The effect of grain size and gouge microstructure on fault slip behavior

John Bedford, & Daniel Faulkner

Published August 15, 2018, SCEC Contribution #8796, 2018 SCEC Annual Meeting Poster #179

Conceptually, brittle fault zones are envisaged to be comprised of a localized fault core, or strands of multiple fault cores, surrounded by a distributed fractured damage zone. The core of fault zones, where the majority of slip is accommodated, generally consist of a granular gouge material formed progressively by cataclasis. Strain within these gouge layers may be highly localized onto discreet slip surfaces and experimental observations have shown that shear localization is a prerequisite for stick-slip instability. Here we investigate the effect of gouge grain size on structural evolution within experimental fault zones and also its effect on the stability of frictional sliding. Different grain size (5, 15 and 30 µm) synthetic quartz gouges are used, a material that is inherently unstable after a small amount of shear strain and characterised by velocity-weakening behaviour thereafter. It is found that the onset of instability in quartz gouge is a function of initial grain size, normal stress and shear strain. For a given grain size, a greater amount of shear strain is required to initiate unstable sliding at lower normal stresses. At a given normal stress, the onset of instability occurs at a lower shear strain for fine-grained gouges. The results show that grain size reduction must occur within the gouge in order to localize deformation and initiate unstable sliding. Initially fine-grained gouges have to undergo less grain size reduction in order to promote the formation of localized slip surfaces than coarse-grained gouges. These results are confirmed by microstructural observations, whereby throughgoing Y-shear planes, which are associated with instability, develop earlier within fine-grained gouge.

Citation
Bedford, J., & Faulkner, D. (2018, 08). The effect of grain size and gouge microstructure on fault slip behavior. Poster Presentation at 2018 SCEC Annual Meeting.

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