skip to main content


Title: Constraints on the Physical Mechanism of Frictional Aging From Nanoindentation
Abstract

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.

 
more » « less
NSF-PAR ID:
10454983
Author(s) / Creator(s):
 ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Geophysical Research Letters
Volume:
45
Issue:
24
ISSN:
0094-8276
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    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
  2. 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.

     
    more » « less
  3. Abstract

    The frictional properties of faults control the initiation and propagation of earthquakes and the associated hazards. Although the ambient temperature and instantaneous slip velocity controls on friction in isobaric conditions are increasingly well understood, the role of normal stress on steady‐state and transient frictional behaviors remains elusive. The friction coefficient of rocks exhibits a strong dependence on normal stress at typical crustal depths. Furthermore, rapid changes in normal stress cause a direct effect on friction followed by an evolutionary response. Here, we derive a constitutive friction law that consistently explains the yield strength of rocks from atmospheric pressure to gigapascals while capturing the transient behavior following perturbations in normal stress. The model explains the frictional strength of a variety of sedimentary, metamorphic, and igneous rocks and the slip‐dependent response upon normal stress steps of Westerly granite bare contact and synthetic gouges made of quartz and a mixture of quartz and smectite. The nonlinear normal stress dependence of the frictional resistance may originate from the distribution of asperities that control the real area of contact. The direct and transient effects may be important for induced seismicity by hydraulic fracturing or for naturally occurring normal stress perturbations within fault zones in the brittle crust.

     
    more » « less
  4. Water ice Ih exhibits brittle behavior when rapidly loaded. Under tension, it fails via crack nucleation and propagation. Compressive failure is more complicated. Under low confinement, cracks slide and interact to form a frictional (Coulombic) fault. Under high confinement, frictional sliding is suppressed and adiabatic heating through crystallographic slip leads to the formation of a plastic fault. The coefficient of static friction increases with time under load, owing to creep of asperities in contact. The coefficient of kinetic (dynamic) friction, set by the ratio of asperity shear strength to hardness, increases with velocity at lower speeds and decreases at higher speeds as contacts melt through frictional heating. Microcracks, upon reaching a critical number density (which near the ductile-to-brittle transition is nearly constant above a certain strain rate), form a pathway for percolation. Additional work is needed on the effects of porosity and crack healing. ▪ An understanding of brittle failure is essential to better predict the integrity of the Arctic and Antarctic sea ice covers and the tectonic evolution of the icy crusts of Enceladus, Europa, and other extraterrestrial satellites. ▪ Fundamental to the brittle failure of ice is the initiation and propagation of microcracks, frictional sliding across crack faces, and localization of strain through both crack interaction and adiabatic heating. 
    more » « less
  5. Abstract

    The constitutive behavior of faults is central to many interconnected aspects of earthquake science, from fault dynamics to induced seismicity, to seismic hazards characterization. Yet, a friction law applicable to the range of temperatures found in the brittle crust and upper mantle is still missing. In particular, rocks often exhibit a transition from steady‐state velocity‐strengthening at room temperature to velocity‐weakening in warmer conditions that is poorly understood. Here, we investigate the effect of competing healing mechanisms on the evolution of frictional resistance in a physical model of rate‐, state‐, and temperature‐dependent friction. The yield strength for fault slip depends on the real area of contact, which is modulated by the competition between the growth and erosion of interfacial micro‐asperities. Incorporating multiple healing mechanisms and rock‐forming minerals with different thermodynamic properties allows a transition of the velocity‐ and temperature‐dependence of friction at steady‐state with varying temperatures. We explain the mechanical data for granite, pyroxene, amphibole, shale, and natural fault gouges with activation energies and stress power exponent for weakening of 10–50 kJ/mol and 55–150, respectively, compatible with subcritical crack growth and inter‐granular flow in the active slip zone. Activation energies for the time‐dependent healing process in the range 90–130 kJ/mol in dry conditions and 20–65 kJ/mol in wet conditions indicate the prominence of viscoelastic collapse of microasperities in the absence of water and of pressure‐solution creep, crack healing, and cementation when assisted by pore fluids.

     
    more » « less