al., 2001). The 1983 Mw 6.5 Coalinga earthquake, The epicenter of the Ms 6.5 Pishan earthquake was located near the Pishan which did not rupture a distinct seismic fault at the surface, caused surface foreland fold and thrust belt (Fig. 1). The Pishan anticline, which has low relief uplift of up to 0.6 m (Stein and King, 1984). Therefore, we suggest that these and has experienced only ~150 m of uplift, is mainly composed of Pliocene normal faults on the Pishan anticline are bending moment faults (Li et al., 2016). mudstone and lower Pleistocene conglomerates at the surface (Fig. 2). A petro- leum industry seismic profile (Fig. 3) shows that this anticline is a typical blind thrust fault anticline. The layers of the anticline have been folded but without ■■ SURFACE DEFORMATION OF THE 2015 Ms 6.5 PISHAN obvious offset. From the seismic profile, we can identify growth strata in the EARTHQUAKE Pliocene layer, which indicate that the fold deformation began in the Pliocene (Chen et al., 2001; Liu et al., 2004). The N-S width of this fold is ~20 km, and the The earthquake intensity in the meizoseismal areas of the Ms 6.5 Pishan detachment is located at a depth of 8–10 km (Liang et al., 2012; Li et al., 2016). earthquake was classified as VIII degrees (Mercalli intensity scale). The long At depth, the Pishan blind thrust fault merges into the range-front fault and axis direction of the meizoseismal area is generally consistent with the strike of then roots beneath western Kunlun (Liang et al., 2012). Based on trigonometric the Pishan anticline (Fig. 5A). Along the active fold, we did not find a coseismic relations, we can estimate an average dip of ~15° for the Pishan blind thrust fault fault. However, near the core of the Pishan anticline, several tensional ground based on a propagation distance of 50 km and a fault depth of 12–15 km at the fissures are present. These fissures are mainly distributed in the western area range front. Figure 3 shows a fault ramp with a dip of ~15° on the northern limb. of the epicentral region (Fig. 4A), where the maximum slip of this earthquake At the crest of the Pishan anticline, a group of northwest-striking fault scarps occurred (Zhang et al., 2016). At the village of Kumuqiake in the town of are present (Fig. 4A; Pan et al., 2007), and an older geomorphic surface with a Pixina, two groups of ground fissures have a right-stepping geometry. The higher scarp (Li et al., 2016) indicates that these faults have been continuously southeastern branch, which is ~18 m long and has a fissure width of 1–3 cm, active. Remote sensing image interpretation and field investigations show is located on a hard asphalt road and trends 310° (Fig. 5B). The northwestern that these faults are distributed over a N-S width of ~9 km. The scarps have a branch, which is 20 m long and has a maximum width of 20 cm, is located on discontinuous distribution and a consistent strike. A trench that we excavated flat ground in an orchard and trends 300° (Fig. 5C). Near the town of Pixina, across the scarp revealed a typical normal fault (Figs. 4B–4D). The tectonic de- several fissures with a total length of ~100 m and widths of 1–10 cm are pres- formation of the blind thrust anticline is characterized by the displacement grad- ent on flat ground. These fissures trend 290° and cut through the hard asphalt ually transforming into layer bending and folding near the surface, and some road (Fig. 5D). Approximately 300 m south of Pixina, a ground fissure with a

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A N D NNE Pishan Trench Guman CENC Fig.2f Coll uvial w edge Fig.5D Pixina Fig.5E Fig.5C P Fig.5B is Activefault ha n Surface fissure an Sand USGS ti cli liquefaction 0  ne 5km Yapu spring Anticline axis Colluvial wedge

B NWW

Trench 0 0.3m

E Bending moment fault

C Fig.4D

Gravellayer Siltylayer

0 2m

Figure 4. Trench profile and sketch map of the blind thrust fault fold and bending moment faults northwest of Pixina. (A) Bending moment faults on the crest of the anticline and ground fissures south of Pishan caused by the earthquake. (B) Fault scarp with a height of ~2 m on the terrace. (C, D) The normal fault revealed by our trench offsets the terrace gravel. Several colluvial wedges can be identified on the profile. (E) Schematic map of the bending moment fault. USGS—U.S. Geological Survey (https://earthquake.usgs.gov/earthquakes/eventpage/us10002n4w/executive); CENC—Chinese Earthquake Networks Center (http://news.ceic.ac.cn/CC20150703090747.html).

