Interdependencies between Landslides, Rock Strength, and Landscape Evolution in the Himalaya, Central Nepal

Abstract

Bedrock strength modulates landslide susceptibility and the relationship between erosion rates and topography, making it fundamentally important to natural hazard assessment and landscape evolution. However, regional patterns in rock strength are rarely known quantitatively due the complex array of factors that induce variability in the shallow subsurface through weathering and fracturing, and the limited spatial scale cast by many of the empirical methods available to make quantitative measurements. This dissertation addresses the above challenges with novel field-based and modeling approaches, and it explores how systematic variations in rock strength manifest in landslide characteristics and orogenic landscape dynamics.

Chapters 2 and 4 apply statistical modeling and inverse slope stability analysis to explore how landslide geometries reflect strength properties and topography. Model results suggest that, all else equal, steeper and deeper landslides reflect higher shear strength along the failure plane, and that the average landslide size reflects a balance between the space allowed by topographic relief and the cohesive strength of hillslope material. These modeling insights are consistent with empirical analysis of eight landside inventories in Chapter 2, leading to the interpretation that landslide size statistics are characteristic of the local rock strength and topographic structure. The landslide back-analysis model presented in Chapter 4 builds upon this result with quantitative strength estimates based on each individual landslide geometry. In applications to two digital landslide inventories, the back-calculated strength properties are found to be relatively low for rock and mostly explained by the frictional resistance between blocks, which is interpreted to reflect the weathered and fractured nature of regolith in the shallow subsurface.

Chapters 3 and 5 explore patterns in rock strength and the factors that control it with field observations in central Nepal. The field surveys use a novel sampling strategy consisting of 1D seismic surveys and engineering rockmass characterizations that are quick to apply repeatedly and provide in situ rock mechanical information over bulk scales (tens of meters). Chapter 3 shows that rock strength varies widely in central Nepal due to variability in chemical and physical weathering - even within the same rock type. While no single factor explains the variability in weathering characteristics, the degree of weathering tends to decrease downslope from ridges to channels. This result is consistent with previous hydrologic and fracture-based models for weathering front advance, but it has not previously been demonstrated to be a dominant factor controlling patterns in weathering over regional scales. Chapter 5 then evaluates the rock strength observations from Chapter 3 against the long term (1-5 Myr) tectonic history and erosion rates across the Himalaya. It is observed that rock strength is highest where weathering is most limited and erosion rates are fastest. Limited weathering is interpreted as the consequence of high erosion rates that reduce the time bedrock spends in the near surface environment. Less weathering results in stronger rock masses, which support steeper slopes and channel gradients. This adjustment to steeper landscapes by increasing rock strength explains how the landscape keeps pace with increasing tectonic uplift rates.