Failure modes of loose landslide deposits in the 2008 Wenchuan earthquake area in China

In this study, a geological investigation and statistical analysis of the post-earthquake slope deposit failures in a meizoseismal area were presented, with a selected example from the 2008 Ms 8.0 Wenchuan earthquake occurred in Sichuan Province in China. The typical slope deposit failures were 15 surveyed in three meizoseismal areas, namely Qingchuan county in Guangyuan city, Beichuan county in Mianyang city, and the epicenter area, Wenchuan county in Aba Tibetan Autonomous Prefecture. According to the movement, material and deformation mechanism of rock or soil, the failures of the post-earthquake landslide deposit could be subdivided into four categories, i.e. slide, rockfall, erosion and flow. This classification of failures of landslide deposit considers the topographic and failure after 20 the earthquake. Besides, some other important factors such as topography, lithology and hydrogeology are also considered. The above mentioned four failure categories are further split into 12 sub-classification. The complicated deformation mechanism and different failure patterns of the slope deposits are analyzed in typical deposits. This classification provides a good reference for the prediction of geological hazards, whereas mitigation of the landslide or debris flows caused by loose 25 deposits in the meizoseismal area is still a difficult task.


Introduction
The failure types of post-earthquake deposits have been examined in several studies, and classification (1938) is primarily based on materials (earth and rock), movement (flow and slip) and velocity (slow or very rapid), without considering the effect of topography, landform, volume and inducing 30 mechanism. Based on the material and type of movement, Varnes (1954Varnes ( ,1978 classified the slope failure into five types, i.e. fall, topple, slide, spread and flow, and this has been the most widely used classification for landslides in the world. According to seismic parameters, materials and geologic environment, Keefer (1984) divided the landslides into 14 types. Considering the landslide shape and geotechnical parameters, Hutchinson (1988) divided the slope deformation failure modes into a creep, frozen ground phenomena, and landslides, but did not consider the trigger mechanism and the effect of the volume. Hungr (2001) classified the landslides into ten types based on genetic and morphological characteristics, which introduced a new category in combination with unsorted material and sorted material. However, the deformation-failure modes, the particularity of the loose post-earthquake main body has not been paid much attention in previous studies, and further studies should be conducted 40 based on these landslide classifications.
The purpose of the new classification proposed in this paper is that landslide deposits can be effectively split into common categories according to deformation mechanisms, which retains the established concept and reveals the deformation and failure trend of landslide events. This is easier to achieve with a statistical analysis of a field survey, without resorting to more complex taxonomic 45 methods. Moreover, understanding deformation and failure mode could help to mitigate and prevent similar geological disasters. Some authors have made good attempts and achieved significant results.
For instance, the "locking section" was used in one study of mechanisms of large-scale landslides occurred in China by Huang (2011) to identify a three-section model which includes sliding, tension cracking and shearing. Using the same apparatus, Yang (2015) also evaluated the post-earthquake 50 rainfall-triggered deposit failure occurred in Lushan area, Sichuan province, China.
The discussion of this paper focuses on the deformation and failure mechanism of loose deposits after the earthquake. Although deformation and damage mechanism of the accumulation body have been preliminarily considered, classification and specialty of the landslide deposits have not been well developed. Wang (1981) found that the after-shock caused the cyclic shear to induce the decrease in 55 the strength of sliding surface shear on rock slope instability. Some researchers used inertia, damping, weakening, and liquefied instability to interpret the instability of the deposit. Seed and Martin (1966) used the regular soil deposit for a laboratory test, with limited effort focused on the large deformation of inclined slope caused by material liquefaction. Kramer (1997) proposed that instability of post-earthquake can be spilt into weakened instability and inertial instability. Based on indoor 60 experiments and field tests, a few researchers studied the liquefaction mechanism and shear deformation of loose deposits after earthquakes in China, Japan, and New Zealand. It was confirmed that liquefaction or shear forces established slope deformation. However, the empirical models for the deformation and failure of loose deposits after such earthquakes have not been proposed.
Nearly 45,000 loose deposits were induced by the 2008 Wenchuan8.0's earthquake in China spreading 65 in 51 disaster areas of 130 thousand km 2 . These include 13,229 landslide deposits, 5,180 rockfall deposits, and 2,400 debris-flow deposits in Sichuan Province, according to the post-earthquake survey (Huang, 2009). Many loose deposits of the Sichuan Post-earthquake areas are susceptible to rainfall or landslides induced by the aftershock. More than 12,000 potential geological hazards were triggered by rainfalls ( Fig.1), which killed hundreds of people (Kirschbaum, 2010;Liao, 2011). A clear classification system of the deformation mode of the accumulation body is more beneficial to the stability evaluation of multifarious geo-hazards. In particular, the geological hazard classification system in strong earthquake areas should consider the effect of multiple factors, such as topography, stratum lithology, material, motion velocity, deformation, and failure mechanism. A practical type of classification based on selected attributes is a good classification and a quick way to solve practical 75 engineering problems. According to the material and sedimentological characteristics,  divided the dam landslides caused by the 2008 Wenchuan earthquake into three categories, which will help the prevention and control of landslide dams in strong earthquake areas, however, there is no classification for loose deposits such as debris flows and rockfall deposits.
In this study, the geological conditions and the type of geo-hazards induced by the 2008 Wenchuan 80 earthquake are first introduced. Subsequently, the classification method and the typical failure mode of the loose deposits occurred since 2008 are discussed. A new classification method for deformation and failure modes of deposits considering various factors such as topography, material, motion velocity, volume, and particle composition is proposed. The formation mechanism and failure modes of the geological disasters induced by 12 loose deposits are analyzed. The proposed new classification 85 of failure modes for loose deposits should also be easily applied to the classification of geological hazards occurred in other strong earthquake zones.

