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^' ∝ 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 ≈ 6.5 m/s²). 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.

Fracture surface energy of the Punchbowl fault, San Andreas system

Fracture energy is a form of latent heat required to create an earthquake rupture surface and is related to parameters governing rupture propagation and processes of slip weakening. Fracture energy has been estimated from seismological and experimental rock deformation data, yet its magnitude, mechanisms of rupture surface formation and processes leading to slip weakening are not well defined. Here we quantify structural observations of the Punchbowl fault, a large-displacement exhumed fault in the San Andreas fault system, and show that the energy required to create the fracture surface area in the fault is about 300 times greater than seismological estimates would predict for a single large earthquake. If fracture energy is attributed entirely to the production of fracture surfaces, then all of the fracture surface area in the Punchbowl fault could have been produced by earthquake displacements totalling <1 km. But this would only account for a small fraction of the total energy budget, and therefore additional processes probably contributed to slip weakening during earthquake rupture.