Far-field stress dependency of the failure mode of damage-zone fractures in fault zones: Results from laboratory tests and field observations of siliceous mudstone

摘要

The macroscopic failure mode (tensile/hybrid/shear) of damage-zone fractures in fault zones may influence the hydrogeological properties of the fault zone. Application of the Griffith-Coulomb failure criterion, combined with the simple assumption that failures are predominantly induced by an increase in differential stresses and/or a decrease in effective normal stresses resulting from stress concentrations generated at the asperities/tips of faults, suggests that (1) only tensile fractures propagate from faults when the effective mean stress is less than the rock tensile strength, (2) tensile/hybrid fractures form when the effective mean stress is less than twice the rock tensile strength, and (3) shear fractures can develop when the effective mean stress is more than twice the rock tensile strength, which suppresses the formation of tensile/hybrid fractures. In this study, thin slots were precut in siliceous mudstone samples and mechanical experiments were conducted under a range of effective confining pressures using core samples with and without precut slots. A comparison of fractures formed in the samples at different applied effective mean stresses gave results consistent with the proposed model. The correspondence of model predictions and results was also corroborated by observations of natural damage-zone fractures observed in the field, in boreholes penetrating the same siliceous mudstone as used in the experiments. The results indicate that fault zones containing numerous tensile/hybrid fractures are limited to domains that have experienced effective mean stresses of less than twice the rock tensile strength.