The evolving energy of propagating strike-slip fault segments

Abstract:

Step-overs and bends along strike-slip faults can serve as impediments to earthquake rupture. Over many earthquake cycles, these irregularities can develop slip deficits so that the local long-term strike-slip rates do not match the slip rates away from the bends and step-overs. Physical analog models show that bends and stepovers propagate new faults and evolve in order to more efficiently accommodate applied strike-slip thereby reducing off-fault deformation. Numerical models permit evaluation of the energy associated with fault propagation and can predict fault evolution over multiple earthquake cycles using work minimization. The algorithm GROW (GRowth by Optimization of Work) honors tensile and frictional failure criteria along pre-existing surfaces and at fault tips, while allowing the orientation of fault growth within the system to be governed by the global energy budget. In GROW, faults evolve in order to minimize deformation energy, thereby maximizing the mechanical efficiency of the entire system. Using GROW, we subject systems of two right-lateral, strike-slip faults that are initially right-stepping, with varying separation distances and strengths, to remotely applied displacement. We track the propagation paths across the intervening releasing bend. We find that fault strength and initial separation control fault linkage propensity and linkage patterns. In order to build insight into the efficiency gains provided by hard- versus soft-linkage, we also track the work budget of these fault systems as they evolve. For example, linkage of faults may reduce internal work while increasing frictional work and seismic work. Assuming that increasing efficiency correlates with increasing potential for a single earthquake to rupture both faults, this approach has utility for assessing the evolution of seismic hazard as segmented faults systems change over geologic time.