Engineering Geology 182 (2014) 88–96

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Engineering Geology

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Characteristics and genetic mechanism of the Cuihua Rock Avalanchetriggered by a paleo-earthquake in northwest China

Yan Lv a, Jianbing Peng a,⁎, Genlong Wang b,⁎a Key Laboratory of Western Mineral Resources and Geological Engineering Ministry, Chang'an University, Xi'an, Shaanxi 710054, PR Chinab Key Laboratory of Geo-hazards in Loess Area, Xi'an Centre of Geological Survey, China Geological Survey, Xi'an, Shaanxi 710054, PR China

⁎ Corresponding authors. Tel./fax: +86 2982339012.E-mail addresses: dicexy_1@126.com (J. Peng), wang2

(G. Wang).

http://dx.doi.org/10.1016/j.enggeo.2014.08.0170013-7952/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Accepted 27 August 2014Available online 2 September 2014

Keywords:EarthquakeDammed lakeRock avalancheRock slopeJoint sets

The Cuihua Rock Avalanche, 30 km south of Xi'an, China, is located within a marvelous geological landscape andwas triggered by an ancient paleo-earthquake. The area is mainly characterized by cliffs, stone seas and adammed lake (Shuiqiu Pool), with a total area of 5 × 105 m2 and a volume of 1.8 × 107 m3. Field investigations,historical records, dating methods and typical seismic profiles indicated that its occurrence could be correlatedwith an earthquake in 780 BC that most likely triggered the landslide. The results also showed that a) the forma-tion of the rock avalanchewas associatedwith high-steep slope created by lifting ofQinlingMountain and cuttingof rock fractures, and b) the high-steep slope and several preferred structural planes play an important role incontrolling the slope instability. Based on conjectures, the failure process of the rock avalanche can be dividedinto four stages: the preliminary stage, the starting-up stage, the accelerating stage, and the accumulating stage.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

As the most important city in Northwest China, Xi'an held theposition of capital city for 13 dynasties over a period of 1120 years. Itwas also the earliest cradle of mankind, and an exchange center forancient western and eastern cultures (Wang, 1990). The CuihuaMountain is located 30 km to the south of Xi'an City, and is famous forrock avalanches caused by paleo-earthquakes (Figure 1). The HeritageSite of the Cuihua Rock Avalanche ranks third in volume in the world(following theUsoi RockAvalanche in Tajikistan and theWaikaremoanaRock Avalanche in New Zealand).

The earliest study of the avalanche can be traced back to a paper byNan and Cui (2000). In it, they reported some characteristics of the rockavalanche in detail, such as rock slope failure in stages and blockmovement with high speeds. After that research, other studies havebeen conducted upon the formation. For example, Wu and Peng(2001), Weidinger et al. (2002), He et al. (2005), Guo (2005), Li et al.(2007), and Lv et al. (2013) studied respectively, the cause, characteris-tics, value and environmental effects of the rock avalanche based onfield investigations from the view of geomorphology. All of this researchhas givenMt. Cuihua great scientific support. However, the results havebeen limited because of the small amount of evidence and dataavailable. In order to provide reasonable and scientific bases for thedevelopment and protection of tourist resources for Mt. Cuihua, the au-thors relied on remote sensing imaging, shallow seismic exploration

006@mail.iggcas.ac.cn

and C14 dating to best define the characteristics related to the geneticmechanism.

2. Characteristics of the Cuihua Rock Avalanche

2.1. Geological setting

The Cuihua Rock Avalanche is in the Dongcha Gully, an easternbranch of Taiyi River. It is within the northern margin of QinlingMoun-tains which is representative of intercontinental orogenesis, magmaticactivities and strong compressional movement during the Mesozoic–Cenozoic. Furthermore, the Mt. Qinling in the south, as well as theWeihe Plain (also known as Guanzhong Plain or Weihe Basin) inthe north, are the two most east–west-striking geomorphic units. TheNorthern Qinling Margin Fault (Figure 2) extends eastward for over310 km from Baoji to Tongguan and is the largest and most intenselyactive fault in this area (Zhang et al., 1995). The formation lithology inthe region of the rock avalanche is mainly composed of migmatiticgranite (Tγ5

1). These intrusive rocks were either generated during thetectonic phase of Y'ngsan or during the phase of Y'ngsi (Zhang et al.,2001). Quaternary deposits (Figure 2) mainly outcrop on the alluvialfan of Mt. Qinling and the alluvial plain of Weihe River.

