AN ANALYSIS OF THE FACTORS THAT CONTROL FAULT ZONE ARCHITECTURE AND THE IMPORTANCE OF FAULT ORIENTATION RELATIVE TO REGIONAL STRESS
The structural style of a fault is generally thought to reflect its mechanical behavior. Stick slip behavior is associated with simple narrow fault zones that have a single high-strain core. These are commonly developed from quartzo-feldspathic protoliths and this structural style is thought to result from the positive feedback between comminution and transformation weakening (Chester et al., 1993). Stable sliding behavior is commonly associated with complex wide fault zones that have multiple anastomosing high-strain cores. These are commonly developed from phyllosilicate rich protoliths, and are thought to reflect strain hardening (Faulkner et al., 2003).
The MW 7.2 El Mayor-Cucapah earthquake of 2010 produced a cascading rupture that propagated through a complex network of intersecting faults that cut metamorphic and plutonic rocks exposed in northern Baja California. Coseismic slip of 1-6 m was accommodated by fault zones displaying the full spectrum of previously mentioned styles, which demonstrates that stick-slip mode is not restricted to a certain type of fault. As fault zone complexity and width increase, coseismic slip becomes more broadly distributed on a greater number of scarps that form wider arrays. Thus the slip of a single earthquake replicates many of the fabric elements of the long- lived fault. In addition to protolith, we find that orientation strongly affects fault zone complexity, which increases with decreasing dip. Projecting regional stress onto individual faults, shows that fault zone complexity increases systematically with resolved normal stress, which is known to increase gouge production in laboratory experiments (Yoshioka, 1986). The progressive rotation of faults is thus a previously unrecognized form of strain hardening. We also conclude that regardless of protolith, all faults should experience alternating cycles of stick slip, gouge production and weakening followed by creep, ductile attenuation of gouge and strengthening.