Formation of a multi-translational reactivated ancient landslide in the Three Gorges 1 Reservoir , China 2

12 The fluctuation of water levels and seasonal rainfall in a reservoir may induce various types of slope 13 movements. Some of these movements are new, whereas others are old but reactivated. The primary 14 aim of this study is to investigate the formation mechanism and process, deposit characteristics, and 15 identification signs of a giant multi-translational reactivated ancient landslide in the Three Gorges 16 Reservoir region based on field observations, on-site surveys, and Electron Spin Resonance 17 experiments. The Outang landslide, located at the south bank of the Yangtze River, has a total 18 volume of approximately 90 million m 3 and can been divided into three independent subzones with 19 an apparent age of 120–130 ka (ka represents a thousand years) for subzone O1, 65–68 ka for 20 subzone O2, and 47–49 ka for subzone O3. The features of mobilized material structure and slip 21 surface morphology in each subzone are similar and are in the form of a spoon. A conceptual model, 22 including sliding, bending, suspending, and accumulating, is deduced to explain the formation 23 mechanism and evolutionary process of this instability. Three types of evidences are proposed to 24 recognize the ancient landslide. Currently, landslide stability is obscure based on the significant 25 landslide movement and reactivated features; more attention and long-term monitoring is necessary 26 in the future. 27


Introduction
The landslide, defined as a wide variety of processes that result in the downward and outward movements of slope-forming materials (Varnes, 1978), is a typical and destructive geo-hazard (Liu et al., 2016;Huang andZhu 2017；Sä ttele et al., 2017;Jacobs et al., 2018).Typically, slope instability can be classified into the occurrence of new landslides and the reactivation of ancient landslides, both of which are extremely prevalent in reservoir areas and may create landslide dams, bury residential houses, and pose a significant threat to the natural environment (Gutié rrez et al., 2015;Gu and Huang, 2016;Huang et al., 2018).A well-known reservoir slope instability event is the 1963 Va jont landslide in Italy, where more than 2,000 deaths were reported and several villages were destroyed.Since then, reservoir-induced landside problems have received particular attention from engineers and geologists (Barla, 2013;Mantovani and Vitafinzi, 2003;Wolter et al., 2016).
For flood control and hydropower generation, one of the largest civil engineering projects in human history-the Three Gorges Water Conservation and Hydropower Project in China-was constructed and completed in 2008.Subsequently, the reservoir water level exhibited a cyclical fluctuation of 30 m (145-175 m) under normal operating conditions, and resulted in a 660-km long and 1.2-km wide (on average) hydro-fluctuation belt in the Three Gorges Reservoir (TGR) region extending from Yichang City to the Chongqing Municipality, China (Fig. 1a).Owing to the complex geological environment, climate condition, and reservoir operation, the TGR region has developed into a landslide-prone area (Wang et al., 2016), where more than 5000 reservoir-induced landslides have been identified since the first trial impoundment in 2003 (Jian et al., 2009, Yin et al., 2016, Miao et al., 2014).Studies regarding the trigger mechanisms and factors, deformation characteristics, and motion modes of reservoir-triggered landslides have been published widely (Sun et al., 2016b;Jian et al., 2009;Hu et al., 2015;Chen et al., 2015).Among the reservoir landslides, problems concerning reactivated ancient landslides including reasons for landslide reactivation, risk of a fast sliding, and available emergency civil protection actions have received significant attention, and were investigated once the old instability was identified, e.g., the Quchi landslide (Gu et al., 2017), Huangtupo landslide (Wang et al., 2016), Anlesi landslide (Jian et al., 2009), and Zhujiading landslide (Hu et al., 2015).
However, the clarification of the reservoir ancient landslide formation mechanism, evolution process, and identification evidence are difficult and challenging because such instabilities vary in size, type, disaster-pregnant setting, and formation age (Zhao et al., 2016).In fact, many reservoir-induced old landslides were formed thousands of years ago and have been subjected to long-term reconstructions or sediment covers; further, the original landslide geomorphic features were blurred and modified (Deng et al., 2017;Zhao et al., 2016;Zhang et al., 2018).Such landslides have hardly been identified owing to significant changes in the original features and failure topography.Many researchers have studied the formation and development characteristics based on geological processes (Cruden and Varnes, 1996), the evolution of river valleys (Xu et al., 2008;Yin et al., 2010;Zhao et al., 2007), paleoclimatology (Xie et al., 1992;Yin et al., 2010).
However, the understanding and cognition on ancient landsides are rather superficial because of the concealment of ancient landslides, the complexities of their formation mechanism, and the limitations of research methods (Zhang et al., 2018).
The objective of this study is to investigate the formation mechanism and multi-translational process of the Outang ancient landslide through a combined analysis of field investigation, geological exploration, and Electron Spin Resonance (ESR) experiments.We anticipate that this study will provide some important insights into the deposit characteristics of the old instability, as well as the corresponding identification signs.

