Congratulations to William Medwedeff who successfully defended his dissertation on Monday January 10th, 2022.
Advisor: Marin Clark
Bedrock strength modulates erosion rates and landslide susceptibility in mountainous topography, making it fundamentally important to landscape evolution and natural hazard assessment. 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 geophysical 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 physics-based slope stability back-analysis to explore how landslide geometries reflect rock 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 landslide inventories in Chapter 2, leading to the interpretation that landslide size statistics are characteristic of the regional rock properties and topographic structure. The landslide model developed in Chapter 4 continues this research direction by inverting the geometries of individual landslides for the strength of the material that failed. In applications to two earthquake-triggered 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 consistent with field observations of bedrock weathering and fracturing in the study areas.
Chapters 3 and 5 explore patterns in rock strength and the factors that control it with a novel field sampling strategy consisting of 1D seismic surveys and engineering rockmass characterizations. 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, which is consistent with previous hydrologic and fracture-based models for weathering front advance. Chapter 5 then evaluates the same rock strength observations against the long term (1-5 Myr) tectonic history and erosion rates, which are inferred from apatite and zircon (U-Th)/He thermochronometry. Counterintuitively, it is found that weathering is most limited - and rock strength highest - where erosion rates are fastest. This observation is interpreted to reflect the inability of chemical weathering processes to keep pace with rapid denudation in the orogenic core of the Himalaya, and it implies that erosion rates in the Himalaya are more sensitive to slope gradients and elevation above base level rather than rock erodibility.