Influence of Effective Stress and Pore Fluid Pressure on Fault Strength and Slip Localization in Carbonate Slip Zones

• Published in: Journal of geophysical research. Solid earth 2020-11, Vol.125 (11), p.n/a
• Main Authors: Rempe, Marieke, Di Toro, Giulio, Mitchell, Thomas M., Smith, Steven A. F., Hirose, Takehiro, Renner, Joerg
• Format: Article
• Language: English
• Subjects: Calcite; calcite gouges; Carbonates; Coefficient of friction; Comminution; Crack propagation; Earthquakes; Effective stress; effective‐pressure law; Fault lines; Fluid pressure; Fluids; Friction; Geophysics; Localization; Mechanical properties; microstructures; Nucleation; Pressure; Pressurization; Pressurized fluids; rotary‐shear experiments; Seismic activity; seismic slip rates; Shear strength; Slip; strain localization; Stress
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2020JB019805

The presence of pressurized fluids influences the mechanical behavior of faults. To test the roles of normal stress and fluid pressure on shear strength and localization behavior of calcite gouges, we conducted a series of rotary‐shear experiments with pore fluid pressures up to 10.5 MPa and difference between normal stress and fluid pressure up to 11.2 MPa. Calcite gouges were sheared for displacements of 0.3 m to several meters at slip rates of 1 mm/s and 1 m/s. Drainage conditions in experiments were constrained from estimates of the hydraulic diffusivity. Gouges were found to be drained at 1 mm/s, but possibly partially undrained during sliding at 1 m/s. Shear strength obeys an effective‐stress law with an effective‐stress coefficient close to unity with a friction coefficient of ~0.7 that decreases to 0.19 due to dynamic weakening. The degree of comminution and slip localization constrained from experimental microstructures depends on the effective normal stress. Slip localization in calcite gouges does not occur at low effective normal stress. The presence of pore fluids lowers the shear strength of gouges sheared at 1 mm/s and causes an accelerated weakening at 1 m/s compared to dry gouges, possibly due to enhanced subcritical crack growth and intergranular lubrication. Thermal pressurization occurs only after dynamic weakening when friction is generally low and relatively independent of normal stress and therefore unaffected by thermal pressurization. The experimental results are consistent with the view that the presence of pressurized fluid in carbonate‐bearing faults can facilitate earthquake nucleation. Key Points Normal stress and fluid pressure equally affect shear strength of calcite gouges at relatively low effective normal stresses (≤11 MPa) The degree of slip localization in calcite gouges sheared at seismic slip rates increases with effective normal stress Thermal pressurization has small effect on shear stress as it occurs after change from pressure‐ to temperature‐controlled slip behavior