Current Research in EEPS: Dr. Behrooz Ferdowsi, University of Houston
How much of the transient rheological behavior of geological shear zones can be explained using granular physics alone?
Geological shear zones exhibit a curious transient response to perturbations in sliding velocity, that comprises an immediate change (“direct effect”) in friction upon perturbations, followed by a gradual evolution of friction toward its steady-state at the new velocity. This transient response goes under the umbrella of Rate- and State-dependent Friction (RSF), and it controls the initiation of numerous geological instabilities with a frictional origin (e.g., earthquake nucleation, landslides, sliding of glaciers). Yet the underlying physics of the transients continues to be debated. The conventional wisdom is that the RSF behavior arises from the time-dependent plastic flow or chemistry at microscopic contact points in the shear zone, however, mathematical models developed based on this idea cannot capture important features observed in the lab. Motivated by observations that faults and other shear zones in the Earth are invariably filled by fragmented rock (gouge) and sediments, here I explore an alternative model in which variations in friction derive simply from granular rearrangements in a localized (gouge) layer, with no rate- or state-dependence at individual grain/grain contacts. I investigate the behavior of the model in simulated laboratory experiments in which a gouge layer is subjected to large variations in slip rate (“velocity-stepping” protocols) and find that the model accurately describes important features (that are almost universally) observed in laboratory velocity-step data. The model also produces some of the most common observations in “slide-hold-slide” protocols, long used to measure the amount of frictional strengthening that occurs during fault “holds”. I next use the energetic properties of the model to explain the emergence of the direct effect seen in the transient response of the model and predict the magnitude of the effect. For the majority of this work, I use a Hertzian contact law (with a non-linear relationship between deformation and force at grain-grain contacts) for grain-grain interactions and such a model produces a rate-strengthening friction in the (quasi-static) range of shear rates explored here. However, I find that a model with Hookean contact law (with a linear relationship between deformation and force at grain-grain contacts) shows a transition from rate-weakening to rate-strengthening at some intermediate shear rates. In the rate-weakening regime, the model also produces stick-slip instabilities as expected in the RSF framework. However, the critical stiffness at which the instabilities emerge is an order of magnitude larger than that predicted by the RSF. I will conclude by discussing the implications of the study for origins of transient friction on faults, and if time allows, will discuss some of my ongoing and future research directions for further probing the physics of mathematical models (of transient friction) commonly used for forecasting near-surface geological processes and hazards.