Relationship between the spatial distribution of landslides and rock mass strength, and implications for the driving mechanism of landslides in tectonically active mountain ranges
Introduction
Rock mass strength is controlled by intact rock strength, the presence of structural discontinuities (joint sets, faults), and rock bridges (Hoek, 1983, Hoek, 1998). At the hillslope scale, rock mass strength plays a fundamental role in landscape evolution (Tucker and Slingerland, 1994; Korup, 2008; Moore et al., 2009; Clarke and Burbank, 2010) and must be considered in studies of slope stability targeted at hazard analysis (Wang et al., 2014). Rock mass strength can be understood as the laboratory strength of a complete rock block of the slope, and also as the strength of the whole rock slope. At the outcrop scale, rock mass strength can be determined by testing representative rock blocks in the laboratory. The measurements from such tests provide inputs into numerical models that are used to estimate, albeit with large uncertainties, the stability of specific slopes (Hoek, 1983; Clarke and Burbank, 2010; Saroglou et al., 2018). The strength of the whole rock slope is very difficult to define in a quantitative way (Hoek, 1983; Clarke and Burbank, 2010; Saroglou et al., 2018). In this study, we focus on the rock mass strength of the entire slope, including the complete rock block and its joints and fissures.
Many researchers have argued that, all other factors being equal, relatively high rock mass strength, even in areas of high relief and steep slopes, results in lower frequencies of landslides (Sklar and Dietrich, 2001; Stock et al., 2005; Molnar et al., 2007; Corominas and Moya, 2008; Moore et al., 2009; Qi et al., 2010; Wang et al., 2014; Wang et al., 2020). In rapidly uplifting mountain ranges, however, high hillslope relief and steep slopes are the hallmarks of high rates of denudation (Montgomery and Brandon, 2002; Clague and Evans, 2003; Korup et al., 2007; DiBiase et al., 2010), which suggests that rock mass strength and landslide rates in tectonically active mountain ranges are positively related, at least on long timescales.
This apparent contradiction may arise from scales of study. Laboratory experiments are based on ‘hand’ samples, typically without fractures and in an environment with a non-varying “climate” (Hoek, 1983; Sklar and Dietrich, 2001; Saroglou et al., 2018). It is true that non-fractured hard rocks are more difficult to abrade or fail than fractured soft rocks, and it is also true that hard rock will host high-relief landscapes. All other factors being equal, high-relief landscapes in hard rock should erode slowly. It therefore follows that high-relief landscapes are prone to erosion due to other factors, for example fracture density, weathering, rainstorms, or rapid uplift (Korup, 2008; Moore et al., 2009; Clarke and Burbank, 2010).
This discussion highlights the need to further examine the relationship between rock mass strength and landslides. This relationship has implications, not only for rock mass strength, but also for external factors such as rock uplift, river incision, earthquakes, and rainfall, which clearly drive landslides in tectonically active mountain ranges. Many researchers have assumed or argued that relatively high rock mass strength, even in areas of high relief and steep slopes, leads to lower landslide frequency (Corominas and Moya, 2008; Qi et al., 2010; Wang et al., 2014). However, if there is a positive relationship between rock mass strength and denudation driven by landslides, the hazard posed by slope failure might be higher than assumed based on strength alone. In this study, we focus on the relationship between rock mass strength at the hillslope scale and landslides in the tectonically active mountains of southeastern Tibet. We document the spatial distributions of 294 large bedrock landslides in three study areas and infer their relations to inferred rock mass strength, relief, and possible triggering factors. Using lidar bare-earth imagery with 10 m resolution and relief and slope values for historical landslides (Figs. 1b and c), we back-calculate rock mass strength at the landscape scale using Culmann's two-dimensional slope stability model (Culmann, 1875; Schmidt and Montgomery, 1995).
Section snippets
Study area
To explore the relationship between rock mass strength at the hillslope scale and landslides, we selected for our study the Langxian (LX), Tongmai (TM), and Lulang (LL) areas, located in the Eastern Himalayan syntaxis in southeastern Tibet (Fig. 1). The LX study area (640 km2) is dominated by phyllite, granite, and conglomerate (Chen et al., 2004; Xu et al., 2015a; Liu et al., 2020) (Fig. 2c); most of the landslides in this area have occurred within phyllite and conglomerate.
Topographic measurements and landslide size distribution
We identified 294 large bedrock slumps and rotational rockslides (minimum area 6 × 105 m2) and 1289 slope units (Fig. 1) using hillshade and slope maps derived from a 2014 10 m-resolution DEM, satellite imagery, and helicopter-based remote sensing imagery. Differences in slope, slope aspect, and roughness allowed us to accurately delineate landslides and to distinguish source areas from deposits (Fig. 2a, b; McKean and Roering, 2004). In plotting the boundaries of landslide and slope units, we
Topographic characteristics of landslides
The constant γ in the relationship between landslide area and volume (V = α×Aγ) is related to the properties of rock or soil (Larsen et al., 2010). We used a value of γ = 1.526 (σ = 0.07) for our dataset (Fig. 3), which is near the upper end of the range of values (1.3–1.6) in a global dataset of bedrock landslides (Larsen et al., 2010). The range of values of volume in the volume-area scaling relationship (V = 0.009 × A1.526, R2 = 0.60) reflects uncertainty in defining the area of landslide
Relationship between rock mass strength and landslide occurrence
Landslide volumes in sub-areas E and F of LX are higher than those in the other four sub-areas (Fig. 8). In addition, rock mass strengths of hillslopes in areas E and F are higher than those of hillslopes in the other sub-areas (Fig. 7). This result is contrary to the previously argued inverse relationship between landslides and rock mass strength (Sklar and Dietrich, 2001; Stock et al., 2005; Molnar et al., 2007; Moore et al., 2009; Wang et al., 2020). Hence, we infer that, at the landscape
Conclusions
Our estimates of rock mass strength in an area of ~1320 km2 in southeast Tibet range from 60 to 770 kPa. We show that more landslides have occurred on the hillslopes with greater rock mass strength at the landscape scale than on hillslopes with lower rock mass strength. We conclude that Culmann's (1875) slope stability model explains this relationship. At the landscape scale, rock mass strength strongly affects the stability of slopes in mountain regions with high rates of uplift. Large
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by the National Key R&D Program of China (No. 2019YFC1509703), the Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (Grant No. 2019QZKK0904), the Foundation of China Scholarship Council.
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