Constraints on the Physical Mechanism of Frictional Aging From Nanoindentation

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. Plain Language Summary The friction coefficient of rocks increases with the length of time they are held in stationary contact, a phenomenon sometimes referred to as frictional aging. However, at low relative humidity (i.e. <5%), frictional aging of silicate rocks does not occur. Some authors have hypothesized that aging is due to creep of the microscopic contacts at which faults actually touch, and that creep is enhanced in humid environments. Here we measured the creep behavior of quartz by indenting it with diamond probe, over a wide range of relative humidity. Our results show that the deformation of quartz is independent of relative humidity. Our results demonstrate that some other mechanism than contact creep must give rise to frictional aging.