1 Introduction

Every year, large-scale landslides and debris flows cause thousands of deaths and economic losses of billions of dollars because of global climate change, rapid infrastructure development, and population growth within mountainous regions (Huang and Fan 2013; Handwerger et al. 2016; Pascal et al. 2020). Numerous researchers have studied the formation mechanism, triggering factors, and development conditions of such landslides. Some studies have revealed that strong earthquakes can directly trigger coseismic landslides (Gorum et al. 2011; Parker et al. 2011; Zhang et al. 2019) and debris flows (Zhang et al. 2014; Fan et al. 2018; Tian et al. 2020b). For example, Zhang et al. (2014) opined that the debris flow caused by coseismic landslides following an earthquake has the following criteria: the height difference between the landslide in the source area and the mouth of the debris flow gully is greater than 350 m, accumulation volume of landslide deposits is more than 1.0 × 106 m3, and slope of the debris flow channel is higher than 27%. Landslides can also be associated with strong earthquakes and heavy rainfall (Malamud et al. 2004; Ram et al. 2019). Rockslides are closely associated with the tectonic environment. For example, studies in the Alps (Carlini et al. 2016; 2017), the Tien Shan Mountains (Strom and Korup 2006), and the Zagros Mountains in northern West Asia (Bahrami et al. 2019) have all revealed that geological factors, such as faults, lithology, back-shaped structures, and extensional structures, control the formation of landslides. However, a unique type of landslide with no evident triggering factors such as strong earthquakes or heavy rainfall, and the occurrence time lags behind the peak rainfall, was called silent large-scale landslide (SLL) here. Our insufficient understanding and underestimation of the potential of SLLs could lead to mass casualties in mountainous areas globally.

Landslides have been classified according to the differences in landslide velocity, volume, landslide-forming material types, and so on (Evans 2011; Hungr et al. 2014). The SLL mentioned here refers to a special class of slow-moving landslides with a volume greater than 1 million m3 and without major triggering factors such as strong earthquakes or heavy rainfall at the time of landslide occurrence. Based on the presence or absence of obvious triggers at the time of landslide occurrence, the authors argue that landslides can be classified as those that were generated after a major triggering event, for example, an earthquake or heavy rainfall, and that occurred without significant single triggering factors. A large number of studies have been conducted on landslides induced by strong earthquakes and heavy rainfall. In contrast, landslides like the Zhaobishan SLL, which occurred without strong earthquakes and heavy rainfall, have only gradually attracted much attention from scholars in recent years. Globally, this kind of catastrophic landslides occurs frequently, particularly in certain regions such as the eastern edge of the Tibetan Plateau, the northwestern edge of the Yunnan Plateau, the Rocky Mountains of North America, the Andes of South America, and the European Alps (Delacourt 2004; Handwerger et al. 2013; Lewkowicz and Way 2019; Bontemps et al. 2020). But it is very difficult to understand how the SLL occurs in the absence of heavy rainfall or strong earthquake, and where sufficient water comes from. In addition, SLL often occurs at unexpected times in unexpected places, which significantly increases the difficulty of landslide warning and mitigation. The study of hazards is fundamental in disaster risk science research, and examining the formation mechanism of landslides can improve forecasting capability and the effectiveness of early warning (Alcántara-Ayala et al. 2017; Shi et al. 2020).