Geological conditions 90
Detailed analyses of the landslide deposits show that the slope deposit failure of the post-earthquake regions in Wenchuan, China are complex. It is important to study the geological conditions in order to recognize potential geological hazards. The specific failure mode is related to the specific topography, deformation, and structure of the rock (soil). This study area has crossed various geomorphic units, covering Qinghai-Tibet plateau, Longmen mountain, Sichuan basin and valley throughout the north to 95 south. The terrain shows high in north and west, but low in south and east. Due to well-developed faults, complicated topography, and various types of rock-soil mass structure and climate change in this area, many post-earthquake loose deposit slopes were accumulated in the potential geo-hazard regions, and it is important to study the failure mode and evolution process of the Wenchuan earthquake area.  Tectonics erodes middle mountainous area of wide valley basin;Ⅱ3: Liangshan tectonic erosion middle mount area. Ⅲ : Mount area in eastern basin in Sichuan; Ⅲ1: Tectonic erosion low mountain hilly in Sichuan Basin; 1 1 Ⅲ : Inclined plain sub-region in the front of western fault depression basin; 2 1 Ⅲ : Mono-clinic low mountain sub-region north of tectonic erosion basin; 3 1 Ⅲ : Table low hilly sub-region south of erosion tectonic basin; 4 1 Ⅲ : Parallelism (low mount) valley (hilly)sub-region in eastern of erosion tectonic basin; Ⅲ2: Michang mount to Dab mount tectonic corrosion bedded middle area;Ⅲ3: Wu mount to Dalou mount strong karst valley middle mountain area.

Seismicity and Rainfall
Several high magnitude earthquake has been recorded in the Longmen Mountain tectonic zone along the eastern margin of Tibetan Plateau (China) in the last few decades. The Ms7.5 Diexi earthquake on 105 August 25, 1933, caused the catastrophic landslide which blocked the Minjiang river and formed three famous " quake lakes " . The rock slide depositions had slipped into a channel and formed landslide dam, then caused deformation and failure, subsequently the water of this lake pour down, and as a result, 2500 people had been killed and more than 6800 houses had been destroyed (Ren, 2017). The Wenchuan earthquake on May 12, 2008, and the Lushan earthquake on April 20, 2013, had 110 magnitudes of Ms 8.0 and Ms 7.0, respectively. These epicenters were located Longmen mountain fault, SW-NE of Chengdu City, and the epicenter was located at the depth from 5 to 20 km, within the Eurasian plate of the Yangtze plate.  It is suspected that the rainfalls and aftershocks have triggered the landslide or debris flow. Rainfall has played an important role in the conversion of loose accumulations into landslides and has also 140 attracted the attention of many research interests. The study area has a subtropical humid climate and usually brings heavy rainfall between June and September. The average annual precipitation in the study area is 4.87×10 12 m 3 , and the annual average rainfall is 1003.1 mm. The Longmen mount fault zone is a concentrated rainfall distribution area with a maximum rainfall of 160 mm in 24 hours, which provides sufficient external dynamic conditions for the loose accumulation failure. In addition, 145 there are more than 1,400 rivers in the study area, and the water flow rate reaches 1.59× 10 4 m 3 per second, which is also an important factor for the deformation and failure of loose deposits (Fig. 3).
Under the combined action of seismic activity and hydrogeological conditions, the loose accumulation slope in this area has a high-risk failure in the earthquake process. These factors must be taken into consideration in loose deposits failure modes classification.