Regional geological background
The Outang landslide (Fig. 1c), a giant reactivated ancient landslide, is located in Anping town of Fengjie County, Chongqing Municipality, China, and is approximately 177 km upstream of the Three Gorges Dam (Fig. 1a).This landslide area belongs to a secondary tectonic unit of the upper Yangtze platform, which is situated on the intersection between the Daba Mountain bow-like folding belt and the east Sichuan folding belt.The strike of geological structures in the study area (marked by rectangle in Fig. 1a and mapped in Fig. 1b) is dominated by the NE-SW direction.The engineering geology map of the Outang landslide is shown in Fig. 2a.The climate in the study area belongs to the monsoon of the subtropical moist climate zone with recognizable four seasons.The average annual air temperature and rainfall are 16.3 °C and 1147.9 mm, respectively.
Rainfall is always concentrated in June to September annually and accounts for approximately 70% of the total annual rainfall in this period.

Spatial geomorphological features
The Outang Landslide, characterized by a reclining bell-shaped surface topography with a maximum length of 1.8 km and a thickness of about 50.8 m, is located at the south bank of the Yangtze River and approximately 12 km upstream away from the Fengjie urban area (Fig. 1b).The instability deposit covers an area of 1.78 km 2 with an estimated volume of 9.0×10 7 m 3 .It extends from the front elevation of 90-102 m above sea level (a.s.l.) to the crown elevation of 705 m a.s.l.along a 75-80° direction from the flow direction of the Yangtze River.The primary slide direction is about 345°.After a careful observation, a chair-like geometry, characterized by a flat and broad terrain at the front section but steep terrain at the middle and upper sections, occurred repeatedly from the water level to the rear part of the landslide in spatial morphology.As illustrated in Fig. 3a, a slope gradient of 5-10° appears and distributes at the elevation ranging from 160 m a.s.l. to 220 m a.s.l., where Anping town is located and the Fengjie-Anping road runs through; subsequently, the steep slope, extending to about 320 m a.s.l., occurs with the slope gradient of 20-35° (occasionally up to 50°).Another flat area (Fig. 3c), located at the elevation of roughly 330 m a.s.l., and a cliff (Fig. 3b, the frontal boundary of subzone O2 and mentioned in the chapter 4.2), were recognized.Similarly, at the altitude of approximately 428-450 m a.s.l., a flat area with a slope gradient of 2-10° (Fig. 3e) and a cliff (Fig. 3d, the frontal boundary of subzone O3 and mentioned in the chaptered 4.3) were also identified.The spatial geomorphological features imply that the Outang landslide might be composed of multiple landslides.

