Collaborative Research: Constraints From Fault Roughness on the Scale-dependent Strength of Rocks

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The strength of crustal rocks is a fundamental factor in tectonic processes: fault motion, mountain building and crustal evolution all affect and are affected by rock strength. Despite its central importance, crustal rock strength is difficult to measure at field scales. Laboratory experiments constrain strength at sub-meter scales, but those results imply that strength is scale-dependent: large rocks are weaker than small ones. This problem is particularly serious in fault zones. Understanding of fault strength is largely based on laboratory experiments. Extending these well-controlled laboratory experimental results to natural faults is one of the major problems of fault and rock mechanics. This project explores a new approach based on the idea that fault surface roughness provides strength estimates at a wide range of scales. The study involves laboratory measurements at very small scales combined with computer modeling and direct observations of fault surfaces. Result will provide a quantitative understanding of fault friction that can be used to predict fault friction for the range of scales and geometries found in the Earth, information essential for the improved understanding of earthquake mechanics. Additional desired societal outcomes of the project include development of a globally competitive STEM workforce through graduate student post-doctoral fellow training.There is an intimate link between fault surface roughness and strength. The yielding of asperities controls surface friction by dynamically adjusting the real area of contact in response to a load. This yielding process can control the topography on the fault surface. This project uses the observed, preserved roughness to infer the yield criteria. Since roughness occurs on multiple scales on faults, the strength (failure criterion) at a variety of scales can be inferred. The goal of this research is to make the link between fault roughness and bulk material strength properties. The first step in investigating the proposed connection between fault roughness and material strength is to measure strength directly on fault surface samples that have the observed roughness relationship. In particular, the researchers aim to understand the scale dependence of both brittle and plastic strength, and to understand the expected transition from brittle to plastic deformation with decreasing length scale. To accomplish these goals, they will use a combination of indentation and nanopillar experiments on natural fault samples to obtain a robust set of strength measurements. These results will be compared to roughness at comparable scales using Atomic Force Microscopy to measure roughness on the same samples. The next step is to establish the relevant modes of failure at various scales on natural surfaces by: (a) predict the dominant failure mode at relevant scales using the laboratory values; (b) use the observation of the minimum scale of grooving to isolate the process that separates failure modes; and (c) investigate smaller scales where the failure mode is determined by the absolute strength of the material. The research team will explore the implications of the measurements for friction by simulating the elastoplastic deformation of a rough fault using the hardness values as measured on the samples and then use the brittle failure criterion inferred from the nanopillar experiments to calculate the shear stress required for motion of the deformed surface and compare the results to typical values of fault friction.