Moreover, SLLs are considered an essential process of geomorphic development in mountainous areas (Korup et al. 2007; Egholm et al. 2013; Wang et al. 2020), and the recurring debris flows that result from their occurrence further increase the risk of disaster. From a spatial perspective, the gravitational erosion process in small watersheds in the complex mountainous areas generally originates from large-scale landslides at the source area of the gully and ends in debris flows at the outlet of the gully. The debris flow generation requires sufficient loose material, and landslides are the most common process that supplies these materials in the debris flows of small watersheds (Samodra et al. 2018; Yang et al. 2023). For example, the sources of debris flows typically include landslides, loose deposits in gully beds, moraine deposits, and loess deposits on the eastern edge of the Tibetan Plateau. Among these, moraine deposits are primarily distributed in glacial areas, and weathered loess deposits mainly accumulate on the Loess Plateau areas. Loose deposits in the gully beds are limited to the transportation area. However, the areas mentioned above are relatively small. Therefore, landslides are considered the most common and critical source of debris flows, thereby controlling their frequency and magnitude (Reid et al. 2003; Zhang et al. 2014). For example, in Yunnan Province, China, over 1 million m3 of landslide source materials are effectively stored in the Jiangjia Gully (48.6 km2), with the highest frequency of debris flows (approximately 10 events per year) being recorded worldwide (Reid et al. 2003; Zhang et al. 2014). Moreover, in Sichuan Province, China, the pivotal source material of the large-scale debris flow that occurred in Zengda Township on 27 June 2019 was a large-scale rockslide. However, few studies have focused on the formation mechanism and disaster effects of a single SLL that can trigger recurring debris flows in a small watershed.

Two research problems are addressed in this study: (1) the mechanism of SLL that lags the peak rainfall under non-heavy rainfall and non-seismic conditions; and (2) the characteristics of the debris flow caused by the landslide and its effect in a small watershed. To examine these problems, the Zhaobishan SLL, a rare but typical landslide that has caused ongoing recurring debris flows in the Lengzi Gully, was selected as a case study. First, the Zhaobishan SLL occurred in the inactive period of the Anning River seismic zone (Ni and Song 2019) and was not directly triggered by heavy rainfall or strong earthquakes. In this area, unlike the typical landslide source materials causing debris flow disasters in the watershed following strong earthquakes (Tang et al. 2012; Zhang et al. 2014; Horton et al. 2019), the Zhaobishan SLL is the only single landslide in the basin that continues to trigger recurring debris flows in the Lengzi Gully. Therefore, this study evaluated the mechanism of Zhaobishan SLL and revealed the geological background and hydrological characteristics that facilitate SLLs. The present results provide a new understanding and perspective on the occurrence of SLLs with no strong earthquakes or heavy rainfall, laying a foundation for the risk mitigation of SLLs and associated recurring debris flows in mountainous regions.

2 Materials and Methods

To comprehend the development and failure mechanisms of the Zhaobishan SLL, this study employed a multidisciplinary approach, integrating geology, hydrology, and geotechnics. The methodology encompassed field investigations, hydrological calculations, laboratory experiments, statistical analyses, and numerical simulations. By analyzing the tectonic, geomorphic, and hydrological features of the SLL area, the study aimed to uncover the complex interactions leading to its formation and failure. Notably, the study revealed a time-lagged relationship between landslide deformation and peak rainfall, shedding light on a crucial aspect of the SLL’s behavior. Moreover, the investigation focused on the combined influence of weak soil (resulting from fractured rock masses) and intense water flow (due to abundant water infiltration) as drivers for the occurrence of the Zhaobishan SLL.

2.1 Description of the Study Area

The study area is located in Chengxiang Town, Mianning County, Sichuan Province, China (Fig. 1a). The Lengzi Gully is a primary tributary on the left bank of the upper reaches of the Anning River (Fig. 1b), and the Zhaobishan SLL is located in the upper reaches of the Lengzi Gully (Fig. 1c). The geographic coordinates at the mouth of the Lengzi Gully are 102°11'23.76'' E, 28°36'22.01'' N. The Zhaobishan SLL is located in the earthquake zone of the Anning River, which is also a relatively dry zone. The entire study area has a subtropical monsoon climate. The annual temperature varies between 10.3 and 22.6 °C, and the average annual precipitation is between 930 mm and 1210 mm. The study area is dominated by medium and high mountains with steep topography (Ni and Song 2019). In addition, neotectonic movements and precipitation have frequently triggered landslides and debris flows in the study area. From 2000 to 2016, the geohazards in Mianning County have caused 168 casualties and economic losses of USD 80.8 million (Tian et al. 2019). Moreover, the population density in the study area is relatively high (Fig. 1b), and therefore, the SLL and debris flows in the Lengzi Gully directly threaten the lives of 966 people and the safety of USD 52.7 million in assets, including infrastructure such as the Daqiao Hydropower Station and roads.

