THE ROLE OF FAULT FRICTION IN DAMAGE ZONE EVOLUTION

Numerical models of a linear fault show the generation of off-fault tensile failure that results from inelastic slip along the fault. We explore models with a constant coefficient of friction (both strong and weak faults) as well as with slip-weakening friction to compare the damage patterns created from the different fault types. Tensile fractures form where tangential stresses along the fault exceed the tensile strength of the rock. These stresses result from locally high slip gradients. We find that static friction faults generate fractures solely at the fault tip in the tensile quadrant of the fault. Alternatively, the slip-weakening fault generates fractures along the entire fault; fractures form mostly at the rupture tip as the rupture migrates along the fault. These results could guide field studies of small faults as to whether the fault failed in small seismic events or in creep.

In addition to the study of fracture development, we investigate the amount of energy available for additional damage generation through a work budget analysis and internal strain energy density maps for each fault type. The strong fault had the greatest proportion of internal work, which indicates excess energy for microcracking and comminution of the host rock, however the work is dispersed throughout the elastic space and not heavily concentrated around the fault. For the weak and slip-weakening fault models, maps of internal work show areas of high strain energy along the fault that are most likely to create pockets of intense damage. These areas tend to cluster at the rupture tip and around off-fault fractures that slip. The pockets of high internal work are asymmetric around the fault.