Effect of pore fluid pressure on the frictional strength of antigorite serpentinite

Triaxial deformation experiments on antigorite serpentinite show a systematic correlation between shear stress and effective normal stress over a wide range of pore pressures (50 < Pp < 143 MPa). Under dry conditions using Ar gas as the pore fluid, the steady-state friction coefficient (μ) is estimated to be 0.66 ± 0.01. In contrast, under wet conditions using distilled water as the pore fluid, μ = 0.51 ± 0.01 (i.e., less than under dry conditions). This difference in μ may be caused by charged water on the serpentine surface, which reduces the frictional resistance of the sliding surface in wet environments. The linear correlation between shear stress and effective normal stress suggests that Coulomb's law applies to sliding surfaces during the injection and ejection of pore fluids. This results in a significant mechanical weakening due to elevated pore fluid pressures; consequently, the locations of asperities on plate boundaries may be controlled by the heterogeneous distribution of fluids on the subducting plate interface.

► Shear stress of serpentinite is linearly correlated with the effective normal stress. ► Coulomb's law applies during the injection and ejection of pore fluids. ► Change of pore pressure might cause fault weakening and instability.