Abstract
AbstractThe 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.
Highlights
Many destructive earthquake ruptures nucleate within or propagate through carbonate rocks, for example, on faults in Italy (Chiarabba et al, 2009; Fondriest, 2014; Ventura & Di Giovambattista, 2013), and in other areas worldwide (Delle Piane et al, 2017; Hartmann & Moosdorf, 2012)
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
Summary
Many destructive earthquake ruptures nucleate within or propagate through carbonate rocks, for example, on faults in Italy (Chiarabba et al, 2009; Fondriest, 2014; Ventura & Di Giovambattista, 2013), and in other areas worldwide (Delle Piane et al, 2017; Hartmann & Moosdorf, 2012). This seismicity in regions dominated by carbonate rocks has motivated a number of experimental studies aiming to understand the factors controlling the strength of faults in carbonates. Due to the complexities of sealing sample assemblies at high velocities (>1 cm/s) (Violay et al, 2013, 2014), many high‐velocity experiments investigating the coseismic stage of fault slip have been conducted at initially saturated conditions, but without any ability to control or measure fluid pressure during the course of the experiments
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