Lithostratigraphy survey
The lithostratigraphy of the landslide was studied through geological explorations and field observations.The mobilized materials can been divided into two layers: shallow colluvium in the top and fractured sandstone in the deep: 1) The top layer is a mixture of clayey soil and rock blocks (approximately 25-66% volume content, sandstone, and siltstone) of sizes of 1-40 cm (Fig. 4a).Its thickness (0.2-35 m) is increased gradually toward the toe.2) Deep in the landslide body, the primary material is a fractured sandstone layer (Fig. 4b) with varying thicknesses between 10 m and 95 m (occasionally exceeding 110 m at the toe) and is cut intensely by two sets of fissures, whose orientations follow 120-150°/55-75° (dip/dip angle) and 40-70°/60-85°.Trench and adit explorations disclosed that dark gray claystone and clayed soil (coal and shale can also been found occasionally) are predominant at three weak interlayers (WIs) with a thickness of 5-20 cm (Fig. 4c).The bedrock at the attitude of 335-350°/18-24° (dip/dip angle) are constituted by sublitharenite of the Xujiahe formation in the Upper Triassic system (T 3xj ) and fine sandstones of the Zhenzhuchong formation in the Lower Jurassic system (J 1z ), with the former overlain by the latter (Fig. 3a).As shown in Fig. 4d, bedding planes that indwell in the outcrops of the bedrock (sandstone) at the rear part develop a downslope with the attitude of 336°/18°.

Weak interlayers
The presence of a shear plane is often assumed as evidence of a slip surface (Hutchinson and Bhandari, 1971;Corominas et al., 2005).In-site investigation found three WIs (numbered WI1, WI2, and WI3, and marked in Fig. 5a).The slip surfaces existing in the WIs were discovered with clear striated polished surfaces by adit (WI1 in Fig. 5b) and trench explorations (WI3 in Fig. 5c) .
The main mineral materials in the WIs are quartz and clay minerals with the average contents of more than 35.2% and 44.7%, respectively.Additionally, the clay minerals, composed of montmorillonite (up to 74%), illite (15%-31%), and kaolinite (5%-11%) are characterized by high swelling, softening potential, and low permeability.To clarify the apparent age of the Outang landslide, the ESR experiment was conducted and eight samples near the slip surface were prepared using a drilling hole, trench, and adit explorations (marked in Fig. 2a).Information on the eight samples is presented in Table 1.The ESR results indicate (in Fig. 2a) that the apparent age of the Outang landslide can been subdivided into 120-130 ka for the low part, 65-68 ka for the middle part, and 47-49 ka for the upper part, thus proving that the ancient instability could be composed of several independent landslides.

Activity signs
Since the first trial impoundment, the instability activity features have became increasingly obvious.At the low part, most of its volume is submerged by the reservoir water owing to its low-flat terrain and local collapse, and failures were observed in the reservoir water fluctuation zone frequently.Figure 6b  some falling sandstone blocks observed on the slope surface.The movement has also resulted in some damages in the Anping town houses (Fig. 6c).At the middle part, a long tension crack of length 74.5 m, width 0.1-75 cm, and visible depth 10-110 cm was observed at an altitude between 350 m and 370 m a.s.l.(Fig. 6d).Meanwhile, the dislocation and cracking of the road at a few places were identified.For example, Fig. 6e shows a typical dislocation with the maximum dislocation of 0.5 m.At the upper part, many signs of reactivation were also exhibited.A fallen telegraph pole was found as a consequence of the continuous surficial movement (Fig. 6f); the newly installed telegraph pole was inclined downslope at an angle of 12° (Fig. 6f).The upslope boundary of the instability is defined by a scarp, where WI3 was exposed (Fig. 6g).

Accumulation characteristics of active parts
In terms of the spatial geomorphology features, ESR results, and slip surface morphology, the Outang ancient landslide can be divided into three independent reactivated subzones (labeled subzones O1, O2, and O3).Generally, the component of the mobilized material in each subzone is similar with that of the top layer is shallow colluvium material (clayey soil and rock blocks), and the deep layer is fractured sandstone.The fractured sandstone structure is spoon like, characterized by an orientation of stratified or stratoid bedding planes (335-350°/18-24°) from the rear to mid-fore part; it changes to nearly horizontal, and curves upward at the toe area (155-170°/0-15°) in each subzone.For example, the orientation of the bedding planes within the fractured sandstone is 340°/21° (Fig. 7b), but changes to 162°/15° (Fig. 4b).Moreover, the variation rule of the fractured sandstone structure in each subzone is similar to its respective slip surface morphology.