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77e 78e 79e A D 38e 38e N Yecheng

Pishan ൜ Moyu

൛ ൚ 37e 37e

0 20 40km 77e 78e 79e

B NE E NE

C ES F S

N

Figure 5. Ground fissures and sand liquefaction caused by the Pishan earthquake, Northwest Tibetan Plateau. (A) The long axis direction of the seismic intensity map is consistent with the strike of the Pishan anticline. The short yellow lines in the VIII degree (Mercalli intensity scale) area (red area) represent ground fissures. (B–E) Photos showing the characteristics of the ground fissures. (F) Sand liquefaction caused by the earthquake.

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strike of ~290° is present, and it extends to the flat, hard ground of a residen- tial yard (Fig. 5E). In addition, sand liquefaction can be widely observed in the earthquake area (Fig. 5F). The sand liquefaction sites are mainly distributed in 0 the vicinity of the anticline core and the reservoir (Fig. 4A). The coseismic fault dislocation of the Pishan earthquake was ~0.6–1.0 m at 5 the epicenter (He et al., 2016; Zhang et al., 2016). However, no coseismic fault km was found at the surface. Inversion results indicate that the top of the fault that

ruptured in the Pishan earthquake is buried 5 ± 2 km beneath the surface and Depth / 10 that the surface uplift was ~10 cm (Zhang et al., 2016). The tensional ground fissures caused by the earthquake are mainly located on flat ground near the core of the Pishan anticline, and their strikes are generally consistent with that 15 of the anticline. Some of the ground fissures extend ~100 m and cut through a 0 5 10 15 20 25 30 hard asphalt road. We suggest that these ground fissures are closely related to Distance / km

tectonic activity and were not caused by the strong ground motion or sloping Figure 6. Focal depths of the main earthquake and the aftershocks after relocation in a profile terrain. Rather, the coseismic displacement was transformed into layer bending (the active anticline deformation characteristics were modified after Daëron et al., 2007; for fig- and folding near the subsurface; therefore, the tensional ground fissures are ure location, see Fig. 2). Aftershocks: Green circles represent earthquakes of Ms 4.0–4.9; yellow circles represent earthquakes of Ms 3.0–3.9; pink circles represent earthquakes of Ms 2.0–2.9; and the surface deformation caused by this earthquake. red circles represent earthquakes of Ms 1.0–1.9. Ms represents Richter magnitude scale. Red star represents the main earthquake.

