A note on fault reactivation

Abstract

Reactivation of existing faults whose normal lies in the σ 1 ′ σ 3 ′ plane of a stress field with effective principal compressive stresses σ 1 ′ > σ 2 ′ > σ 3 ′ is considered for the simplest frictional failure criterion, τ = μσ n ′ = μ( σ n − P), where τ and σ n are respectively the shear and normal stresses to the existing fault, P is the fluid pressure and μ is the static friction. For a plane oriented at θ to σ 1 ′, the stress ratio for reactivation is ( σ 1 ′/ σ 3 ′) = (1 + μ cot θ)/(1 − μ tan θ). This ratio has a minimum positive value at the optimum angle for reactivation given by θ ∗ = 1 2 tan −1 (1/μ) but reaches infinity when θ = 2θ ∗ , beyond which σ 3 ′ < 0 is a necessary condition for reactivation. An important consequence is that for typical rock friction coefficients, it is unlikely that normal faults will be reactivated as high-angle reverse faults or thrusts as low-angle normal faults, unless the effective least principal stress is tensile.