NON-LINEAR SLIP-WEAKENING IN A ROTARY GOUGE FRICTION EXPERIMENT

摘要

Two families of friction law are classically introduced to describe mechanicalfault instabilities: slip weakening and rate and state friction laws. Generallyopposed, we propose here, on the basis of our experimental results, to combinethem in a single unified law. Using a large-displacement ring-shear apparatusfor thick gouge samples (confinement: 0.1-0.5 MPa), we observe that slip-weakening largely dominates rate and state effects. The effective friction coef-ficient μ_(eff)is found to undergo a power-law decrease with imposed slipδ: μ_(eff)= μ_0+Aδ~(-β),with β = 0.4. Although no characteristic length scale really exists, the main decrease ofeffective friction occurs over about 50 cm of slip. This appears quantitativelyconsistent with seismological data both in terms of typical weakening distancesand characteristic rupture energies. Rate and state effects are involved over sig-nificantly smaller scales: dc ≈ 100 μm. On the micro-scale, grain attrition exists.An image correlation technique reveals that the active interface is significantlywider than suggested by the microstructures. Slip-weakening appears caused bya progressive mechanical decoupling between a shear band and the bulk of thesample. Direct normal stress measurements along the shear interface have beenpossible and show a negligible influence of hoop stresses. Implications for faultsare finally discussed.