Tectonic activity in the Weihe Basin is obvious. Since the Oligocene,when the Weihe Basin (i.e., Guanzhong Plain) began to sink and acceptsediment, the activity of the Qinling Piedmont Fault (also known as thenorthern QLmargin fault) along the basin margin had been very strong.Moreover, Zhang et al. (1995) reported that the activity of the QinlingPiedmont Fault is outstanding in the Quaternary, especially sincehuman activity began. The vertical differential movement rate of the

Fig. 1. Site of the Cuihua Rock Avalanche inWeihe Basin. According to historical records, two large earthquakes occurred in 780 BC, and in 1556. Bothwere thought to havemagnitudes oflarger than 8.0. And both epicenters were located in the Weihe Basin, near the Qinling Piedmont Fault.

Fig. 2. Geological map of the Cuihua Rock Avalanche and the surrounding areas. The formation lithology in the region of the rock avalanche is mainly composed of migmatitic granite.Quaternary deposits mainly outcrop on the alluvial fan of Mt. Qinling and the alluvial plain of Weihe River.

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Fig. 3. Remote sensing image of the Cuihua Rock Avalanche and the surrounding areas. The rock avalanche is about 1000 m in length from south to north and approximately 500 m inwidth, with a mean thickness of 50–80 m, and occupies a total area of 1.5 × 105 m2 and contains a total volume of rocks about 1.8 × 107 m3.

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fault reached 1.7–3.4 mm/a in the Holocene (Li et al., 1992; Peng et al.,1992). Meanwhile, a number of active faults are distributed inthe Weihe Basin, and render it a geologically unstable area (Figure 1).According to historical records, two large earthquakes occurred in780 BC, and in 1556. Both were thought to have magnitudes of largerthan 8.0. And both epicenters were located in the Weihe Basin, nearthe Qinling Piedmont Fault (Figure 1). It is believed that the occurrenceof the rock avalanche could be correlatedwith the earthquake of 780 BCthat might be responsible for triggering the landslide (Weidinger et al.,2002).

2.2. Characteristics of the rock avalanche

According to field investigations, the largest fall within the CuihuaAvalanche is about 400 m. Its deposit is about 1000 m in length fromsouth to north and approximately 500 m in width from east to west.

Fig. 4. Panoramic photo of the rock avalanche represented with bluffs (steep back scarps), stavalanche are present in residual peaks and precipices, which are generally 160–220 m high, a

The formation is shaped in the form of a reverse funnel (the front iswider than the rear part) with a mean thickness of 50–80 m, andoccupies a total area of 1.5 × 105 m2 and contains a total volume ofrocks about 1.8 × 107 m3 (Figure 3). The back scarps of the rock ava-lanche are present in residual peaks and precipices, which are generally160–220m high, and reach a dip of 60–80° (Figure 4). The deposit bodystrikes east to west, and forms a barrier lake (Shuiqiu Pool) which hasan irregular rectangular area of 1.2 × 105 m2, with a length of 700 m, awidth of 90–320m and amean depth of 7m (the deepest is up to 11m).

This lake, Shuiqiu Pool, is the only existing natural mountain lake inthe north slope of Mt. Qinling. The deposit body mainly consists ofgranite blocks with varied sizes and shapes cut by joints and fissures.The blocks are angular and irregular (Figure 4), and most of themmea-sure 5 to 30 m, some of them even 70 to 100 m. Some blocks werescattered on the gentle slope of the Cuihua Mountain, and others fellover the low-lying lands. As a result, the slope surface is rugged and

one seas (deposit of the rock avalanche) and huge boulders. The back scarps of the rocknd reach a dip of 60–80°.

Fig. 5. Statistics of different-sized blocks in the area of the rock avalanche. The statisticsindicated that almost half of the blocks range from 1 to 5 m in diameter. The percentagesof blocks from 5 to 10 m, 10 to 20 m and 20 to 50 m account for 27.8%, 14.8% and 6.8%respectively.

Fig. 7. Rosette diagrams illustrating orientation frequency distribution of huge blockswithin the region of the rock avalanche. The direction of the blocks' macroaxis isconcentrated in 0–30° and 50–70°.

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rough (Figure 4). In summary, the Cuihua Rock Avalanche is mainlyrepresented with cliffs (steep back scarp), stone seas (deposit of therock avalanche) and a mountain lake (Figure 3).