Fig. 1
figure 1

Location and the geomorphologic features of the Zhaobishan silent large-scale landslide (SLL). a The study area in Sichuan Province, China. b Topography and settlements of the study area. c Orthoimage of the Zhaobishan SLL via unmanned aerial vehicle. ZK01, ZK02, and ZK03 represent the locations of three boreholes with different elevations in the area of the Zhaobishan SLL

Tectonically, the study area is located at the edge of the Sichuan-Yunnan rhombic block in the northeastern section of the Hengduan Mountains on the eastern margin of the Tibetan Plateau (Fig. 2a) and belongs to the middle section of the north-south seismic belt in China (Yang et al. 2020). According to the Tibet Autonomous Region Geology and Minerals Bureau (1988), there are three faults from east to west in the study area (Figs. 2 and 5), which pass through the upstream, middle, and downstream reaches of the Lengzi Gully, respectively. The hydrogeological conditions are highly active, and spring water seeps from the slope during our on-site investigation. Lithologically, the rocks comprised of dark gray rhyolitic and tuff in the Suxiong Formation (\({\mathrm{Z}}_{\mathrm{ax}}\)) of the Lower Sinian System are exposed at the top area of Zhaobishan SLL, and the rock mass was relatively broken. The Permian Emeishan basalt (\({\mathrm{P}}_{\upbeta }\)) is exposed at the mouth of the Lengzi Gully. Gray-black feldspar–quartz–sandstone, siltstone, and mudstone intercalated with carbonaceous mudstone of the upper Triassic Baiguowan Group (\({\mathrm{T}}_{3}\)-\({\mathrm{J}}_{1\mathrm{bg}}\)) are widely exposed in the study area (Fig. 2). The rocks in the Lengzi Gully vary greatly due to active tectonism. Moreover, there are semi-diagenetic clay rocks and siltstone intercalated with fine sandstone of the **geda Formation in Tertiary (\({\mathrm{N}}_{2\mathrm{x}}\)) in the upper and middle reaches of the Lengzi Gully.

Fig. 2
figure 2

Geology of the study area. a EHS Eastern Himalayan syntaxis; MFT Main frontal thrust; YZS Yarlung-Zangbo suture; LSB Lhasa block; BNS Bangong-Nujiang suture; QTT Qiangtang terrane; XSF **anshuihe fault; SGB Songpan-Ganzi block; LMF Longmenshan fault; XJF **aojiang fault; RRF Red River fault; NJF Nujiang fault; SYRB Sichuan-Yunnan rhombic block; b AA’ is the profile line of Fig. 6b. \({\mathrm{Q}}_{\mathrm{p}}\) and \({\mathrm{Q}}_{\mathrm{h}}\) represent the deposits and alluvial deposits in Quaternary. \({\mathrm{J}}_{2\mathrm{x}}\) is the mudstone and siltstone of the Middle **ncun Formation in Jurassic. \({\mathrm{T}}_{3}\)-\({\mathrm{J}}_{1\mathrm{bg}}\) is the feldspar-quartz-sandstone, siltstone, and mudstone intercalated with carbonaceous mudstone of the upper Triassic Baiguowan Group. \({\mathrm{P}}_{\upbeta }\) is the Permian Emeishan basalt. \({\mathrm{Z}}_{\mathrm{ax}}\) is the rhyolitic and tuff in the Suxiong Formation of the Lower Sinian System. \({\mathrm{N}}_{2\mathrm{x}}\) is the semi-diagenetic clay rocks and siltstone intercalated with fine sandstone of the **geda Formation in Tertiary