Investigation Methods
Field investigations were performed to understand the geological features in the area and the mechanism of the landslides deposits. Methods include outcrop observations and topographical measurements, as well as the use of drilling, trenching and pit exploration to investigate the internal conditions of loose deposits. Some giant loose deposits also used geological drilling and standard 155 penetration testing (SPT) to study the particle composition. Due to the complex and diverse lithology of the landslide loose deposits, the engineering geological profile of typical loose deposits is drawn based on the investigation and analysis of the lithology of the strata. Finally, the deformation and failure mechanism are analyzed. The main field survey site and research object are the most representative large deposit body within 50 km wide along the Longmen mount fault zone (Fig.3). 160 The field investigation results reveal that the typical lithology of the deposit is the bedrock which consists of weakly weathered, moderately weathered and strongly weathered coarse and fine granite, limestone and sandstone. Under weathering or post-earthquake weathering, the bedrock is covered with a large amount of loose clay, broken rock mass or their mixture, which is the main component of landslide sediments. 165 According to field investigation statistics for the Wenchuan earthquake area in 2010 (CGS), these deposits can be classified into four types based on the topographic and type of movement (Cruden and Varnes, 1996), i.e. slide, rockfall, erosion, and debris flow type representing for 62.74%, 24.57%, 11.38%, and 1.31% of the deposits, respectively. The ratios of slide, rockfall, erosion and flow type are of 41:29.1:28.6:0.4 in plateau mountain areas. In high to medium mountains in a transitional zone 170 from the plateau to basin, the slide of landslide deposits induced by Wenchuan earthquake is the main failure mode (up to 65.3%), followed by erosion mode with 26.6%, rockfall type with 6.5%, and the debris flow with 1.6%. But in basin and mountain area in Sichuan province, the ratios of slide, rockfall, erosion and debris flow type are 66.9:31.1:0.5:1.5 (Table 1).

Slide
The slide type of deposits is usually caused by the reconstruction of rock or soil slopes. Under the action of external geological forces, e.g. rainfall, aftershocks and human engineering activities, the loose deposits move along the weak surface or sub-surface. According to topography, material 180 materials, motion characteristics, and on-site investigation, we classify the slide into four categories, i.e. reactivation of old landslide, slide along the weak soils or rocks, the shallow slide of deep deposits and integral sliding on bedding rock.

Reactivation of old landslide
Stable or almost stable ancient landslide deposit body is induced by the earthquake, and subsequently 185 global or partial reactivation may occur to lead to deformation and failure of accumulation body under the effects of rainfall conditions, aftershocks and human project activities. For instance，the Xindianzi landslide, located in Yinxiu town, Wenchuan county, obviously the epicenter of the 2008 Wenchuan earthquake, is a typical reactivation of old landslide (Fig. 4). The source area of the Xindianzi old landslide is nearly 0.8 km long and 0.5 km wide, while the old slope angle is 25°～30°. The angle of 190 old main scarp behind deposits is steep (45°～75°). The estimated volume of the deposits is 6×10 6 m 3 and their material is a single and homogeneous, mostly the loose medium granular soil. Slides along with the weak soils or rocks usually occur in deposits with weak interlayers. The main body consists of loose deposits, broken rocks, and their mixtures. The weak interlayer consists of plastic-soft clay or clastic sediments, and the bedrocks are usually consist of fully weathered-fully 210 weathered shale, mudstone or sandstone. Before the deformation of the rock and soil in the weak interlayer occurs, the landslide generally moves slowly, and the moving speed is usually less than 0.1m / 1a. Whereas under the influence of earthquakes, rainfall and human engineering activities, the loose deposit will suddenly accelerate in the case of the transfixion of weak interlayer or the weak zone (Huang, 2011). 215 Fenghuang Mountain landslide locates in Ershe village, Leigu town, Beichuan county, with a total square volume about 1.08 × 10 6 m 3 . It is the slide on the weak interlayer with the following main features: the landslide deposit is nearly 420 m long and 1560 m wide, with the average slope angle of 25°, which is affected by deformation. Its main scarp is 25 m high in average, presenting two moving steps, with the horizontal distance of 167 m and the height of 80 m. The middle of deposits is 111.6 m 220 thick, 94 m thick in the slope toe and 58 m thick in the slope head. Most of the material of this landslide deposits are composed of limestone, carbonaceous shale, silty clay, crushed stone or pebbly clay. The soil sample exposed by drilling is characterized by kneading and water absorption, suggesting that the soil sample is subjected to high compression and grinding. According to geological hazard monitoring, the slip velocity of this accumulation body is 0.08 m/1year. Excavation of the road 225 at the toe of the slope resulted in the rapid down move of the deposit along the weak interlayer (Fig.   5).

