Fault stability transition with slip and wear production: laboratory constraints

Large earthquakes take place on mature faults with hundreds of meters to kilometres of cumulative slip. At shallow depths, the fault zone is generally composed of non-cohesive rock wear products, often referred to as gouge. Seismic and aseismic slip occur in this fault gouge and fracture/brecciation of the wall rock and damage zone can add to the fault gouge as part of the wear process. Gouge thickness generally increases linearly with the cumulative fault shear displacement and laboratory work shows that gouge tends to stabilize fault frictional stability. Previous works show that frictional stability of simulated fault gouge varies as a function of shear displacement. The stability evolution is interpreted as a consequence of the degree of shear localisation within the simulated fault gouge: the more the deformation is localized, the more the fault slip is unstable. This implies that for bare rock surfaces, unstable behaviour is expected as the deformations are forced to be localized at the interface between the two sheared surfaces.

On natural faults at large shear displacement (or for faults having a high gouge production rate), a competition must take place between 1) the localization of the deformation at rock-on-rock surfaces, 2) the delocalization of deformation due to gouge production and wall rock brecciation, 3) fault zone lithification and frictional healing and 4) shear localization within the gouge and wear material. The competition and interaction between these phenomena are modulated by cumulative fault slip, temperature and fluid chemistry. In turn, this competition may influence the frictional stability of faults with increasing shear displacement, and thus, their potential seismic activity.

To characterise the influence of shear displacement on fault stability, constant velocity and velocity step experiments were performed to large displacement. Two initially intact rocks were chosen as starting material: a high porosity Fontainebleau sandstone and a low porosity quartzite. These samples represent very different resistances to abrasion (i.e., wear production with slip) for the same initial mineral composition (< 95% quartz), which allows us to investigate wear and wear rate on fault stability. Additionally, simulated quartz gouge was tested for comparison. Mechanical data are analysed within the rate-and-state framework, and post-mortem microscopic analyses of the sample were performed. For initially bare surface experiments a threshold shear displacement is required to transition from stable to unstable sliding. Stick-slip events (laboratory earthquakes) evolve systematically as a function of fault zone shear displacement. The inversion of the rate-and-state parameters shows that shear displacement has a dominant influence on both (a-b) and D_c. For all the faults tested, (a-b) decreases with increasing shear displacement. For high wear rates and simulated gouge, D_c decreases with increasing shear displacement. However, for low wear rate faults, D_c is constant within the tested shear displacement. These results demonstrate that, under the tested boundary conditions, fault stability varies systematically with fault maturity and in particular that shear displacement and strain localization are the dominant parameters controlling fault slip stability.