■■ FOCAL PARAMETERS AND AFTERSHOCK DISTRIBUTION Cut and paste is an effective method for inverting focal mechanisms be- To study the relationship between the Pishan earthquake and the structure, cause of its insensitivity to lateral differences in the velocity structure (Zhu and we relocated the main earthquake and its aftershocks using the double dif- Helmberger, 1996). In this paper, we calculated the fitting error function between ferential location method. A total of 1793 aftershocks recorded by more than theoretical and actual full waveforms after removal of low correlation coefficient three stations within 400 km (Fig. 1B) with six seismic phases were utilized. We through assigning different weights to Pnl and S waves, and obtained the opti- modeled the one-dimensional velocity structure (Table 1) based on the seismic mal solutions using grid search method. We utilized CRUST 2.0 (http://igppweb. reflection results (Li et al., 2001), geologic mapping (Si et al., 2007), and deep ucsd.edu/-gabi/crust2.html)​ to obtain one-dimensional velocity structure. The drilling data (Geological and Mineral Bureau of Xinjiang Uygur Autonomous focal mechanisms of the main earthquake and 13 aftershocks that were calcu- Region, 1992) in this area, which can constrain our structural model. In this lated based on at least 8 seismic stations in each inversion are shown in Table 2. velocity model, the crustal thickness is ~52 km, which is generally consistent The focal mechanism of the main earthquake shows that the seismogenic with receiver function results (e.g., Liu et al., 2011). Finally we obtained 1549 structure strikes northwest (Fig. 2) and that the earthquake was a typical thrust relocated earthquakes (Figs. 2 and 6). Based on the conjugate gradient method, fault rupture. The relocated depth of the Ms 6.5 Pishan earthquake is 8.4 km, the average relocation errors in the N-S, E-W, and U-D directions are 0.8 km, which is consistent with the fault depth revealed by the seismic reflection pro- 0.8 km, and 0.7 km, respectively, and the average relocation residual is 0.07 s. files (Jiang et al., 2013). An inversion result based on teleseismic body waves and interferometric synthetic-aperture radar measurements also indicates that the rupture occurred at a depth of ~7–9 km (Zhang et al., 2016). The main TABLE 1. CRUSTAL VELOCITY MODEL OF earthquake was located on the fault ramp of the Pishan anticline, and its dip of THE WESTERN KUNLUN RANGE FRONT, ~15–20° is consistent with the dip of the fault ramp. The relocated aftershocks NORTHWEST TIBETAN PLATEAU are densely distributed along the Pishan anticline in a zone ~45 km long from –1 Depth/km P-wave velocity/km·s VP/VS northwest to southeast and 20 km wide from northeast to southwest (Fig. 2). 0.0 4.0 1.75 The aftershock distribution in map view (Fig. 2) and profile (Fig. 6) indicate that 10.0 6.1 1.75 the deformation occurred throughout the entire area of the anticline and was 20.0 6.3 1.75 not concentrated along the fault belt. The temporal and spatial distributions of 24.0 6.7 1.75 the main shock and its aftershocks indicate that the Pishan earthquake rupture 35.0 6.8 1.75 52.0 8.1 1.75 propagated from the southeast (epicenter) to the northwest. The focal depths of the Ms ≥ 1.0 aftershocks show that the tectonic deformation mainly occurred Note: V /V —ratio between P-wave and S-wave velocity. P S in the Cenozoic sedimentary layers at depths of 3–12 km. The envelope of the

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TABLE 2. FOCAL MECHANISM SOLUTIONS OF THE Ms 6.5 PISHAN EARTHQUAKE AND SEVERAL AFTERSHOCKS, NORTHWEST TIBETAN PLATEAU Magnitude Depth Longitude Latitude Section plane I Section plane II (Ms) (km) (°E) (°N) (°) (°) Strike Dip Rake Strike Dip Rake 6.5 8.4 37.53 78.17903080282 61 96 4.3 9.5 37.45 78.18340 32 111136 60 77 3.9 8.3 37.57 77.95182 58 31 74 64 144 4.6 8.8 37.48 78.074150–60 17948–121 3.4 6.5 37.57 78.08251 35 –826155–96 3.3 7.1 37.59 78.10192 59 –101 33 33 –72 3.7 10.2 37.47 78.02289 20 –698771–97 3.7 9.4 37.53 77.95295 64 –799128–111 3.3 8.2 37.59 78.04291 37 106915578 4.2 10.0 37.45 78.05107 30 80 2996196 4.0 7.5 37.58 77.83298 34 117866073 3.7 7.7 37.65 77.87260 22 51 12173104 4.5 8.9 37.50 78.04128 27 90 3086390 3.7 9.1 37.48 78.109959–91 28131–88 Note: Ms represents Richter magnitude scale.