2.2 Methods

Because landslides are controlled by soil and water conditions (Shoaei and Sidle 2009; Luo et al. 2021), a detailed study of the tectonic setting and hydraulic properties were conducted. Based on the case of the Zhaobishan SLL, the phenomenon and initiation mechanism of SLL were examined through field investigation, meteorological analysis, geological exploration, remote sensing image interpretation, and numerical simulation. The topographic characteristics were obtained by analyzing the digital elevation model (DEM) data of ALOS-12.5 m DEM and the DEM obtained by unmanned aerial vehicle (UAV) in the study area. Historical insights into the landslide-prone region were acquired through on-site interviews with local residents. During these interviews, a minimum of six local residents were engaged. The questions posed predominantly pertained to the occurrence of past landslides or debris flows in the area, their respective timelines, the severity of the hazards, the depth of mud accumulation, and the distinctive characteristics of the slurry within debris flows, among other aspects. Additionally, our investigation encompassed extensive field work, incorporating activities such as field sampling, cross-section measurements, flow velocity assessments, drone-assisted aerial photography, and an evaluation of land use within the watershed. Characteristic attributes such as the presence of large boulders in the gully were quantified through meticulous measurements as part of our comprehensive survey.

Regarding the tectonic setting, the average fault density index refers to the fault length per unit area (km/km2), which was obtained by the ArcGIS software in combination with the the geological map of the study area (Tibet Autonomous Region Geology and Minerals Bureau 1988). Based on the data from the United States Geological Survey (USGS) National Earthquake Center (NEIC),Footnote 1 we located historical earthquakes from 1900 to the time that the Zhaobishan SLL occurred and set the Zhaobishan SLL as the center with a 420 km search radius. Then the model proposed by Keefer (1984) was used to identify the earthquakes that had an impact on the landslide.

To determine the hydrogeological characteristics of the study area, the Soil Conservation Service Curve Number (SCS-CN) model proposed by the United States Department of Agriculture was used to calculate runoff, and the CN weighted values were determined based on the land use types (Mishra and Singh 2003). The land use types of the study area were obtained by Google Earth image analyses and field investigation. Groundwater characteristics were determined through geological exploration (Fig. 1) and field investigation. Rainfall data were obtained from the meteorological station in Chengxiang Town, which is about 300 m away from the study area.

Based on the geological model of the Zhaobishan SLL derived from the field investigation, the SEEP/W and SLOPE/W modules of the Geo-Studio software were used to analyze the slope instability under the effects of seepage. SEEP/W uses a two-dimensional Richard equation to calculate pore water pressure (PWP). SLOPE/W uses the limit equilibrium method to calculate the factor of safety (Fs) and automatically performs the required Monte Carlo calculation. During the simulation, the PWP values were directly correlated from SEEP/W (Geo-Slope International Ltd. 2007a, 2007b). This method is widely used to simulate landslide stability, particularly in analyzing the instability mechanism of a landslide caused by geotechnical seepage (Chen et al. 2020; Guo et al. 2020). The occurrence of the Zhaobishan SLL was caused by seepage, and this is the reason that we chose this model for our study. Therefore, a finite element model comprising quadrilateral cells and small amounts of trilateral transitional cells was used to study the failure mechanism of the Zhaobishan SLL. The geological profile (Fig. 7 in Sect. 4.2) was selected as the numerical calculation model. The model’s length and height were 3,400 and 780 m, respectively. The bottom boundary is the waterproof boundary, and both sides of the model were zero-flow boundaries.

The soil-water characteristic curve (SWCC) was determined by the sample function embedded in the software and the saturated water content. Then, the permeability function was estimated using the Van Genuchten permeability function in SEEP/W. The physical and mechanical parameters of the rock and soil, and their permeability coefficients under saturated conditions (ks), were determined by field and laboratory experiments. The governing equation in SEEP/W is Richards’ equation (Richards 1931), which describes the two-dimensional flow in unsaturated soils, as shown in Eq. (1). The extended Mohr-Coulomb failure envelope (Fredlund et al. 1978) was used to define the shear strength criteria as shown in Eq. (2):

$$\frac{\partial }{\partial x}\left({k}_{x}\frac{\partial h}{\partial x}\right)+\frac{\partial }{\partial y}\left({k}_{y}\frac{\partial h}{\partial y}\right)+Q=\frac{\partial \theta }{\partial t}$$
(1)

where x and y are spatial coordinates; θ is the volumetric water content; h is the hydraulic head; kx and ky are a function of θ and represent the hydraulic conductivities in the x and y directions, respectively; Q is water flux; and t is time.

$${\tau }_{f}={c}{\prime}+\left(\sigma -{u}_{a}\right)\mathit{tan}\,{\varphi }{\prime}+\left({u}_{a}-{u}_{w}\right)\mathit{tan}\,{\varphi }^{b}$$
(2)

where c' is cohesion; φ is the total normal stress on the failure surface; ua and uw are net normal stress and matrix potential, respectively; φ' is the internal friction angle related to the net normal stress state variable \(\left(\sigma -{u}_{a}\right)\); and φb is the rate at which the shear strength increases with the matrix suction \(\left({u}_{a}-{u}_{w}\right)\).

