  Potential energy stored during the inter-seismic period by tectonic loading
around faults is released during earthquakes as radiated energy, heat and
fracture energy. The latter is of first importance since it controls the
nucleation, propagation and arrest of the seismic rupture. On one side,
fracture energy estimated for natural earthquakes (breakdown work) shows a
clear slip-dependence. On the other side, recent experimental studies
highlighted that, fracture energy is a material property limited by an upper
bound value corresponding to the fracture energy of the intact material
independently of the size of the event. To reconcile these contradictory
observations, we performed stick-slip experiments in a bi-axial shear
configuration. We analyzed the fault weakening during frictional rupture by
accessing to the near-fault stress-slip curve through strain gauge array. We
first estimated fracture energy by comparing the measured strain with the
theoretical predictions from Linear Elastic Fracture Mechanics and a Cohesive
Zone Model. By comparing these values to the breakdown work obtained from the
integration of the stress-slip curve, we show that, at the scale of our
experiments, fault weakening is divided into two stages; the first one
consistent with the estimated fracture energy, and a long-tailed weakening
corresponding to a larger energy not localized at the rupture tip, increasing
with slip. Through numerical simulations, we demonstrate that only the first
weakening stage controls the rupture initiation and that the breakdown work
induced by the long-tailed weakening can enhance slip during rupture
propagation and allow the rupture to overcome stress heterogeneity along the
fault. We conclude that the origin of the seismological estimates of breakdown
work could be related to the energy dissipated in the long-tailed weakening
rather than to the one dissipated near the tip.
