Branch faulting in subduction and strike-slip settings

Earthquake ruptures encounter a variety of geometric complexities as they propagate. I focus on the rupture behavior at a branching intersection between a branch fault and a main fault that is generally planar through the intersection. Under a variety of conditions, rupture does not propagate straight ahead on this main fault, but rather branches off onto a secondary branch or splay fault, and the main fault continuation can be ignored. Branching geometries are prevalent in strike-slip settings where I am concerned with faults linking up with one another, and potentially causing a much larger earthquake than if the rupture was confined to an individual fault. These geometries also occur in subduction zones where splay faults cut through the overriding accretionary prism. These splay faults pose hazards due to their close proximity to the coast and could cause an earlier than expected arrival of a tsunami if these faults rupture. To determine the likelihood of branch or splay faulting, I model the dynamic rupture propagation and the behavior of the rupture at a branching junction. There exists a basic framework for understanding the activation of these branched faults. I add to this simple model by quantifying the effect of a variety of real world occurrences including material contrasts across faults, off-fault plastic deformation, and variations in the strength of faults. Material contrasts across mature faults exist and can alter the likelihood of branch fault activation and influence the preferred direction of rupture propagation when off-fault plastic deformation is accounted for. The initial stress state and plastic deformation during rupture also influence the rupture path. Additionally, I consider contrasts in strength between the main and branch faults as suggested by critical taper theory, which has been widely applied to subduction zones. Since it is not certain if subduction zone splay faults rupture during major earthquakes, I look to the 2004 Sumatra-Andaman earthquake and associated tsunami records for additional evidence of splay fault rupture. I examine the morphology of the lead wave, the local uplift patterns and aftershock distribution and these suggest that a splay fault may have ruptured during this event.