Subzone O1
Located at the low part of the landslide, subzone O1 has a reclining bell-shaped surface geometry with a primary slip direction of roughly 345° (Fig. 2a).The elevation of the frontal part of subzone O1 is approximately 90-102 m a.s.l.(submerged by water completely); the crown elevation is 300-370 m a.s.l. and is covered partly by subzone O2.It has a maximum length of 880 m, a width of 1100 m, and an average thickness of 70.3 m.This zone has an area of 9.22×10 5 m 2 and a volume of 6.48×10 7 m 3 .As mapped in Fig. 7a, the shallow colluvium material has an uneven thickness of 10 to 35 m; the layer of fractured sandstone exhibits an average thickness of 62 m.Two local strong deformation areas distributed at both sides of the toe of O1 (mapped in Fig. 2a clarified with a total volume of 4.1×10 6 m 3 .Slip surfaces with clear sliding traces for the two local strong deformation areas were also revealed by geological exploration (Fig. 7c-d).Moreover, adit exploration revealed that the slip surface of this subzone existed in WI1, where the main components were claystone (size of 0.5-6 cm) and clayed soil (content of 60-80%), whose colors are dark gray and gray, respectively (Fig. 4c).

Subzone O2
Subzone O2 is located at the middle of the landslide.It extends from 250-300 m at its toe to 400-530 m a.s.l. at its rear section, where it is wrapped partly by subzone O3.This zone has a length of approximately 440 m, width of 650 m, area of 3.16×10 5 m 2 , and volume of 1.02×10 4 m 3 .
As illustrated in Fig. 8a, the thickness of the deposit material in this subzone is approximately 32 m, and the thickness of the shallow colluvium is less than 6 m.The slip surface of subzone O2 also exists in WI1; it was revealed by a drill hole (numbered 5 in Table 1) of thickness 3-15 cm and was dark gray.The field investigation shows a cliff with a vertical dislocation of 8.5 m, and the exposed fractured sandstone is the frontal boundary of this subzone (bounded in Fig. 3a and illustrated in Fig. 3b); the lateral boundary is a ridge in the west (Fig. 1c and Fig. 3c) and a gully in the east (Fig. 1c).Owing to the blockage of the rear part of subzone O1, the flat area appears at the mid-fore part of subzone O2 (marked in Fig. 3a and displayed in Fig. 3c).The ESR experiment shows that the apparent age of subzones O1 and O2 are 120-130 ka and 65-68 ka, respectively, indicating that both subzones occurred at different times; specifically, subzone O2 occurred later than subzone O1.

Subzone O3
Subzone O3 was seated at the upper part of the Outang landslide with a length of 0.64 km, width of 0.83 km, and an average thickness of 27.2 m that increased downslope.It extends from 400-530 m a.s.l. to 705 m a.s.l., with an entire planar area of 0.54 km 2 and a volume of 1.45×10 7 m 3 .The shallow colluvium is extremely thin (less than 1.2 m, as shown in Fig. 7a).Cliff daylighting with broken rock mass and bloating with many shear and tension-shear cracks was found at an altitude of roughly 408 m a.s.l., which is the frontal boundary of this subzone (labeled in Fig. 3a and illustrated in Fig. 3d).The flat area located at the mid-fore part of subzone O3 was also recognized (marked in Fig. 3a and shown in Fig. 3e).Trench exploration disclosed that the (Fig. 4c).Moreover, the rear part of this subzone is bounded by the scarp of the daylighting of WI3 (Fig. 6g).The ESR experiment indicates that the apparent age of subzone O3 is the latest compared to that of subzones O2 and O1.

