The variations in the response of different state evolution laws to large velocity increases can dramatically alter the style of earthquake nucleation in numerical simulations. But most velocity step friction experiments do not drive the sliding surface far enough above steady state to probe this relevant portion of the parameter space. We try to address this by fitting 1–3 orders of magnitude velocity step data on simulated gouge using the most widely used state evolution laws. We consider the Dieterich (Aging) and Ruina (Slip) formulations along with a stress‐dependent state evolution law recently proposed by Nagata et al. (2012). Our inversions confirm the results from smaller velocity step tests that the Aging law cannot explain the observed response and that the Slip law produces much better fits to the data. The stress‐dependent Nagata law can produce fits identical to, and sometimes slightly better than, those produced by the Slip law using a sufficiently large value of an additional free parameter c that controls the stress dependence of state evolution. A Monte Carlo search of the parameter space confirms analytical results that velocity step data that are well represented by the Slip law can only impose a lower bound on acceptable values of c and that this lower bound increases with the size of the velocity step being fit. We find that our 1–3 orders of magnitude velocity steps on synthetic gouge impose this lower bound on c to be 10–100, significantly larger than the value of 2 obtained by Nagata et al. (2012) based on experiments on initially bare rock surfaces with generally smaller departures from steady state.

The transition from static to sliding friction is mediated by rapid interfacial ruptures 1-5 propagating through the solid contacts forming a frictional interface 6 . While propagating, these ruptures correspond to true shear cracks 7 . Frictional sliding is initiated only when a rupture traverses the entire interface 1 ; however, arrested ruptures can occur at applied shears far below the transition to frictional motion 8-17 . Here we show, by measuring the real contact area and strain fields near rough frictional interfaces, that fracture mechanics quantitatively describe rupture arrest and therefore determine the onset of overall frictional sliding. Our measurements reveal both the local dissipation and the global elastic energy released by the rupture. The balance of these quantities entirely determines rupture lengths, whether finite or system-wide. These results confirm a fracture-mechanics-based paradigm 7,15,18 for describing frictional motion and shed light on the selection 18-21 of an earthquake's magnitude.A frictional interface is formed by the interlocked solid asperities of rough surfaces in contact, whose area is much smaller than the nominal one 6 . Failure of the asperities via rupture fronts is the fundamental mechanism responsible for the transition from static to sliding frictional motion 1,4 . Ruptures can propagate well before the onset of global sliding, and then arrest before spanning the entire interface. Rupture arrest can, for example, result from inhomogeneous stress distributions along the interface 8-10 . As no overall motion of the contacting bodies is induced by such events, they are often called precursors to sliding motion. Arrested events are analogous to earthquakes, which are dynamic ruptures of finite extent within pre-existing natural faults; the boundary between contacting tectonic plates 22,23 . Predicting the length of precursory ruptures is, therefore, closely related to the question of what determines the size of an earthquake [18][19][20][21] .Since their initial discovery 8 , a rich variety of models has been dedicated to the dynamics of precursory ruptures in frictional systems. Aimed at reproducing nucleation and arrest, these include minimalistic one-dimensional (1D) models 9 , discrete contacts descriptions 11,13,16 , rate-and-state friction laws 17 and fracture mechanics 12 . These models are able to reproduce the existence of arrested ruptures but they provide no explicit predictions of where and how arrest occurs in real systems. Recent theoretical work 15 explained the available data 8 by explicitly demonstrating how fracture mechanics can be used to predict rupture arrest. Here we describe new experiments that confirm these theoretical predictions and show that this general framework indeed enables us to understand the selection of rupture length for any system geometries and loading conditions.Recent experiments have shown that the strain fields driving ruptures along frictional interfaces are described 7 by the linear elastic fracture mechanics (LEFM; ref. ...

We study rupture fronts propagating along the interface separating two bodies at the onset of frictional motion via high-temporal-resolution measurements of the real contact area and strain fields. The strain measurements provide the energy flux and dissipation at the rupture tips. We show that the classical equation of motion for brittle shear cracks, derived by balancing these quantities, well describes the velocity evolution of frictional ruptures. Our results demonstrate the extensive applicability of the dynamic brittle fracture theory to friction.

The onset of sliding motion is conditional on the propagation of rupture fronts that detach the contacting asperities forming a frictional interface. These ruptures, when propagating over a fault surface, are the most common mechanism for an earthquake. Experimentally, the transition from static to sliding friction takes place when a rupture traverses the entire interface. But ruptures can also arrest before reaching the end of the interface. The determination of the mechanisms responsible for rupture arrest is of particular interest for understanding an earthquake's magnitude selection. Propagating ruptures have been shown to be true shear cracks, driven by singular fields at their tip, and fracture mechanics have been successfully used to describe rupture arrest along homogeneous frictional interfaces. Performing high temporal resolution measurements of the real contact area and strain fields, we demonstrate that the same framework provides an excellent quantitative description of rupture arrest along interfaces with heterogeneous fracture properties and complex stress distributions at a macroscopic scale. This work unravels the different mechanisms responsible for rupture arrest along model laboratory faults. This fracture‐based paradigm opens a window to a wide range of possible consequences for frictional behavior along any two contacting bodies; from the centimeter scale to the scale of natural faults.