Friction, Earthquakes and Everything in Between

Friction is generally described by a single degree of freedom, a ‘friction coefficient’. We experimentally study the space-time dynamics of the onset of frictional motion when two contacting bodies start to slide. We first show that the transition from static to dynamic sliding is governed by rupture fronts (essentially earthquakes) that break the contacts along the interface separating the two bodies. Moreover, we show that the structure and dynamics of these ‘laboratory earthquakes’ is quantitatively described by singular solutions originally derived to describe the motion of rapid cracks under applied shear. We demonstrate that this framework essentially replaces the idea of a ‘friction coefficient’ as a description for the onset of frictional motion. Moreover, this framework quantitatively describes both ‘laboratory earthquake’ motion and arrest. The results establish a new (and fruitful) paradigm for describing friction and provide a fundamental understanding of how earthquakes develop, progress and eventually arrest.