Normal faults, normal friction?

Abstract

Debate continues as to whether normal faults may be seismically active at very low dips (δ < 30°) in the upper continental crust. An updated compilation of dip estimates (n = 25) has been prepared from focal mechanisms of shallow, intracontinental, normal-slip earthquakes (M > 5.5; slip vector raking 90° ± 30° in the fault plane) where the rupture plane is unambiguously discriminated. The dip distribution for these moderate-to-large normal fault ruptures extends from 65° > δ > 30°, corresponding to a range, 25° < 𝛉_r < 60°, for the reactivation angle between the fault and inferred vertical σ₁. In a comparable data set previously obtained for reverse fault ruptures (n = 33), the active dip distribution is 10° < δ = 𝛉_r < 60°. For vertical and horizontal σ₁ trajectories within extensional and compressional tectonic regimes, respectively, dip-slip reactivation is thus restricted to faults oriented at 𝛉_r ≤ 60° to inferred σ₁. Apparent lockup at 𝛉_r ≈ 60° in each dip distribution and a dominant 30° ± 5° peak in the reverse fault dip distribution, are both consistent with a friction coefficient μ_s ≈ 0.6, toward the bottom of Byerlee's experimental range, though localized fluid overpressuring may be needed for reactivation of less favorably oriented faults.