The impact of pre-existing weaknesses on strike-slip fault evolution: insight into strain partitioning of the 2019 Ridgecrest earthquake

Abstract. The 2019 Ridgecrest mainshock produced a complex rupture pattern at the northwest end of the dextral rupture where a set of disconnected pre-existing faults that trend perpendicular to the strike of the mainshock fault had sinistral slip. This highly segmented geometry of active faulting may reflect immature faulting that provides insights into the early development of strike-slip faults. Physical experiments that simulate upper crustal deformation using scaled analogue materials, such as wet kaolin, allow us to control loading and material rheology, and directly document the complete evolution of strike-slip fault systems that grow in material with pre-existing weaknesses. To assess the impact of pre-existing weaknesses on strike-slip fault evolution we vary initial orientation and spacing of the vertical surfaces, and nature of basal shear loading (localized and distributed). Weaknesses oriented 60˚ and 90˚ from the applied dextral loading showed negligible slip while weaknesses oriented 120˚ developed sinistral slip and weaknesses oriented 150˚ had dextral slip. Experiments that developed sinistral slip along cross-faults (120˚) also showed distributed dextral strain between the faults that contributed to significant rotation of material, including the cross-faults, within the shear zone. When the sinistral faults were rotated to orientations unfavourable for continued slip, new dextral faults developed.  This finding suggests that strike-slip systems with active cross faults represent immature stages of evolution that will become reorganized upon further strain accumulation. The amount of off-fault deformation and shear zone width depends on the presence of pre-existing weaknesses (even if they had low slip) and the persistence of fault irregularities that arose from slip along and interaction of new faults with the pre-existing weaknesses. Understanding how the orientation of pre-existing weaknesses influences the early evolution of strike-slip faults and strain localization over geologic time scales can inform future seismic hazard assessments of regions with pre-existing structures.