Additional details can be found in the Emergency Investigation Report of Debris Flows in the Lengzi Gully (Sichuan Huadi Construction Engineering Co. Ltd. 2016) and previous studies (Zienkiewicz and Stagg 1969; Schoeller 1977; Domenico and Schwartz 1998; Weight and Sonderegger 2001), and key parameters of numerical simulation were determined as shown in Table 1. To study the attenuation characteristics of soil strength during the Zhaobishan SLL, the back-analyzed slope stability was conducted. Based on the slope instability conditions, the strength of the soil measured in indoor experiments was reduced until the safety factor of the slope was less than 1.

Table 1 Parameters used in numerical studies

3 Characteristics of the Zhaobishan Silent Large-Scale Landslide (SLL) and Debris Flows in the Lengzi Gully

On-site interviews and the Emergency Investigation Report of Debris Flows in the Lengzi Gully (Sichuan Huadi Construction Engineering Co. Ltd. 2016) revealed historical evidence that there have been multiple geological disasters in the Lengzi Gully, resulting in severe losses of life and properties (Table 2). According to the records, there have been three landslide events in the Zhaobishan SLL area, which belongs to the upstream portion of the Lengzi Gully. The largest and most recent landslide event occurred on 18 July 2000 and is considered the SLL event in this study. The main sliding direction was 313°, and the volume was about 3.37 million m3 (Chen et al. 2012). Notably, it was the largest landslide that has occurred so far, and the recurrence interval is approximately 70–80 years in this area. Considering this time scale, the rare large-scale Zhaobishan landslide has led directly to the occurrence of ongoing, high-frequency debris flows in the Lengzi Gully—after the Zhaobishan SLL that occurred on 18 July 2000, debris flows began to occur in the gully in August 2000. Since that time, debris flows have occurred every year in the rainy season, at least once a year, and up to as many as 5–6 per year. To date, debris flow activity has not ceased. Among the debris flow events, the two largest occurred during the rainy seasons in 2003 and 2007.

Table 2 Historical landslide and debris flow disasters in the study area

The Zhaobishan SLL developed in the upper reaches of the Lengzi Gully, and its overall shape was “chair-like.” The geographic coordinates at the main scarp of the landslide are 102° 11′ 53.31″ E and 28° 26′ 21.72″ N. The landslide area presents an irregular fan shape in plan view (Fig. 3a), which is conducive to the confluence and runoff. At this stage, the elevation of the steep wall formed by the back cliff of the Zhaobishan SLL is 2195–2290 m a.s.l. (meters above sea level), and the elevation of the shear outlet at the toe of the landslide is approximately 2,030 m a.s.l. To date, the landslide accumulation is 300–570 m long and 160–380 m wide, with an average thickness of approximately 20 m, and a total volume of approximately 270.4 × 104 m3.

Fig. 3
figure 3

Landform characteristics of the Zhaobishan silent large-scale landslide (SLL) and the Lengzi Gully. a The yellow shaded part represents the area of the Zhaobishan SLL. b Red lines indicate the debris flow check dams. c Red areas indicate the accumulated silt behind the debris flow check dams