Shallow slide of deep deposits
A shallow slide on the deep earthquake deposits generally occurs in highly consolidated deep rock and soil. The velocity is extremely high (often greater than 0.1 m/a), and sometimes the surface 230 fragmentation of the soil accelerates with the rise in slope increases throughout the movement. This type of failure is caused by earthquake, rainfall or human activities. It leads to the deterioration of the structure and strength of the shallow surface of the stratum, followed by the creep and sliding deformation of the shallow of deposit body (Fig. 6).

235
Majiapo landslide locates in Yuli town, Beichuan county, which is nearly 330 m wide and 230 m long.
Its volumes are nearly 4×10 5 m 3 and less than 10 m thick of the main body. The landslide deformation was very slow before the Tangjia Mountain earthquake lake was formed. However, after the toe of these deposits was submerged by the water, the shallow landslide moved quickly. The landslide deposits have a steep (25°～45°) slope angle about 28 m high. The source of the deposits is largely 240 composed of gravelly soils with highly weathered phyllite and slate (takes up 50-60%). Likewise, these shallow landslides are known to occur both on the surface land and under the earthquake lake water.

Integral sliding on bedding rock
Integral sliding on bedding rock generally occurs in loose rock deposit with a forward gentle laminar 245 rock layer. The topography of this failure mode is characteristic by V-shaped or U-shaped valleys.
These slopes are composed of medium-to-sloping layered rocks. They may slide along the bedding plane under the action of their own weight or load, or they may be deformation and failure caused by external loads such as rainfall or earthquakes. The formation lithology in the landslide deposit primarily consists of sandstone of the Triassic system 255 (T) and Quaternary residual slope alluvial soil(Q). The angle of bedding rock is steep (more than 35°), and the main body is 9.6 m height on average, of which the main scarp is 8 in height. Remaining unstable landslide height 8 m may slide suddenly in the future. According to field reconnaissance, the velocity of this landslide is a 0.5 m/1 year, and the rainfall infiltration and becomes a weak surface along the bedding limestone are the main failure factors (Fig.7). 260

Rockfall
Rockfall is produced in steep slope deposits which under external forces, including gravity, earthquake, weathering denudation or human activities. It is a single or compound movement with sharp fall, caving, sliding, rolling, jumping, and other special forms, sometimes they hit each other in the process of movement, then pile in the slope toe (Rens, 2008). Most of the rockfall sources are rock 265 deposits with low shear strength and 2-3 groups of penetrating fractures. Whether or not rock fall occurs depends on deposits steepness and deposit stability. Based on the rockfall travel velocity and movement way, rockfall type can be split into the following three sub-types.

rockfall-slide
Xinmo catastrophic rockfall-sliding rock avalanche is the recent famous massive rock rockfall in 270 Wenchuan earthquake area, causing 10 people died and 73 people missing. This massive deposits located in Xinmo village, Diexi town, Mao County, Sichuan province. It may be originated from the fissure stretched downward and finally passed, and then the massive rock mass traveled more than 2 275 km. The total volume of the rock mass deposits is about 4.5×10 6 m 3 , about 210 m long and 300 widths, and the fastest traveling velocity of the massive loose landslide deposits is about 74.6 m (Fig. 8) Fang, 2017;Meng, 2018). Massive rockfall-sliding is one of the catastrophic disasters that pose threats to the people's lives in 280 the earthquake area. If the loose deposits consisted of densely structured rocks and joint fissure which had an unstable effect on rocks extensive distributed, fractures would be formed through a plane.
Subsequently, under the action of multiple earthquakes and long-term gravity, the aging deformation is generated. When the rainfall accumulates several months and overall the stability of the loose deposits, the catastrophic landslide may be produced suddenly. 285

