Sliding and healing of frictional interfaces that appear stationary

Frictional interfaces are found in systems ranging from biological joints to earthquake faults. When and how these interfaces slide is a fundamental problem in geosciences and engineering1–20. It is believed that there exists a threshold shear force, called static friction, below which the interface is stationary4,10, despite many studies suggesting that this concept is outdated1,21–28. By contrast, rate-and-state friction formulations1,26,27 predict that interfaces are always sliding29, but this feature is often considered an artefact that calls for modifications30. Here we show that nominally stationary interfaces subjected to constant shear and normal loads, with a driving force that is notably below the classically defined static friction for which creep is known to occur9,27–29, are sliding, but with diminishingly small rates down to 10−12 m s−1. Our precise measurements directly at the interface are enabled by digital image correlation18,31,32. This behaviour contradicts classical models of friction but confirms the prediction of rate-and-state friction1,26,27. The diminishing slip rates of nominally stationary interfaces reflect interface healing, which would manifest itself in higher peak friction in subsequent slip events15,27,33, such as earthquakes and landslides, substantially modifying their nucleation and propagation and hence their hazard3,12,13,34. Digital image correlation measurements show that nominally stationary interfaces subjected to constant shear and normal loads are sliding at extremely small rates, confirming the predictions of rate-and-state friction formulations.