The empirical constitutive modeling framework of rate‐ and state‐dependent friction (RSF) is commonly used to describe the time‐dependent frictional response of fault gouge to perturbations from steady sliding. In a previous study (Ferdowsi & Rubin, 2020), we found that a granular‐physics‐based model of a fault shear zone, with time‐independent properties at the contact scale, reproduces the phenomenology of laboratory rock and gouge friction experiments in velocity‐step and slide‐hold (SH) protocols. A few slide‐hold‐slide (SHS) simulations further suggested that the granular model might outperform current empirical RSF laws in describing laboratory data. Here, we explore the behavior of the same Discrete Element Method (DEM) model in SH and SHS protocols over a wide range of sliding velocities, hold durations, and system stiffnesses, and provide additional support for this view. We find that, similar to laboratory data, the rate of stress decay during SH simulations is in general agreement with the “Slip law” version of the RSF equations, using parameter values determined independently from velocity step tests. During reslides following long hold times, the model, similar to lab data, produces a nearly constant rate of frictional healing with log hold time, with that rate being in the range of ∼0.5 to 1 times the RSF “state evolution” parameter
In rate and state friction (RSF) theory, time‐dependent evolution (represented by the Aging law) and slip‐dependent evolution (represented by the Slip law) both succeed in explaining some features of the friction curves while failing in explaining others. Nevertheless, experimental results provided strong evidence for the two ideas and suggested that they are both critical components in friction evolution. Making a first attempt toward reconciling these two ideas, we developed a new friction model for RSF evolution assuming combined physical mechanisms that highlight asperities' plastic behavior: the time‐dependent growth of asperity contact area and the slip‐dependent enhancement (slip strengthening) of asperity intrinsic strength. Our model adopts a two‐scale mathematical structure developed by Li and Rubin (2017;
- NSF-PAR ID:
- 10455920
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 124
- Issue:
- 2
- ISSN:
- 2169-9313
- Page Range / eLocation ID:
- p. 1838-1851
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract b . We also find that, as in laboratory experiments, the granular layer undergoes log‐time compaction during holds. This is consistent with the traditional understanding of state evolution under the Aging law, even though the associated stress decay is similar to that predicted by the Slip and not the Aging law. -
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Abstract This letter compares the predictions of two expressions proposed for the porosity evolution in the context of rate and state friction. One (Segall & Rice, 1995,
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https://doi.org/10.1002/essoar.10509831.110.1002/essoar.10509831.1 ). Values of the frictional stability parameter (a –b ) determined from oscillations reveal dominantly velocity‐weakening behavior across the entire range of experimental conditions. However, values of (a–b ) determined from velocity steps in the same experiments yield velocity‐strengthening behavior. We also show that the elastic stiffness of the ice‐rock system depends on the temperature, and is unlikely to be explained by changes in the elastic properties of ice. Load point velocity oscillations induce oscillations in applied shear stress. Many natural fault systems exhibit slip behaviors that depend on harmonic oscillations in applied tidal stresses. Our new method provides a way to study how frictional properties directly depend on parameters relevant to tidal forcing, and how oscillatory loading must be considered when extracting friction parameters. -
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Abstract Rate‐ and state‐dependent friction (RSF) equations are commonly used to describe the time‐dependent frictional response of fault gouge to perturbations in sliding velocity. Among the better‐known versions are the Aging and Slip laws for the evolution of state. Although the Slip law is more successful, neither can predict all the robust features of lab data. RSF laws are also empirical, and their micromechanical origin is a matter of much debate. Here we use a granular physics‐based model to explore the extent to which RSF behavior, as observed in rock and gouge friction experiments, can be explained by the response of a granular gouge layer with time‐independent properties at the contact scale. We examine slip histories for which abundant lab data are available and find that the granular model (1) mimics the Slip law for those loading protocols where the Slip law accurately models laboratory data (velocity‐step and slide‐hold tests) and (2) deviates from the Slip law under conditions where the Slip law fails to match laboratory data (the reslide portions of slide‐hold‐slide tests), in the proper sense to better match those data. The simulations also indicate that state is sometimes decoupled from porosity in a way that is inconsistent with traditional interpretations of “state” in RSF. Finally, if the “granular temperature” of the gouge is suitably normalized by the confining pressure, it produces an estimate of the direct velocity effect (the RSF parameter
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