Crack-slide rockfall
A crack-slide rockfall is a form of a steep slope, characterized by steep and vertical fractures on the crown of the slope, occurring when loosely cemented material or rock layers move on a short distance and dump at the toe of the slope (Tarbuck.1998). Though the surface of the slope displacement is small, deep crown cracks had been formed by rain infiltrated, earthquake, or weathering (Fig.9). 290 Moreover, the gravity of overburden deposits based on the weak layer increases in the process of rainfall, thereby making deposits falls down gradually along a parallel surface. This deformation mostly occurs in the consequent bedding landslide deposits. Jiguan mountain crack-slide rockfall occurred on July 9, 2018, which is about 40 km south of the 295 epicenter of 2008 Wenchuan earthquake. Fig.10 gives an aerial photograph of the rockfall. At the crown of the rockfall, there were several vertical cracks about 2.5 m deep. The amount of the rockfall deposits was about 250 m wide and 560 m long, with the total volume of about 3.8 × 10 6 m 3 . Most materials of the deposit were primarily composed of silty sandstone and limestone that formed from the Mesozoic era, the Triassic (T). In the area where the rockfall occurred, the artificial slope was 7.5 300 m high with an over 70 degrees angle and covering considerable underlying rocks on the consequent bedding sandstone layer.

Toppling rockfall
Toppling failure is one of the most common failure forms of rock deposit slope in the strong earthquake area. The main failure mode of the toppling failure is bending and overturning, which is 305 caused by bending stress. Toppling generally occurs in steep rocks with vertical joints. Moreover, the soft rock and hard rock interlaced sedimentary rock often occurs toppling failure. When the lower soft interlayer is weathered or eroded by rainfall, the upper loose accumulation body would be suspended, falls, rebounds or rolls down under the action of gravity. Toppling rockfall is characterized by breaking rocks and discontinuous structural cracks, usually triggered by earthquakes or human 310 activities (e.g. hydropower stations building, highways building and other works) (Guo,20l7). Besides, effective inter-granular stress would decrease in deposit material due to the increase in internal seepage pressure and the decrease in pore water pressure, thereby causing a rockfall.. This deformation failure model can be defined as toppling rockfall. Because the rock has been falling for 85 years, the rockfall deposits are approximately 1000 m wide 320 and 1500 m long, the rockfall rock traveling distance more than 1400 m (Huang, 2009). After the 2008 Wenchuan earthquake, the average thickness of the rockfall deposits were over 180 m, and the total volume of were over 2.1 × 10 8 m 3 . Such loose deposits are mostly composed of Quaternary(Q), Triassic metasandstone, crystalline limestone(T) (Fig.10).

Erosion 325
Erosion often occurred in loose deposit body induced by rainfall or flow in the area with undulating landscape. This mode of motion is usually a spatial continuous motion, and the deposit is carried away by the current from high to low. These processes contributed to the formation of unstable rock and soil masses in the surface of gullies during the different courses of geological erosion (J. Dvorak, 1994), deformation and destruction, and finally the deposits moved with the grading movement of mud (sand) 330 flow, which depends on the water content, mobility and movement evolution.

Scouring and lateral erosion
Scouring and lateral erosion have two main mechanisms: scouring and lateral erosion. River erosion is the direct removal of soil particles by the current. The rate of scouring is determined by the impact of the flow and the erosion resistance of the bank's loose deposit material. When the weight of the upper 335 deposit is greater than the strength of the slip zone, the failure will occur subsequently, resulting in lateral erosion. The process depends on many factors, including the particle composition of the slope material, the water content and the coverage of the vegetation. These two erosion processes are interrelated because the scouring at the bottom of the river bank produces steeper slopes or overhanging clods that are more unstable and may be laterally eroded (Fig.11) 340 This type is primarily formed on the surface of loose deposits body, and usually, both sides of the slope have U-shaped or V-shaped canyon. They will be strengthened if they occur on a hillside with less vegetation or both sides of the gullies that have been lost vegetational by earthquake or mining deforestation. Under heavy rain and extreme rainfall conditions, the upstream water continuously 345 washed away the loose deposits, thereby caused the slopes on both sides of the valley to be washed repeatedly, and the valley section gradually expanded and deepened, and finally causes the slope failure (Fig. 11). For instance, the deposits of scouring and later near Baihe Village, Qiangchuan county, Sichuan province, China, 2014, which destroyed 15 houses and caused 3 death, is underlain sericite phyllite of the Silurian system(S). After thousands of years of erosion, the erosion efficiency 350 determines the speed of the material in the rockfall process, so the erosion accelerated after the Wenchuan earthquake.

