On the Scale Dependence in the Dynamics of Frictional Rupture

When an earthquake nucleates in the earth crust, the potential energy accumulated during the inter-seismic period is released into breakdown work, heat energy and radiated energy. Often the breakdown work is considered a seismological equivalent of the fracture energy. However, discrepancies related to the definition of the two are not yet fully solved. To this end, we reproduced frictional ruptures in the laboratory to study the relationship between these two energies. A dual strength weakening is observed, reflected in a scale dependent evolution of breakdown work with fault’ slip, contrarily to fracture energy which is, by definition, scale independent. This behavior shows to be probably caused by thermal weakening (i.e. flash heating) activated during slip and to be well described by the recently developed unconventional theory of frictional ruptures (i.e. rupture driven by a non-square root singularity). Importantly, these results highlight, from an experimental point of view, the presumable unconventional nature of earthquakes, solving the discrepancies between breakdown work and fracture energy. Moreover, it suggests that an analysis of the propagating rupture in the framework of linear elastic fracture mechanics could prove to be not always sufficiently exhaustive when frictional weakenings occur, as it is expected along crustal faults.