The formation of the Zhaobishan SLL has produced the conditions of high potential sediment yield, strong potential sediment erosion, and high risk of blocking the river. The drainage area of the Lengzi Gully is 0.62 km2, the length of the main gully is 2 km, and the gully itself presents an overall morphology of gentle slopes in the upstream and downstream sections and steep slopes in the middle section (Fig. 3a). The elevation range of the basin is 1813–2405 m a.s.l., while the relative elevation difference is about 592 m and the average longitudinal slope is 29.6%. With an area of 0.62 km2, the Lengzi Gully, however, has formed an accumulation fan with an area of 0.1 km2. Notably, in the Lengzi Gully, the area of the accumulation fan divided by the area of the watershed is 0.16, far exceeding the ratio of 0.01–0.08 in the ** after being softened by water, which likely played a role in reducing the rainfall threshold required for landslides and debris flows (Xu and Liu 2011). Therefore, the landslide deposits of the Zhaobishan SLL could easily participate in multiple debris flow events in the Lengzi Gully over a longer time because the critical rainfall intensity was not very high. Moreover, based on the situation that the Zhaobishan SLL was at risk of further sliding, it is necessary to conduct detailed monitoring work in the area.

Fig. 13
figure 13

Deposits of the Zhaobishan silent large-scale landslide (SLL) accumulated on the semi-diagenetic clay rocks (Nqx)

5.2 Failure Mechanism Comparing to Other Types of Landslides

The water-soil interaction was well known as a necessary condition for landslides, but there are differences in the formation mechanisms of different landslides. First, for the coseismic landslides or strong earthquake-triggered landslides, researchers studied their mechanism by analyzing detailed coseismic landslide inventories after strong earthquakes (Gorum et al. 2011; Tanyas et al. 2017) or by numerical simulations and shaking table tests (Podolskiy et al. 2015; Jibson 2011). The response of the slope to earthquakes mainly includes the reduction of elastic modulus, the formation or widening of cracks at the top of the slope, and the generation of excess pore water pressure, which further leads to the reduction of the soil strength and the occurrence of coseismic landslides (Wang et al. 2001; Rivière et al. 2015; Fan et al. 2019). Second, regarding the rainstorm-induced landslides or heavy rainfall-induced landslides (often associated with extreme rainstorms triggered by typhoons), the infiltration of rainwater from landslide areas leads to the increase of soil water content, pore water pressure, and buoyancy force, the formation of the saturated sliding surface, the decrease of soil strength, and finally, the occurrence of landslides (Xu et al. 2016; Wu et al. 2017; Wang et al. 2019). The erosion of river banks by floods formed by heavy rainfall can also lead to slope failure (Chen et al. 2014). Third, the mechanism of the landslide examined in this study, that is, weak-soil and strong-water jointly controlling the occurrence of SLL under the active tectonic and complex hydrological background, is consistent with existing studies. For example, studies have found that the coupling of small to medium earthquakes and hydrological conditions is conducive to the occurrence of landslides (Bontemps et al. 2020). Ambrosi and Crosta (2006) pointed out that tectonic, lithology, groundwater fluctuations, and weathering effects significantly impact large landslides in the central Italian Alps. Therefore, strong earthquake-induced landslides and heavy rainfall-induced landslides are directly triggered by strong earthquakes or heavy rainfall; SLLs, on the other hand, are caused by the amplified seepage under the active tectonic and complicated hydrological conditions.

5.3 Characteristics and Control Strategy of Risks Induced by Silent Large-Scale Landslides (SLL)

To reduce the disaster risk of SLL hazards, we summarize the spatial and temporal characteristics and discuss the risk-control strategy of SLL based on the results of our field investigation, numerical simulation, and other existing studies.

First, the timing of SLLs is often unexpected because they are not directly triggered by heavy rainfall or strong earthquakes. The timing of SLLs is different from conventional landslides, and the unexpected timing increases the disaster risk of landslide hazards. For example, the 2018 Baige SLL along the **sha River in Tibet occurred in late October and early November, which is not the rainy season, and there was no obvious rainfall process when it occurred (Deng et al. 2019). Although the 2017 **nmo SLL in Maoxian, Sichuan Province in China occurred in June, there was no obvious rainfall process when it occurred, and the daily rainfall on the day of the landslide was only 2–5 mm (Xu et al. 2017). In this study, the Zhaobishan SLL and the previous peak rainfall also showed time-lagging characteristics. The large water confluence area behind the main scarp of the landslide and the lag of amplified runoff determined the delayed occurrence of the landslide. Therefore, the initiation mechanism of SLL proposed here may be used to explain the hysteresis phenomenon between landslides and peak rainfall. To mitigate the disaster risks of SLL, special attention should be paid to the hysteresis characteristics between the time of potential landslides and peak rainfall to improve the early warning system. Awareness of local residents also should be enhanced through education and training. In addition, the prediction of these landslides should not be conducted solely based on rainfall intensity, and vigilance against landslides and deformation monitoring must be increased after heavy rainfall.

