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 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- Award ID(s):
- 1848192
- NSF-PAR ID:
- 10492571
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 25
- Issue:
- 2
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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