Collaborative Research: Rock Friction, Nanoindentation, and Atomic Force Microscope Experiments Focused on Understanding Earthquake Mechanics

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Significance and importance of the project. The initiation of earthquakes on faults in the Earth?s crust is controlled, incredibly, by physical processes that occur at microscopic contacts between rough surfaces of rock that touch along the fault. Despite the success of empirical friction equations in describing the results of laboratory friction experiments and producing a variety of earthquake-related phenomena in computer models of earthquakes, these equations lack a physical basis. That is, the precise identity and nature of the physical mechanisms that occur at microscopic contacts, and give rise to the observed friction effects in experiments, remain unknown. The empirical nature of the descriptions reflects the difficulty of isolating and studying the physical processes at microscopic fault contacts. Without a sound physical understanding, we remain limited in our abilities to reliably apply the equations to earthquakes in nature, to obtain a better general understanding of the earthquake process, and to ultimately make reliable predictions of earthquakes. In this transformative study, we will make use of state-of-the-art materials science testing methods, namely atomic force microscopy and Nanoindentation, to provide a physical basis for friction observations at a coarser scale and thereby gain a much improved understanding of the earthquake process. This work may allow us to learn whether we are likely to be able to detect accelerating creep on faults just prior to an earthquake and thereby predict earthquakes days to hours before an earthquake, which would save many lives and mitigate damages to the built environment. From the perspective of the scientific fields of mechanics and materials science, these new insights, gained by identifying and connecting frictional behavior across many length scales, have potential application well beyond geophysics, for example, in many engineered systems, including silicon-based micromechanical devices. Technical description. The overarching goals of the proposed studies are to isolate and identify the physical mechanisms that occur at asperity contacts at frictional interfaces. A more specific major goal of our study is to understand the origin of the friction state ?evolution? effect in rate and state friction, the simplest manifestation of which is an increase in ?static? friction with the time of quasi-stationary contact. To that end, we will conduct a coordinated, interdisciplinary collaboration that will employ laboratory experiments that investigate frictional phenomena over a wide range of length scales. One outcome will be to develop constitutive equations that will allow extrapolation of these mechanisms to the elevated temperatures and longer times relevant for earthquakes. We will perform macro-scale friction experiments on rocks at Brown University, micro-scale to nano-scale indentation creep, adhesion, and friction experiments in the Nanoindenters at Oak Ridge National Laboratory, and nano-scale adhesion and friction experiments in atomic force microscopes at the University of Pennsylvania. To make connections between the different types of experiments and to isolate different origins of the state evolution effect, we will vary the same environmental conditions in all three sets of experiments. These include tests as a function of humidity, pH (in liquid), and temperature. All three environmental factors have been demonstrated to influence the evolution effect in macroscopic rock friction experiments. Nanoindentation and AFM measurements should allow us to determine the processes on an asperity scale responsible for the macroscopic behavior.