Strength of Calcite-Bearing Faults in Fluid Pressure-Controlled Experiments

Monday, 15 December 2014
Giulio Di Toro, University of Padua, Padua, Italy, Elena Spagnuolo, National Institute of Geophysics and Volcanology, Rome, Italy, Marie Violay, ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland and Stefan Bjorklund Nielsen, University of Durham, Durham, United Kingdom
Fluid pressure Pf  is a major factor controlling natural and induced seismicity. According to the Terzaghi's principle, the effective normal stress σeff = σn (1-α Pf) decreases with increasing Pf, with σn the normal stress and α the Biot coefficient. At α is usually attributed a value of 1, though it may vary with fluid composition and permeability of the fault zone.

Here we conducted 10 experiments on pre-cut (experimental fault) hollow-cylinders (50/30 mm ext/int diameter) of Carrara marble (99.9% calcite) installed in the rotary shear apparatus SHIVA. The hollow-cylinders were inserted in a fluid vessel filled with water (drained conditions) and subjected to σn of 15 MPa. After this, the Pf was raised to 5 MPa and the shear stress τ to 6.5 MPa, corresponding to an effective friction coefficient μeff = τ / σeff = 0.65 or close to the instability of the experimental fault; then Pf in the vessel was increased stepwise of ΔPf = 0.5 MPa every 10 to 40 s to induce fault instability. The main instability was preceded by short-living slip bursts. Assuming α = 1, the increase in Pinduced the main instability for μeff = 1.3 in the case of stepwise intervals of duration ~10 s and for μeff = 0.7 in stepwise intervals of ~40 s: the time threshold td that resulted in the two different μeff (0.7 and 1.3) was ~30 s.

We interpret our experimental observations as due to the permeability decrease of the fault slipping zone which resulted in a smaller α. Wear and comminution during the slip bursts that preceded the main instability produced a low-permeability ultrafine calcite gouge layer in the experimental fault. The gouge layer isolated the fluid in the slipping zone from the fluid in the vessel. As a consequence, the ΔPf increase imposed on the fluid in the vessel was delayed inside the experimental fault. For each ΔPf increase, the Pf in the slipping zone could re-equilibrate with the Pf in the vessel provided that the duration of the stepwise increase was long (40 s) with respect to the fluid diffusion time. Given td ~30 s and the half width of the ring-shaped experimental fault l = 5 mm, the hydraulic diffusivity κ (= l2/td ) was ~ 10-8m2s-1, compatible with κ of a low permeability shale. We conclude that fault strength was controlled by slipping zone permeability and by the magnitude and duration of the fluid pressure increments.