Furthermore, when the number of different-sized blocks in the re-gion of the avalanche was counted, the results indicated that almosthalf of the blocks range from 1 to 5 m in diameter. The percentages ofblocks from 5 to 10 m, 10 to 20 m and 20 to 50 m account for 27.8%,14.8% and 6.8% respectively (Figure 5). Statistically, a large number ofboulders with different sizes were separated from their original rock,and were cut again along joints and fissures (Figure 6a, b, c and d) inorder to achieve that regularity.

Due to great potential energy (more than several hundredmeters ofheight difference), the fallen blocks that were triggered by the earth-quake rolled, bounced and collided, causing new cracks in the bouldersin the process. Especially in the center of the deposit, many of these

Fig. 6. Typical photos of huge blocks (a, b, c, d: cracked rocks); e, f: chaotic block arrangementoriginal rock, and were cut again along joints and fissures in order to achieve that regularity.

large boulders lie distributed. Weidinger et al. (2002) reported thatthe largest blocks reached dimensions of 80–100 m in diameter(Figure 6a). Field investigations have confirmed that the large, crackedrocks are very common and spectacular (Figure 6).

The sizes of these stones are various with the edges and corners ob-vious. Most of these huge boulders were chipped away or dislodgedfrom previous structural planes such as joints and fissures that, alongwith overlaidmassive stones, accumulation andmutual support formedthe interconnected underground spaces (Figure 6d, e and f), that havebecome the famous geological relics such as Wind Cave (Figure 6e),Ice Cave, Bat Cave, Stone Bridge, Stone Peak and Drunk Stone. Theloose structures of blocks formed the barrier dam and the lake, ShuiqiuPool, and these structures have avoided a dam break in the past twothousand years.

Apart from the above mentioned characteristics of mutual supportfor the deposit, we also speculated that the chaotic block arrangementmore or less can retain and reflect some kinematic and dynamic

). Statistically, a large number of boulders with different sizes were separated from their

Fig. 8. Near east–west shallow seismic profile and geological explanation for the rock avalanche. The maximum thickness corresponding to the western part of the profile is about 20 m,and the eastern part is a little thicker with a maximum thickness of about 40 m.

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information. Generally speaking, the macroaxis' direction of the blocksin the avalanche's region suggests the bearing of a landslide. Therefore,we investigated hundreds of blocks in the avalanche zone. The resultindicated that the direction of the blocks' macroaxis is concentrated in

Fig. 9.Near south–north shallow seismic profile and geological explanation for the rock avalanclittle deeper than the southern part.

0–30° and 50–70° (Figure 7). In other words, the vast majority of blocksare directionally arranged approximately along a northeast sliding path.Most of the blocks' major axis' direction appears to be northeast. It alsorepresents the main direction of the avalanche (Figure 3).

he. The profile which is along the west side of the lake indicates that the northern part is a

Fig. 10. Primary joints with dip angle of 65–80°. They occur as a result of multi-periodicand complicated fault activities in Qinlin Mountains, and are well developed in the rockmasses.

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To uncover the deposit thickness of the avalanche, a high-precisionshallow seismic prospecting method (corresponding to the surveylines of I–I′ and II–II′ in Figure 3) worked well. The profile I–I′(Figure 8) shows that the bottom boundary of the avalanche clearlyreflects some characteristics of the thickness. As shown in Fig. 8, themaximum thickness corresponding to the western part of the profileis about 20 m, and the eastern part is a little thicker with a maximum

Fig. 11. The main joint sets cut the granite, and formed the steep back scarps. The J1 dips to

thickness of about 40m. The thickness of themiddle part varies greatly,with the shallowest thickness measuring only about 16 m. The profileII–II′ (Figure 9), which is along the west side of the lake, indicates thatthe northern part is a little deeper than the southern part. Accordingto field works and seismic profiles, we can infer that the northern endof the avalanche is deeper in thickness, with an average thickness ofabout 80–100 m; and the southern end is thinner, only with an averagethickness of about 10–30 m. The maximum thickness of the deposit isup to 100–120 m corresponding to the location of the barrier dam.

3. Mechanism of the Cuihua Rock Avalanche

3.1. Correlation between the avalanche and joint sets

As has been shown in previous research, if the rockmass is hard andintact, landslides are not easily triggered under seismic loading. Howev-er, joints have a strong impact on rock slope stability and some structur-al planes (i.e., joints) often play an important role in controlling slopeinstability. Adamellite intrusion (hard granite of Indo-Chinese epoch),which has a variable degree of magnetization, can be found at the bot-tom of the fallen accumulation. The twomain joint sets occur as a resultof multi-periodic and complicated fault activities in Qinling Mountains,and are well developed in the rock masses. One set trends to nearlywest–east and dips to the north, and the other trends to nearlysouth–north and dips to the east. Both of them have a high dip angleof 65–80° (Figure 10). As a result, the rock masses in Mt. Cuihua weredivided into several parts by these steep dip joints, and in fact theymade an essential preparation for rock falls.