Mechanism and process
Based on the investigation and analysis of the basic characteristics of the Outang landslide, and in conjunction with the information about the engineering geology and ESR experiments, a conceptual model for understanding the formation mechanism and multistage sliding process of this instability is deduced and shown in Fig. 8.The detailed descriptions of these processes are as follows: 1.At the stage of Early Pleistocene, the whole TGR has suffered from intermittent uplift, tilt, and continuous action of intensive river incision (the fluvial incision rate up to 92.5 cm/ka between the river section of Chongqing and Fengjie) as he effect of the Himalayas' movement, causing the appearance of steep-sided valleys that provided space for the slope movement (Fig. 8a-b).
2. A weak interlayer (WI1), and some tectonic and weathering cracks within the rock mass occurred.During heavy rainfall, rainwater could penetrate the rock mass along the cracks and converge to the WI1.On one hand, the WI1 would suffer from the action of softening or argillation, and then slip or squeeze out to the free surface (Zhao et al., 2012).On the other hand, hydrostatic pressure caused by water flow could induce a larger uplift pressure (Zhao et al., 2015).Both factors contribute to the overlying rock stratum creep-slide along WI1 and result in tensile cracks (Fig. 8b).Because WI1 is not exposed at the lowest part of the slope, where the sliding resistance mass is strong, stress will be concentrated at the toe area of the slope because of the slope creep deformation, thus resulting in a slight bending and upward uplift of the rock stratum at this part (bounded by rectangle in Fig. 8b).tensile crack and extending it toward WI1 (Fig. 8c).Additionally, the fracture and flexure upward of the rock mass become increasingly acute at the toe, accompanied by numerous fissures and relaxed rock mass (bounded by rectangle in Fig. 8c).The orientation of the fractured sandstone next to the toe area (Fig. 4b) of subzone O1 is the best evidence to justify the processes of bend and fracture of the rock stratum.These processes further cause rainwater to seep into the slope easily and generate a potential curved shear surface with a tendency to join with the WI1 gradually (Fig. 8c).
4. As time progressed and owing to the infiltration of rainwater, the tensile structural plane and potential curved shear surface became gradually connected with WI1.Approximately 120-130 ka ago, as induced by heavy rain and other factors, a translational landslide (subzone O1) occurred along the shear rupture surface (slip surface) (Fig. 8d).Thus, the formation mechanism of subzone O1 can be summarized as slide-bending.During the sliding process, under the influence of the reservoir buttress effect (circled in Fig. 8d) (Paronuzzi et al., 2013) and slip surface morphology (Fletcher et al., 2002;Sun et al., 2016), the moved subzone O1 stopped progressively. 5.After the generation of subzone O1, a new free surface (Fig. 8d) occurred at the front of the rock stratum at the middle part of the slope consequent, where WI1 is exposed.The mechanical property of WI1 would further deteriorate under the adverse effect of water; moreover, the rock stratum at the middle part exhibits a large tendency to move along WI1 on account of the long-term gravitational deformation (Deng et al., 2017).Approximately 65-68 ka ago, another translational landslide (subzone O2) was induced along WI1 (Fig. 8e).Therefore, the formation mechanism of subzone O2 can be summarized as a planar slide.
Owing to the blockage of subzone O1, the moving subzone O2 gradually stopped with the rear area of subzone O1 being covered by subzone O2.In this process, part of the moving rock mass at the front part of subzone O2 is crushed, and accumulates along the slope surface of subzone O1, whereas most parts of the rock stratum remain on the slope and/or hang in air (circled in Fig. 8e).Moreover, controlled by the slip surface morphology and hindrance of the rear part of subzone O1, the dip of the rock stratum at the front part of subzone O2 is typically opposite to that of the slope, and the flat topographical area can been recognized (Fig. 3c).
6.The phenomena of unloading, rebound, and attenuated sliding resistance mass at the toe of the upper part of the slope are inevitable after the emergence of subzone O2; they are in favor of the creep deformation of the upper part of the slope above WI3 and give rise to the bending and upward uplift of the rock stratum at the toe of this part (bounded in Fig. 8e).When the possible rupture surface was created and connected with WI3 gradually, as induced by heavy rain and other factors, a landslide (subzone O3) occurred (47-49 ka ago (Fig. 8f)).The formation process of subzone O3 is similar to that of subzone O1; the formation mechanism can be classified as slide-bending.Moreover, for the reason of the blockage of subzone O2, the moving subzone O3 gradually stopped with the rear area of subzone 2 covered by subzone O3.The structure of the rock stratum and its morphological features are also similar to those of subzone O2, as well as a flat topography area can also be found (Fig. 3e).
For subzone O1, after the landslide, a relatively stable period characterized by a thicker top layer formed, and the morphology and location of the frontal boundary changed due to the humid and rainy climate (Fig. 2a and/or Fig. 6a).For subzones O2 and O3, the rock stratum that is pushed out is suspended without support at its bottom (Figs.8e-f).Owing to its own gravity and the weather, local cracking and the collapse failure of rock mass occurred; this may explain the occurrences of cliffs with large dip angles and many fissures at the front parts of subzones O2 and O3 (labeled in Fig. 2a and shown in Fig. 2b and Fig. 2b, respectively).Meanwhile, the failed mass would move downslope and accumulate on the slope surface; this may have caused the thickness of the top layer for each subzone to follow the order of O1＞O2＞O3.Another reason may be the apparent age; this implies that the earlier the subzone occurs, the thicker is the top layer.For the Outang landside, the earliest is subzone O1, followed by subzone O2 and then the latest is the subzone O1.Overall, with the evolution of the Yangtze River, the long-term geologic force has evidently changed the features of the original slope.Hence, the Outang landslide is an ancient landslide that has experienced the long process of sliding, bending, accumulating, and remolding.
Since the occurrence of the old landslide, it has undergone a long-term geological transformation (human activities, weathering, erosion, etc.).Consequently, many original landslide characteristics have vanished, and the original topography and geomorphology have varied, thus causing significant difficulties in landslide identification (Zhao et al., 2015).However, in the specific case of the Outang ancient landslide, some evidences remain that can be used to identify such an old instability.

