The granular physics contribution to rate- and state-dependent fault friction

Princeton University, Princeton NJ

Investigators

Abstract

Friction plays a critical role in many areas of societal interest, including transportation and manufacturing. In Earth Sciences, understanding friction is critical for a better understanding of the hazards associated with earthquakes and landslides. The friction properties of materials have been studied for centuries, but the physics and chemistry underlying their time dependence remain obscure. Yet, small fluctuations in friction properties during earthquakes and landslides can have tremendous effects on the size and speed of these events. Friction on sliding interfaces such as tectonics faults are usually described by the so called "rate- and state-dependent friction" laws. These empirical laws account for the sliding speed ("rate") and for the evolving properties of the interface termed "state"; this latter, a function of the slip history, is difficult to observe directly. The rate-and-state framework is widely used to model frictional sliding. But the corresponding laws fail to accurately describe laboratory observations for a range of conditions relevant to earthquakes. Here, the team aims to better understand the physics underlying the frictional properties of rocks. The researchers use computer simulations to model the behavior of granular layers of finely-ground rock, called gouge, that are present along tectonic faults. The goal is to test whether rock friction and its time dependence is governed at the grain scale by grain-to-grain interactions. The simulation outputs are constrained by experimental observations: in many cases they describe them better than the most successful rate-and-state friction laws. The team, thus, gradually unveils the physics underlying the behavior of earthquake-generating faults. In addition to its strong societal relevance, this project provides support for an early career scientist as well as training for undergraduate students. To model the behavior of the gouge, the researchers employ Discrete Element Method simulations. They use model geometries and loading conditions designed to mimic standard rock-friction experiments, such as "velocity-step" and "slide-hold-reslide" protocols. They test the hypothesis that rock friction as observed in the laboratory is governed by time-independent properties at the grain-grain contact scale. This innovative approach differs from more traditional ones which assume that time-dependent plasticity or chemical bonding at microscopic contacts are the source of the rate-and-state dependence of friction. The granular simulations are consistent with the most successful rate-and-state-dependent friction equations for sliding protocols where those equations accurately describe experiments ("velocity-step" and “slide-hold” protocols). They better match laboratory data for sliding protocols where those equations fail (e.g., the reslides following "slide-hold" protocols). Furthermore, output of the granular simulations allows investigating the source of the rate-and-state-dependent friction-like behavior of the model. The team finds that if the kinetic energy of the gouge particles is suitably normalized by the confining pressure, it produces an estimate of the velocity dependence that is consistent with the simulations and within the ballpark of laboratory data. The researchers continue exploring the granular flow model by comparing it to a wider range of sliding protocols that are not well explained by existing equations (e.g., "slide-hold-reslide" and "normal-stress-step" experiments). They also compare the compaction/dilation of the gouge layers in the simulations to experimental observations; the goal is to evaluate the role of porosity on the gouge sliding behavior. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.