aftershocks is similar to the shape of the fold (Fig. 6), and the aftershocks near migrated northward at least ~50 km into the interior of the Tarim Basin (Fig. 1). the surface are normal faulting focal mechanisms (Fig. 3), indicating tensional The foreland fold and thrust belt, including the Pishan anticline, shows evidence stresses near the surface. The aftershock distribution is consistent with the of strong tectonic activity because the late Quaternary sediments and river structural deformation characteristics of a blind thrust fault and fold, which terraces have been faulted (Fig. 4; e.g., Li et al., 2016). The Tekilik fault, which indicates that the Pishan earthquake was a typical folding earthquake event. is a range-front fault zone in western Kunlun, has been inactive since the late Quaternary, indicating the outward growth of the piedmont. In contrast to the thrust movement and crustal shortening in the range front, strike-slip faults ■■ DISCUSSION accommodate the differences in horizontal displacement between the tectonic blocks within the plateau. The focal mechanisms in the interior of the western In contrast to the linear distribution of aftershocks along a steeply dipping Kunlun Range all show obvious strike-slip motion (Fig. 1). Because the India fault (Stein and Yeats, 1989), the aftershocks of the Pishan earthquake were plate downthrusts beneath Tibet and the Tarim Basin forms an obstruction densely distributed over a width of ~20 km with a length-to-width ratio of ~2 in the north, Tibet is laterally extruded due to India’s penetration into Asia (Figs. 2 and 6). The seismic profile (Fig. 3) reveals that the seismogenic fault (Matte et al., 1996). The northward propagation of the western Kunlun results dips at ~15° (Li et al., 2016; Lu et al., 2016), and the coseismic uplift occurred in the southward subduction of the Tarim Basin beneath Tibet (Matte et al., along the entire Pishan anticline (Zhang et al., 2016). The energy of the Pishan 1996). The N-S convergent motion between Tibet and the Tarim Basin gener- earthquake was released over a wider area compared to high dip-angle faults. ated the Ms 6.5 Pishan earthquake. In general, the high-angle strike-slip faults In addition, the shallow depth of only ~8–9 km for the main shock and the ba- within the high range accommodate lateral motion, and the low-angle thrust sin amplification effect increased the earthquake effects. Therefore, although faults in the foreland mainly absorb the crustal shortening, which results in this earthquake was not the largest earthquake in Xinjiang in recent years, it slip partitioning in this region (Fig. 7). The Pamir, which is also located along caused significant destruction. the northwestern margin of the Tibetan Plateau, is dominated by thrusting As the northwestern margin of the Tibetan Plateau, the western Kunlun along the range front and strike-slip motion within the plateau (e.g., Burtman Range has experienced intense tectonic deformation and uplift. Previous stud- and Molnar, 1993; Zubovich et al., 2010). Since the Cenozoic, the Pamir has ies have indicated that the tectonic deformation in the western Kunlun Range penetrated ~300 km northward into Eurasia (e.g., Burtman and Molnar, 1993; piedmont is that of a typical foreland fold and thrust belt in which the tectonic Cowgill, 2010). We propose that the northward penetration of the Pamir can activity and crustal shortening are migrating from the range front toward the be explained by northward fault propagation. basin (e.g., Matte et al., 1996; Zheng et al., 2000; Pan et al., 2007; Liang et al., The seismic reflection profile (Fig. 3) shows that the tectonic deformation 2012; Jiang et al., 2013). The thrust deformation and horizontal shortening have mainly occurs in the Cenozoic layers from depths of ~8–12 km to the surface

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colluvial wedges revealed by our trench across the fault scarp (Figs. 4C, 4D) N also indicate that several large earthquakes have occurred along the foreland structure. The Pishan event only ruptured part of the Pishan anticline, and the aftershock distribution also shows that the energy was mainly released along

tern Kunlun the Pishan anticline. However, the main locked part between the Tekilik fault s

Tarim Basin and the Pishan anticline did not rupture. In addition to the concentrated distri- We 0 bution of aftershocks in the vicinity of the main shock, a group of small shocks Mz Q in the western region, which occurred along the Tekilik fault, was possibly N-E triggered (Fig. 1). A static stress study showed an increase in the Coulomb 10 Pishan Ms 6.5 stress of 471 Pa on the western Kunlun range front (Jin et al., 2017). Based

