On the rotation and frictional lock-up of normal faults: Explaining the dip distribution of normal fault earthquakes and resolving the low-angle normal fault paradox

Classical rock mechanics predicts that normal faults should form at 60-65 degrees, rotating to 30-40 degrees before locking, so widespread slip at around 20 degrees is considered paradoxical. Furthermore, the dip distribution of normal fault earthquakes has a distinct, unexplained peak at similar to 45 degrees. For both problems, some combination of low friction, high fluid pressures, stress rotation and efficiency of low-angle slip have been suggested but provide at best a partial solution. A simple quantitative model for normal fault rotation (iterating between slip on faults and distributed strain) predicts that faults spend more time at lower angles. Combining this result with Mohr-Coulomb analysis of the range where seismogenic normal faults should lock up predicts the dip distribution of seismogenic normal faults and matches both the range and 45 degrees peak observed for normal fault earthquakes. As even the lowest dips of normal fault ruptures fall within the limits of fault reactivation, slip on seismogenic low-angle normal faults is not paradoxical. A new Mohr-Griffith solution to the limits of fault reactivation extends the analysis into the top few km, where low-angle slip is best constrained by field observations, and shows that low cohesion normal faults can remain active at <20 degrees without recourse to very low friction, high fluid pressures or stress rotation. This analysis thus resolves the long-standing paradox of slip on low-angle normal faults. Where continued rotation or changes in mechanical properties cause low-angle faults to lock in the shallow subsurface, a new fault propagates upward from the point of lock-up, transferring a hanging wall slice to the footwall, to be rafted up and out as a rider block. By recording the dip/depth of lock-up, riders atop detachments constrain the mechanics of faulting and of lock-up.