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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.

Earthquakes are natural phenomena that cause ground shaking and damage to people and infrastructures. Despite significant progress achieved in understanding how earthquakes start, propagate, and arrest, many aspects of their physics and mechanics remain not fully detailed due to their intrinsic complexities. While a seismic rupture shares many characteristics with a propagating crack, it can also be described as a sliding process governed by friction. These two frameworks (fracture and friction), which appear to be independent at first glance, may interact in the behavior of frictional ruptures. However, several aspects of this potential interaction are not yet fully explored. Through an experimental approach, this thesis aims to contribute to a better understanding of the aforementioned dual nature (friction and fracture) of seismic ruptures and to study their scale dependence and its impact on the emergence of rupture complexities. The first part investigates, how off-fault measurements can aid in detecting the precursory phase of earthquakes by monitoring the temporal evolution of seismic properties. The second part studies laboratory earthquakes as frictional ruptures within the context of fracture mechanics. The influence of lubricants (representative of both natural and industrial fluids permeating natural faults) was investigated and found to promote fault reactivation, increase nucleation length, and decrease fracture energy characterizing rupture propagation. Moreover, the difference between fracture energy and breakdown work under dry conditions is explored, with the first corresponding to an interface property and the second exhibiting a slip-dependent feature, as a result of on-fault frictional weakening. This mismatch can be reconciled through the emergence of unconventional singularities, caused by the activation of frictional weakening (flash heating), which can have a significant impact on rupture dynamics. Finally, the last section investigates, thanks to the newly developed large biaxial apparatus, the scale effect of frictional ruptures and the complexities emerging when they are reproduced on fault systems greater than the characteristic nucleation size.