Mobilized material structure characteristics and slip surface
The landslide is located at the southeast wing of the Guling syncline, where the orientation of the bedrock is 335-350°/18-24°.The dip in the bedding planes within the mobilized materials is the same as that of the bedrock from the rear to the mid-fore (335-350° in Fig. 7b), but opposite to that of the bedrock at the toe of each subzone (155-170° in Fig. 4b).This implies that the attitude of the mobilized material is variable, as characterized by the dip angle decreased nearly horizontal and curved upward at the toe of each subzone, which is analogous to its respective slip surface.
The slip surface, particularly of the striated polished surface, interpreted as a result of relative displacements among the displaced materials and the bedrock, was revealed by geological survey (Fig. 5b-c) with the orientation similar to that of the mobilized material.Thus, the material structure characteristic and rupture surface are strong and clear evidences for identifying the ancient landslide.

Landform characteristics
As previously mentioned, the Outang landslide has experienced the long process of remodeling with a steep topography (varies from 20° to 45°) under the elevation of 160 m a.s.l.; however, it changes to 5-10° immediately at the altitude of 160-220 m a.s.l. with the occurrence of a flat and broad area (a slope of 5-10°), where human activities (e.g., building roads (Fengjie-Anping road) and reclamation projects (Anping Town)) were frequent and intensive.Analogously, two cliffs at the elevation of about 290 m a.s.l. and 462 m a.s.l.(Fig. 3b and Fig. 3d, respectively), and the flat terraces at the mid-fore parts of O2 and O3 (Fig. 3c and Fig. 3e, respectively) have attracted a substantial amount of attention, thus rendering the landslide area significantly different from the surrounding mountains and easily recognized.Thus, topography saltation occurring in the landslide area will be the important evidence of landslide identification in field investigations.

Underground water characteristics
For the Outang landslide, many cracks distributed at the ground surface provided better access for rainfall infiltration.As shown in Fig. 7a, the thickness of mobilized materials decreased substantially (less than 1.2 m for shallow materials and approximately 26 m for the fractured sandstone layer at sliding mass O3).Moreover, the permeability for the fractured sandstone is large (roughly 3.35×10 -3 cm/s).These factors allow rainwater to sweep easily into the rock mass, thus causing increasing underground water level that appear in the form of a spring at the front part of each subzone.Typically, this type of spring is characterized by a large flow during the heavy rainfall season (Fig. 9a) that decreases substantially during the dry season (Fig. 9b).

