Abyssal-hill faults are reactivated on the outer slope of trenches when they strike within 25° from trench parallel; otherwise, new faults form parallel to the trench. We use the observed transition angle (25°) and a three-dimensional failure analysis to determine the coefficient of friction on reactivated abyssal-hill faults. The stress state in the outer slope is modeled as the superposition of the overburden stress and bending-induced plane strain deformation. If new trench-parallel faults fail according to a linear failure relationship with no cohesion, then determination of the coefficient of sliding friction (μs) on reactivated faults is independent of the absolute value of the principal stresses and depends only on the Poisson ratio of the crust and the slope of the failure law for new faults (μf). We find that reactivated faults, dipping at 45°, are ∼30% weaker than surrounding crust (e.g., μs = 0.6 for μf = 0.85). These results suggest that the variation in the strength of oceanic crust due to the seafloor spreading fabric is small, and that the coefficient of sliding friction on oceanic faults is consistent with that observed in the laboratory.

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