As mentioned above, the Mt. Cuihua is composed of migmatiticgranite. Field investigations indicated that several sets of jointswere de-veloped in the rock masses. Among them, three main joint sets formedby multi-periodic and complicated fault activities in the Qinling Moun-tains. These have beenwidely developed in the granites. The threemainjoint sets are J1, J2 and J3 in Fig. 11. The J1 dips to 60–80° with an angleof 50–75°, and it is almost parallel to the slope of Cuihua Peak(Figure 11). Therefore, J1 played a very important role in the CuihuaAvalanche. The J2 dips to 250–270° and a dip angle 50–70°, and it isalmost perpendicular to the slope. Therefore, J2 played the role of thelateral cutting plane. The J3 dips to 160–175° with an angle of70°–85°, and it also has a significant effect on the slope stability.

60–80° with an angle of 50–75°, and it is almost parallel to the slope of Cuihua Peak.

Fig. 12.Avalancheswith zonal distribution are east–west strike, and basically parallel to theQinling Piedmont Fault in a buffer zone of 3–8 km. These rock avalanches are all located on thefootwall of the northern Qinling Margin Fault, and are related to this fault.

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3.2. Correlation between the avalanche and the earthquake

Apart from the Cuihua Rock Avalanche, several rock avalanches havebeen found in the region fromTaiping Gully to DaGully (a zone of about38 km along the north slope ofMt. Qinling). These zonal distribution av-alanches run on an east–west strike, and continue parallel to the Qinling

Fig. 13.Another typical rock avalanche (Shinaogou RockAvalanche),which is 15 kmwest awayhigh, 10 m wide and 11 m long.

Piedmont Fault in a buffer zone of 3–8 km (Figure 12). Field investiga-tions also indicate that these rock avalanches are all located on thefootwall of the northern Qinling Margin Fault, and are related to thisfault. Among them, Shinaogou Gully, Baolong Gully and Xisi Gullyhave large-scale rock avalanches. Avalanches in Big Gully, Small Gully,Taiping Gully, Zige Gully and Feng Gully are very common (Figure 12).

from the TaiyiyuGully, is similar to theCuihuaRock Avalanche. The biggest blocks are 12m

Fig. 14. A paleo-seismological section in Zige Gully near the Qinling Piedmont Fault. It has indicated that the young pluvial and alluvial sediments (6300 years agomeasured by means ofC14 dating) were cut and caused about 80 cm displacement by the fault.

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Shinaogou Rock Avalanche has another typical rock avalancheformation, 15 km west away from the Taiyiyu Gully. Also, Shinaogoupossesses the largest fall of these formations, about 350 m, as well asthe deposit with the largest diameter of about 1200 m in length fromsouth to north and approximately 200 m in width from east to west,containing a total volume of rocks of about 3 × 106 m3 (Figure 13).The biggest blocks are 12 m high, 10 m wide and 11 m long.

Based upon zonal distribution of features and the mass resistance ofavalanches, those avalanches in eachof those areaswere likely triggeredby the same geological event. But onlyMt. Cuihua formed barrier lake aswell as art stones, and the rockslide's appearance has been nearlypreserved, making it a precious wonder of Mt. Qinling.

Recently, neotectonic structures within the region have been gener-ated and activated by a newearthquake. The Cuihua RockAvalanche hasrecorded the faulting and earthquake activities in the Quaternaryand reflected the continuous Qinling orogenic movement since theCenozoic. Recently, additional evidence came to light to demonstratethe correlation between the avalanche and the paleo-earthquake. Asshown in Fig. 14, a small-scale fault cutting through Holocene strata(mixed pottery pieces) has been found around the Qinling PiedmontFault. It has indicated that the young pluvial and alluvial sediments(6300 years ago measured by means of C14 dating) were cut andcaused about 80 cm displacement by the fault.

In addition, a typical palaeo-seismological section has been found atLaoyu Gully near the Qinling Piedmont Fault. It can be seen in Fig. 15that the fault dislocated theHolocene system (youngpluvial and alluvial

Fig. 15.A paleo-seismological section at Laoyu Valley near theQinling Piedmont Fault. Thefault dislocated the Holocene system (young pluvial and alluvial sediments). According tofield investigations, the foot wall was found to be composed of Holocene sands andgravels, and the hanging wall of the fault was formed by the late Paleozoic Mylonite.

sediments). According to field investigations, the foot wall was found tobe composed of Holocene sands and gravels, and the hangingwall of thefault was formed by the late Paleozoic Mylonite. These signs of faultactivity may be associated with landslide events.