Evolution of stability
The earliest geological survey report, provided by the Sichuan Geology and Mineral Bureau in August 1988, indicated that the Outang landslide is stable or quasi-stable, which was further confirmed by the Comprehensive Survey Bureau of the Yangtze Water Resources Committee in December 1995.However, the deformation and failure of old landslide have been discovered frequently and have received particular attention by local residents and authorities; therefore, a landslide disaster prevention project (installing anti-slide piles, etc. labeled in Fig. 2a and mapped in Fig. 2b) was proposed by the Yangtze Institute of Survey, Planning, Design, and Research, and was completed in November 2003.Since then, landslide stability has been improved significantly.
Unexpectedly, after the TGR dam was completed in 2008, the landslide reactivated signs, including the damage of houses and roads, broadening of cracks, failure of local collapse, etc., were increasingly evident.Meanwhile, striated polished surfaces were also discovered by geological exploration, and two local strong deformation areas with a total volume of 4.1×10 6 m 3 distributed at both sides of the toe of O1 was recognized as well.Thus, the landslide stability decreased substantially and exhibited the tendency of a complete failure, as reported in November 2012.Although another remediation project, including backfill toe weight and lattice revetment in the east strong-deformation area (bounded in Fig. 2a and shown in Fig. 2c) and masonry revetment in the west strong-deformation area (marked in Fig. 2a and demonstrated in Fig. 2d), was completed in 2013, the landslide is in a state of continuous creep deformation with increasingly evident activity signs hitherto (including road and house damages, ground fissures, local collapses, etc.).Further, the landslide stability is ambiguous (Huang and Luo 2018, under review).

Conclusion
The Outang landslide could be divided into three subzones with an apparent age of 120-130 ka for subzone O1, 65-68 ka for subzone O2, and 47-49 ka for subzone O3, among which the change rules of the deposit material attitude in each subzone were similar and the same to its respective slip surface morphologies.Additionally, two local strong deformation areas were identified at both sides of subzone O1.
This landslide deposit has evolved from multiple ancient translational sliding masses with the formation mechanism of slide-bending for subzones O1 and O3, and planar sliding for subzone O2.
Moreover, the local collapse and accumulation downslope, and the apparent age of each subzone could be the reasons for the change in the thickness of the top layer.
The material structure characteristics, rupture surface, topography saltation, and seasonal variation of groundwater exposure could be regarded as valid proofs in identifying ancient landslides during an on-site investigation.
Currently, although the landslide has undergone two remedial measures, its stability remains uncertain based on the significant landslide deformation and reactivated features.Therefore, long-term monitoring and emergency civil protection actions are necessary.
illustrates the representative local collapse with a material of volume of 2.3×10 4 m 3 sliding into the river, and the front boundary of the landslide retreated nearly 6 m with Nat.Hazards Earth Syst.Sci.Discuss., https://doi.org/10.5194/nhess-2018-399Manuscript under review for journal Nat.Hazards Earth Syst.Sci. Discussion started: 11 January 2019 c Author(s) 2019.CC BY 4.0 License.

Fig. 1 .
Fig. 1.Outang landslide in the Three Gorges Reservior area.a The Three Gorges Reservoir area.b Landslide location map of the study area.c A closed-up view of the Outang landslide

Fig. 2 .
Fig. 2. Outang landslide and some pictures of landslide treatment in 2003 and 2013.a Engineering geological map of the landslide.b Partial images of the landslide disaster prevent project in 2003.c-d Partial photos of the landslide remediation project in 2013

Fig. 3 .
Fig. 3. Outang landslide and its geomorphological features.a Geological cross section B-B of the landslide (see location in Fig. 2a).b and d Partial photographs of cliffs (see location in Fig. 3a).c and e Images of flat areas (see location in Fig.3a)

Fig. 7 .Fig. 8 .
Fig. 7. Outang landslide.a The thickness of main body varies with the horizontal distance derived from Fig. 6a.b The orientation of the bedding planes within the fractured sandstone exposed by adit exploration.c-d Slip surfaces with clear sliding trace for two local strong deformation areas

Fig. 9 .
Fig. 9. Seasonal variation of groundwater exposure.a Spring at wet season.b Spring at dry season