Earthquake Nucleation: The formation of rupture fronts and the influence of local conditions

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, whose propagation characteristics are fully described by the framework of fracture mechanics.

However, the formation of these fronts - the nucleation process - is not yet fully understood. This process is not included in the fracture mechanics framework, which describes only cracks that are above the critical (Griffith) length needed for propagation, and does not provide us an explanation on how a small defect grows and reaches this critical point. In laboratory experiments, obtaining a detailed description of nucleation is a challenge, due to its unpredictable nature.

We use an experimental system in which the real contact area between 2 PMMA blocks is continuously imaged into a fast camera to record the dynamics at the onset of frictional motion and to monitor the propagation of the rupture fronts. In order to overcome the difficulties of the unpredictable nucleation process, a unique experimental technique is used to dictate the nucleation location, thus enabling the direct measurements of the nucleation time and local stresses at the nucleation point. In these controlled experiments, the dynamics of the nucleation process during the slow expansion of a nucleation patch are recorded in detail, as well as the transition to the fast propagation of the newly formed front.

We find that the expansion of the nucleation patch is qualitatively different than the propagation of the fully formed rupture front. It occurs at extremely slow and constant velocities, and it is 2D in nature. Some of the features of this expansion, like self-similar evolution and timescales that are stress-dependent, are general. However, the details of this process are governed by the local conditions at the nucleation region. Due to the slow rates of expansion, local variations in the surface toughness (the ‘fracture energy’) can influence characteristics such as the exact nucleation point, the shape of the patch, and the stress threshold that is needed for nucleation to occur.

As nucleation is not described by the usual frameworks that are used to explain rupture propagation, understanding the driving mechanism of it is of fundamental importance to questions ranging from earthquake nucleation and prediction to processes governing material failure. We propose a possible mechanism for this process and discuss it.