Normal faults, normal friction?

Debate continues as to whether normal faults may be seismically active at very low dips (d , 308) in the upper continental crust. An updated compilation of dip estimates (n 5 25) has been prepared from focal mechanisms of shallow, intracontinental, normal-slip earthquakes (M . 5.5; slip vector raking 90 86 308 in the fault plane) where the rupture plane is unambiguously discriminated. The dip distribution for these moderate-to-large normal fault ruptures extends from 658 . d . 308, corresponding to a range, 258 , ur , 608, for the reactivation angle between the fault and inferred vertical s1. In a comparable data set previously obtained for reverse fault ruptures (n 5 33), the active dip distribution is 108 , d5u r , 608. For vertical and horizontal s1 trajectories within extensional and compressional tectonic regimes, respectively, dip-slip reactivation is thus restricted to faults oriented at ur # 608 to inferred s1. Apparent lockup at ur 608 in each dip distribution and a dominant 30 86 58 peak in the reverse fault dip distribution, are both consistent with a friction coefficient ms 0.6, toward the bottom of Byerlee’s experimental range, though localized fluid overpressuring may be needed for reactivation of less favorably oriented faults.