Steam Bank erosion
It often occurred at the toe of the loose deposits, and it would be damaged and degraded by the stream, 355 river and lake. Due to the scour, dredging and erosion of current, the upper part of the deposit is not balanced, resulting in local downward cutting or rockfall of the deformation mode. This study has a typical example for the stream bank erosion of the slope deposit in Soqiao village, Wenchuan County, Sichuan province, China (Fig.12) Accordingly, it is speculated that landslides will occur in future heavy rain or earthquake conditions.

Debris flow cutting
Debris flow cutting typically occurred in a slope of loose deposit body with a slope up to 45 degrees, usually initiated during heavy rainfall, which upstream materials driven by a rainstorm or debris flow. 370 When the water accumulates rapidly in the upstream, a debris flow will form in the middle and lower reaches, Subsequently rushed out of the channel, and cut the slope foot result in a steep air surface.
The existence of these loose materials on the slope and the development of heavy rainfall events are the main reasons for the deformation and failure of these deposits (Xu, 2012). The slope falls of the secondary platform (1300 m) and the tertiary platform is relatively small (140.3 ‰ and 322.5 ‰ , respectively), whereas the ditch is deep and narrow and the accumulation body exhibits a large loose thickness, which makes it extremely easy for the erosion and erosion cutting deformation and failure.

Debris avalanche
The debris avalanche was originated from the collapsing material caused by the earthquake. Because of the steep slope, scarce vegetation and extremely loose structure of the deposit, combined with exterior geological force (e.g. aftershocks and human activities), debris flow material in a superficial layer of loose deposit slipped downward with high speed, accompanied by the flow of dust 395 and tumbling sounds of tumbling rocks. Since 2008, there are hundreds of debris avalanche induced by rainfall or aftershock in Wenchuan earthquake area. The speed of the avalanche chute to the steep channel is usually more than 10 meters per second, whereas some of the landslide flows are much faster. For instance, the Mengjiacao debris 400 avalanche, located in Mianzi town where about 10 km south of Wenchuan county, Sichuan Province, it is a typical avalanche flow in this area. Because of the consequent rockfall flow since 2008, the rock or soil has been accumulated in the toe of the slope, and the total volume of this deposits was over 2.5 × 10 6 m 3 . The material of this landslide-debris flows contains characteristic by the loose coarse and fine particles that distributed in the different rockfall area. The velocity of this landslide-debris in the 405 steep channels usually attain speeds over 12 m/s (Fig.14).

Debris flow
Though the number of the debris flow in Wenchuan earthquake area all deposits is a small proportion(1.31%), it has aroused the huge attention from geologists and government because of its fast movement, great harm, difficult prevention and control For instance, the Hongchun gully Debris 410 flow occurred in near the Yinxiu town, Wenchuan county, Sichuan, in 14 August 2010, caused 17 people missing. The debris flow has battered the new 213 National Highway, blocked the Min river, then wiped out Yinxiu town (Fig.15). Hongchun gully debris flow is one of the 72 debris flows near the Beichuan-Yinxiu Fault in August 415 2010, which is characterized by the amount of loose deposits, the steep drop in the shape of gullies and critical rainfall (Tang, 2009). The total volume of this debris flow is nearly 80.5× 10 4 m 3 , all of these loose materials of the debris flow are composed of granular soil (60%), Boulder (25%), rubble (10%) and sand (5%). The channel catchment area covers 3.35 square kilometers, the main channel length is 3.6 kilometers, and the average longitudinal slope of the channel reaches 35. 8%. The top of 420 the slope is 2168.4 m asl, and the gully mouth of debris flow is 700 m asl. The debris flow materials mainly come from three branches in the upper reach of the Hongchun gully, among which 52 are landslide or rockfall deposits, and the total amount of the loose solid material is 3.57×10 6 m 3 . Besides, since the rainfall "8.14" debris flow in Hongcun gully was 16.4 mm per hour and total rainfall reach to 162.1 mm/34 hours before debris flow outbreak, the heavy rainfall is the inducing factor of debris 425 flow outbreak (Gan, 2012).

