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 engineering^1-20. It is believed that there exists a threshold shear force, called static friction, below which the interface is stationary^4,10, despite many studies suggesting that this concept is outdated^1,21-28. By contrast, rate-and-state friction formulations^1,26,27 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 occur^9,27-29, 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 correlation^18,31,32. This behaviour contradicts classical models of friction but confirms the prediction of rate-and-state friction^1,26,27. The diminishing slip rates of nominally stationary interfaces reflect interface healing, which would manifest itself in higher peak friction in subsequent slip events^15,27,33, such as earthquakes and landslides, substantially modifying their nucleation and propagation and hence their hazard^3,12,13,34.