The increase in the frictional strength of rocks with the time of quasi‐stationary contact, known as frictional aging, may ultimately determine whether unstable slip (i.e., earthquakes) can nucleate. In spite of its importance, the physical mechanism that underlies frictional aging in rocks is still uncertain. The widely held view is that aging results from an increase in contact area due to asperity creep. Here we show via nanoindentation testing that the hardness and creep rate of quartz are independent of relative humidity from <10−4% to 50%. This contrasts strongly with the standard interpretation of previous friction experiments on quartz tested over a similar humidity range, which reveal an absence of frictional aging for humidity <5%. Our results demonstrate that frictional aging in quartz cannot result from asperity creep and instead argue in favor of other mechanisms, including time‐dependent chemical bond formation or slip‐induced strengthening.
The evolution of fault friction during the interseismic period affects the mechanics of a future earthquake on the same fault patch. Frictional aging has been previously tied to time‐dependent contact area growth through observations made on rock analogs. However, our understanding of the processes that control frictional aging is limited and is dependent on experiments that explore only numerous mechanisms. We conduct slide‐hold‐slide experiments with a dual‐axis nanoindenter on single‐crystal surfaces of quartz and calcite. Our results show that frictional aging in diamond‐quartz contacts is independent of time and contact area, in stark contradiction to past experiments done on quartz‐quartz contacts in rocks. Diamond‐calcite contacts show modest frictional aging, but still well below previous reported values from calcite‐calcite contacts. These results suggest that frictional aging of like‐on‐like minerals may be of chemical origin, as suggested in recent studies with atomic force microscopy and molecular dynamics simulations.
more » « less- NSF-PAR ID:
- 10495706
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 51
- Issue:
- 4
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract 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;
https://doi.org/10.1002/2017JB013970 ), where the specification of the surface distribution of asperity ensembles and their geometry facilitates the numerical construction of the state variable in the RSF equation. Results show that this new model's fit to the slide‐hold‐slide experiments is similar to the best fits of the Slip law, while it provides an improved physical picture; however, for velocity steps it fails to match the symmetry of step ups and downs, although the general fit is acceptable. By introducing two new parameters to specify the slip‐hardening mechanism, we allow the model to incorporate as a subset the pure time‐dependent model discussed in Li and Rubin (2017). Although failing to completely reconcile time‐ and slip‐dependent friction evolution, this study produced useful insights for future research; for example, a granular numerical description of the frictional surface might be convenient for modeling the physics of friction. -
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Abstract 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
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 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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