Second, the location of SLLs has special characteristics because these landslides are jointly controlled by weak-soil (fractured rock mass) and strong-water (abundant water replenishment) conditions under the impact of active tectonism and complex hydraulic properties. For example, the water confluence area behind the main scarp of the Zhaobishan SLL is 3.16 times the area of the slope body. Water in the Bogong Gully, with a catchment area of 23.2 km2, behind the main scarp of the Baige landslide, refilled the long-term seepage within the slope of the Baige landslide (Tian et al. 2020a, b). Similarly, on 25 April 1974, the Mantaro landslide in Peru occurred because of the increased seepage caused by the infiltration of the Pumaranra River behind the slope (Berrocal et al. 1978). Therefore, SLLs usually occur on slopes with strong tectonic uplifts, high-density faults, frequent historical strong earthquakes, and large water confluence areas behind the main scarp. These characteristics help enhance the accuracy of SLL identification, which is crucial for landslide warning and disaster risk reduction. Consequently, we also need to pay attention to the following aspects in the mitigation of disaster risk: (1) the long-distance groundwater replenishment in the slopes should be considered in the prediction and warning of landslides; (2) the impact of geological and geomorphic features on the runoff and groundwater seepage should be analyzed during the early identification of landslides; and (3) remote sensing images should be used to analyze land use conditions, and special attention should be paid to the water confluence and infiltration from farmland.

Finally, for the watersheds with semi-diagenetic clay rocks of the **geda Formation, recurring debris flows following massive landslides also should be focused, and high-efficiency sediment drainage should be conducted to avoid the outburst floods caused by the blockage of the main river.

5.4 Limitation

There has been increasingly more attention to SLL and some studies reported that tectonic activity and groundwater play an essential role in the occurrence of landslides and debris flows (Wei et al. 2019; Bontemps et al. 2020), which is largely consistent with our results. However, the mechanism through which the amplified runoff affects the rock mass strength after infiltration, quantification of the influence of tectonic activity on landslides, and physical process of transforming rare landslides into recurring debris flows, are all topics necessitating further research.

6 Conclusion

Taking the Zhaobishan SLL, that has been causing recurring debris flows since 2000 in the Lengzi Gully on the eastern edge of the Tibetan Plateau, as a case study, the coupling effect of weak-soil (fractured rock mass) and strong-water (abundant water replenishment) on the occurrence of SLL was revealed through field investigation, remote sensing image analysis, geological exploration, hydrological calculation, and numerical simulation. The results explain why SLL lags the peak rainfall and provide new insights into the risk mitigation of these distinct landslides and debris flows in vulnerable mountainous areas.

The Zhaobishan SLL was associated with high sediment yield, substantial sediment erosion, and high risk of river blocking in the Lengzi Gully. Strong tectonic uplift, high fault density, and multiple historical strong earthquakes had established weak-soil conditions, and the combined effect of special lithology, antiform, large regional water confluence area, and cultivated land had established strong-water conditions, which are both conducive to the occurrence of the landslide. Extensive runoff and long-distance water replenishment created the instability and lagging effect of the Zhaobishan SLL. The coupling effect of strong-water and weak-soil on the slope body eventually attenuated the rock and soil strength to approximately 0.4 times its initial strength, triggering the landslide. The Zhaobishan SLL combined with the exceptional setting of semi-diagenetic clay rocks of the **geda Formation also contributed to the recurring debris flows in the Lengzi Gully. To reduce disaster risk of SLL in vulnerable mountainous areas, the water confluence area behind the main scarp of landslides, the hysteresis characteristics between the landslides and peak rainfall, and the long-distance water replenishment of slopes should be further considered, and recurring debris flows following massive landslides also should be focused.