Depth / km Pre-Cenozoic on the study by Ziv and Rubin (2000), a small change in the Coulomb stress 20 may affect seismicity on active faults. In the piedmont of the western Kunlun, no large earthquake with a magnitude greater than 7 has been recorded in 0 50 100 Distance / km the last 200–300 years. Therefore, this area may have a high seismic risk that should be the focus of further attention and research. Figure 7. Tectonic framework of the western Kunlun, Northwest Tibetan Plateau. The upper crust-scale tectonic interpretation was based on Matte et al. (1996), Li et al. (2001), and Liang et al. (2012). The profile reveals the slip partitioning in the western Kunlun and the northward propagation of the range front. Q—Quaternary; ■■ CONCLUSION N-E—Neogene-Paleogene; Ms represents Richter magnitude scale; Mz—Mesozoic. The Ms 6.5 Pishan earthquake, which occurred on the south-dipping fault ramp of the Pishan blind thrust fault anticline, was a typical folding event with (Liang et al., 2012; Li et al., 2016). Consistent with the tectonic deformation of a focal depth of 8–9 km. This earthquake did not generate an obvious coseis- the piedmont, the Ms 6.5 Pishan earthquake was a typical thrust rupture event mic surface fault. A series of northwest-striking tensile fissures on the crest (Li et al., 2016; Lu et al., 2016; Zhang et al., 2016). Our study also shows that the of the anticline represent the surface deformation caused by this earthquake. main shock occurred at a depth of 8–9 km and that the deformation caused by The tectonic deformation of the Pishan blind thrust anticline is mainly the Pishan earthquake mainly occurred in the Cenozoic layers. In recent years, characterized by layer folding and bending. The normal fault at the top of several moderate to strong thrust earthquakes have occurred along the active the anticline is a bending moment fault that developed because of the local foreland structural belt (Fig. 1), indicating that the foreland fold and thrust belt tensile stress. The deformation caused by this earthquake mainly occurred is the main structure that accommodates the N-S crustal shortening (Jiang et in the Cenozoic sedimentary layers at depths of 3–12 km. The foreland folds al., 2013). A series of bending moment faults on the crest of the Pishan anticline and thrust faults are the main structures that accommodate the N-S crustal indicates that the deformation caused by large earthquakes during the late Qua- shortening across the western Kunlun range front. ternary was dominated by layer folding and surface uplift in the upper crust. Based on a seismic reflection profile, Li et al. (2001) reported average crustal thicknesses of ~60–65 km beneath the western Kunlun. Compared with the ~50 ACKNOWLEDGMENTS km crustal thickness on the southern margin of the Tarim Basin, we estimate This research was supported by the National Science Foundation of China (41672208, 41590861, 15 km of excess crustal thickness beneath a region as wide as ~80 km from the 41661134011) and a fund from the State Key Laboratory of Earthquake Dynamics (LE1413). High-res- Kangxiwar fault to the Tekilik fault. Thickening due to shortening of a 50-km- olution remote sensing images were obtained from Google Earth. This paper commemorates the victims of the Pishan earthquake. thick crust would require ~24 km of crustal shortening, which is consistent with the crustal shortening of 24.6–54 km calculated based on the balanced section (Jiang et al., 2013; Lu et al., 2016). Therefore, we suggest that the tectonic defor- REFERENCES CITED mation in the western Kunlun area mainly occurred in the upper crust and that Avouac, J.P., 2003, Mountain building, erosion, and the seismic cycle in the Nepal Himalaya: Ad- the growth of the plateau is driven by upper crustal shortening and thickening. vances in Geophysics, v. 46, p. 1–80, https://​doi​.org​/10​.1016​/S0065​-2687​(03)46001​-9. Previous studies have shown that the foreland thin-skinned nappe structure Avouac, J.P., Tapponnier, P., Bai, M.X., You, H.C., and Wang, G., 1993, Active faulting and folding in is a seismogenic system that can generate large earthquakes (Avouac et al., the northern Tian Shan and rotation of Tarim relative to Dzungarian and Kazakhstan: Journal 1993; Avouac, 2003; Zhang et al., 1994; Feldl and Bilham, 2006; Liu et al., 2015). of Geophysical Research. Solid Earth, v. 98, p. 6755–6804, https://​doi​.org​/10​.1029​/92JB01963. Brunel, M., Arnaud, N., Tapponnier, P., Pan, Y., and Wang, Y., 1994, Kongur Shan normal fault: Based on the fault-plane area, Li et al. (2016) proposed that the foreland thrust Type example of mountain building assisted by extension (Karakoram Fault, eastern Pamir): system can generate large earthquakes with magnitudes greater than 7. The Geology, v. 22, p. 707–710, https://doi​ .org​ /10​ .1130​ /0091​ -7613​ (1994)022​ <0707:​ KSNFTE>2​ .3​ .CO;2.​

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