According to “Chinese History”, Emperor Wudi of the Han Dynastybuilt a palace near the lake in 109 BC, to offer a sacrifice to Taiyi God(a man in a Chinese mythological tale). Hence, the lake has been inthis location at least since 2120a. Xie and Xiao (1991) reported that aquantity of Xanthoria elegans (commonly known as the elegantsunburst lichen, and easily recognized by its bright orange or redpigmentation) was found growing on the rocks of the avalancheformation's back scarp; and lichen dating shows that the age of the av-alanche is about 3000a. Wu and Peng (2001) collected a group of soilsamples in the landslide body of the avalanche, and inferred that the av-alanche occurred before 2400a by means of C14 dating.

The region's most important earthquake, which probably triggeredthe rock avalanche, occurred in 780 BC. According to historical records,there was a great earthquake that occurred at the second year (780 BC)of the You King's period of West Zhou Dynasty in westernWeihe Basin.And in that year, the earthquake resulted in the Jinghe River and theWeihe River dried up and Qishan Plateau collapsed. Based on theabove analysis, we presumed that the avalanche should have occurredin 780 BC and would have been triggered by a large earthquakewith magnitudes of 8 or more, such as one strong enough to collapse aplateau.

3.3. Failure process of the rock avalanche

Fig. 16 is the conjectural failure process of the Cuihua Rock Ava-lanche, and the total process can be divided into four stages. The firststage is the preliminary stage in which it was characterized by strongactivity of the Qinling Piedmont Fault, rapid incision of the Taiyi Valleyand rapid uplift of the mountains on both sides. During the process,the granites generated a lot of joints and fissures. Among of them, thejoint set with a dip direction of 60–80° and an angle of 50–75° is criticalto hillside stability. The second stage is the starting-up stage in which itwas characterized by stress concentration along the structural planes(joints and fissures) under earthquake loading. During the process, thestructural planes generated plastic deformation and were pulled apartgradually. The third stage is the accelerating stage in which a giant ofblocky stream of rocks rolled and bounced for a long distance fromSWW to NEE due to tremendous kinetic energy. During the process,these big rocks with high speed collided with each other and began todisintegrate. The final stage is the accumulating stage in which thevelocities of these blocks decreased gradually after the earthquake. Atlast, the numerous blocks formed the landscapes of stone sea, andthey blocked up the river channel and formed a dammed lake.

Fig. 16. Failure process of the Cuihua Rock Avalanche. a) In the preliminary stage, the granites generated a lot of joints and fissures because of the differential uplift and subsidence in thearea along the Qinling Piedmont Fault. b) In the starting-up stage, the structural planes generated plastic deformation andwere pulled apart gradually under earthquake loading. c) In theaccumulating stage, the numerous blocks formed the landscapes of stone sea, and they blocked up the river channel and formed a dammed lake.

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4. Conclusions

Field investigations, historical records, dating methods and typicalseismic profiles have proved that the rock avalanche was anearthquake-induced landslide. From the evidence, the age of theavalanche's occurrence would be ancient, and might be related to the780 BC, earthquake. As a well-preserved geological hazard, the rockavalanche is also a beautiful natural park. The rock avalanchewasmain-ly represented with cliffs, stone seas, barrier dam, and a lake, occupyinga total area of 5 × 105 m2 and containing a total volume of rocks about1.8 × 107 m3. The formation of the rock avalanche was associated withhigh-steep slope created by lifting of Qinling Mountain and cutting ofrock fractures. The high-steep slope and several preferred structuralplanes (i.e., joints) play an important role in controlling the slope insta-bility. Based on field investigations and conjectures, the failure processof the rock avalanche can be divided into four stages: a) the preliminarystage, b) the starting-up stage, c) the accelerating stage, and d) theaccumulating stage.

Acknowledgments

The authors acknowledge the financial support from the NationalBasic Research Program of China (973 Program) (grant no.2014CB744702), the Key Program of National Natural Science Foun-dation of China (grant no. 41130753), and the Opening Fund of KeyLaboratory of Western Mineral Resources and Geological Engineeringof Ministry of Education (Chang'an University) (no. 2013G1502008).

The author would like to thank Professor X.J. Feng, X.S. Xie, and S.R. Sufor their valuable comments on an earlier draft of this paper.

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