Mechanics of Fault-Tip Deformation in Brittle and Ductile Faults: Laboratory Test of Off-Fault Yield Models & Fracture Energy Budget

Taka Kanaya, & Greg Hirth

Published August 14, 2018, SCEC Contribution #8563, 2018 SCEC Annual Meeting Poster #178 (PDF)

Off-fault damage was characterized for propagating fault tips in experimentally deformed granular rocks, in conjunction with fracture mechanics analyses of fault-tip stress fields. (1) Tip regions of dynamic brittle faults formed at 20°C show microcrack damage consistent with compressional yielding through Hertzian cracking. Microcracks increase in density towards the fault, with higher densities and fault-parallel fabrics on the compressional side, in agreement with those expected for faults stressed at low angles. In contrast, quasistatic ductile fault-tips formed at 900°C display anastomosing shear bands emanating from the tip with limited Hertzian cracking. (2) For dynamic faulting, the observed damage zones are orders of magnitude thinner than predicted using a Mohr-Coulomb yield criterion. We propose that strengthening mechanisms, such as rate-dependence, may be considered for the formation of damage zones involving extreme loading rates. Conversely, for quasistatic faulting, the observed extent of shear damage agrees with that expected for shear yielding using a yield criterion constrained from our high-temperature tests. (3) Energetic analyses of fault tips suggest that, for brittle faulting, microcracking accounts for almost all fracture energy, in which dissipation occurs mostly within the central gouge layer, but little outside. In contrast, the large fracture energy associated with ductile faulting results from frictional dissipation along anastomosing shear bands. We conclude that increasing temperature induces a transition in fault rupture mode from dynamic to quasistatic, because the activation of major fault-tip yielding results in an increase in the fracture energy. Furthermore, our findings - the temperature dependence of damage zone dissipation - have an important implication for whether fracture energy increases with fault size. For brittle faulting, the negligible damage zone dissipation observed suggests that, with increasing fault length, fracture energy does not increase as fast as energy release rate, resulting in earthquake runaway. In contrast, for ductile faulting, the significant damage zone dissipation observed suggests that, with increasing fault length, fracture energy increases as damage zone expands, leading to rupture self-arrest and slow slip.

Citation
Kanaya, T., & Hirth, G. (2018, 08). Mechanics of Fault-Tip Deformation in Brittle and Ductile Faults: Laboratory Test of Off-Fault Yield Models & Fracture Energy Budget. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)