Discussion
Previous studies suggested that different types of accumulation body have significantly different deformation and destruction mechanism and failure modes (Zhang,2012;Cui, 2014;Huang,2015).
Controlled by various factors (e.g. rock and soil mass structure, geological structure, rainfall and 430 geographical and geomorphology) of the study area, the accumulation body presents different deformation and failure modes, and its movement type, speed, scale, geomorphology and landform, failure modes,etc. are also different (Table 2). It is worth noteworthy that topography is a factor significantly affecting the failure of landslide deposit.
It also determines the scale, the shape and the deformation and destruction mode of these accumulation slopes. Macroscopic topography controls the development and distribution deposit body.
Slopes with different gradients, heights, shapes and vegetation significantly effect the disaster mode of 440 landslide deposit.
Besides, there was not a clear relationship between the failure mode of the deposits and particle size to be observed. Deposits are composed of fine particle soil (e.g. sandy soil, gravel soil and clay) that can be occur sliding, erosion and debris flow. Deposits are composed of the medium and coarse particle that can also occur such failure as long as there is sufficient rainfall. The precipitation process, rainfall 445 and rainfall intensity significantly affect the formation of debris flow. This study suggests that the continuous rainfall and rainstorm can lead to different failure modes through the same deposits with the same particle size. Vegetation and its root system can weaken and protect the accumulations of erosion from being eroded by rainwater. Investigation statistics reveal that deposit with well-developed vegetation primarily formed slip type deformation and destruction, whereas it is 450 unlikely to develop into erosion or rockfall. In contrast, rockfall or erosion deformation and destruction often occur in places with poor development or underdeveloped vegetation in landslide deposition.
Moreover, the formation of accumulation was controlled by geological structure. The closer the distance to Longmen mountain seismic fracture zone, the greater the seismic force and the structure of 455 accumulation became loose to form debris flow, which may likely be transformed into rockfall type and erosion if landslide deposit produced in much closer to fracture zone. Investigation statistics reveals that the failure of landslide deposit in Wenchuan earthquake area was primarily developed in rock and rock-soil (e.g. granite, quartzite, dolomite and limestone). Integral sliding on bedding rock mostly occurred in rock deposit which is composed of hard rock at the top and weak rock at the 460 bottom. Deposits largely composed of rocks at the top with highly compacted density and weak structural bedding surface, thereby inducing a slide on weak soil or rocks easily. Most giant landslide deposits located in the steep slope near Longmen mountain fault belt, and it was extremely easy to produce catastrophic landslide or debris flow.

Conclusion 465
Previous classification studies on loose deposits were based primarily on material, velocity, water content, geotechnical parameters, and other geological hazards, and the effects of topography, landform, volume, and triggering mechanisms are generally not considered. This paper presented a world-recognized classification improvement from the perspectives of topography, velocity, material, volume and triggering mechanism of loose deposits in the strong earthquake area. Thus, the basis of 470 this factors of this classification here is more comprehensive, especially suitable for the actual classification of geological disasters in the meizoseismal, which help to lay a scientific basis for the prevention and control of geological disasters.
According to the results of field investigation and statistical analysis, there were four main types and 12 subcategories of failure modes in loose deposits after 2008 Ms8.0 Wenchuan earthquake area, 475 where are as follows: (1) Slide, covering the reactivation of old landslide, Slide on weak soil or rocks, shallow sliding of deep deposits and integral sliding on bedding rock; (2) rockfall, including rockfall-slide, cracking-sliding rock rockfall, topping soil rockfall and debris flow cutting; (3) Erosion, e.g. scouring and lateral erosion, stream bank erosion; (4) Flow, e.g. debris avalanche and debris flow.
The investigation statistics on hotspots in Wenchuan earthquake area, Sichuan province, suggests that 480 the failure mode of loose deposit was mostly of the slide, some of them may occur rockfall and erosion, and the fewest of them will occur debris flow.
The category of failure modes in landslide deposits proposed here can serve as preliminary of hazard & risk assessment. More reliable assessment should consider the geotechnical investigation method and means under various conditions, and also rely on the accurate geological analysis of landslide 485 deposit. These massive deposits are still highly likely to induce geological disasters under the effect of rainfall, earthquake or human activities. Accordingly, the prediction and stability evaluation of the deformation and damage of loose deposits formed by strong earthquakes remain a matter of great concern.