In recent years, there has been important progress in the experimental study of earthquakes (‘laboratory earthquakes’). Much has been learned about the character and dynamics of rupture fronts propagating along a frictional interface. These fronts are the vehicle with which the contacts composing a frictional interface break, therefore enabling slip. These fronts were shown to be identical to shear cracks.
This work only considered propagating rupture fronts after their nucleation, i.e. an already existing crack. The nucleation mechanisms of interface ruptures and the creation of the initial crack remain fairly unknown, as data is nearly nonexistent. The experimental difficulties stem from the difficulty to predict when, where, and how rupture nucleation takes place. As a result, detailed measurements of these important processes at the necessary spatial and temporal scales are nearly impossible.
We present experiments in PMMA where the real contact area defining a frictional interface between two blocks is imaged in real time. We dictate the nucleation location ahead of time, by the imprinting narrow (high fracture energy) barriers at which propagating ruptures first arrest. The singular stress field formed by the static arrested crack, will later drive rupture nucleation at the other side of the barrier. Once nucleation takes place an ‘ordinary’ rupture will eventually propagate away from the nucleation region. In this way, we control the location of the nucleation, measure it in detail, and study its characteristics. In particular, we measure the exact location of the nucleation and the ‘nucleation time’ required for a rupture to develop. We also use the imaging of the surface to examine the transition of the (2D) nucleation 'patch' into a propagating (1D) front