G: Fracture energy, friction and dissipation in earthquakes

Recent estimates of fracture energy G (') in earthquakes show a power-law dependence with slip u which can be summarized as G (') ae u (a) where a is a positive real slightly larger than one. For cracks with sliding friction, fracture energy can be equated to G (f) : the post-failure integral of the dynamic weakening curve. If the dominant dissipative process in earthquakes is friction, G (') and G (f) should be comparable and show a similar scaling with slip. We test this hypothesis by analyzing experiments performed on various cohesive and non-cohesive rock types, under wet and dry conditions, with imposed deformation typical of seismic slip (normal stress of tens of MPa, target slip velocity > 1 m/s and fast accelerations ae 6.5 m/s(2)). The resulting fracture energy G (f) is similar to the seismological estimates, with G (f) and G (') being comparable over most of the slip range. However, G (f) appears to saturate after several meters of slip, while in most of the reported earthquake sequences, G (') appears to increase further and surpasses G (f) at large magnitudes. We analyze several possible causes of such discrepancy, in particular, additional off-fault damage in large natural earthquakes.