THE IMPACT OF PRE-EXISTING WEAKNESSES ON STRIKE-SLIP FAULT EVOLUTION: RUPTURE MAPS FROM THE 2019 RIDGECREST EARTHQUAKE INSPIRE SCALED PHYSICAL EXPERIMENTS

The Eastern California Shear Zone of Southern California, USA, has hosted several historic ground rupturing earthquakes along complex strike-slip faults. For example, the 2019 Ridgecrest mainshock produced a complex rupture pattern of cross faults 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. The highly segmented geometry of active faulting at the northwest end of the rupture may reflect very immature faulting that provides insights into the early development of strike-slip faults. Why did the rupture reactivate these NE striking sinistral faults rather than developing a new NW trending dextral fault that could accommodate the coseismic strain? Understanding how the orientation of pre-existing cross-faults can influence 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. Physical experiments that simulate upper crustal deformation using scaled analog materials, such as wet kaolin, allow us to control loading and material rheology, and directly document the complete evolution of fault systems. We vary three aspects of the experimental set up to assess the impact of pre-existing weaknesses on strike-slip fault evolution: initial orientation and spacing of the vertical surfaces, and nature of basal shear loading (localized and distributed). Cross faults oriented 60˚ and 90˚ from the applied dextral loading showed negligible reactivation while cross faults oriented 120˚ reactivated with sinistral slip and cross faults oriented 150˚ had dextral slip. Experiments that developed sinistral slip along cross-faults (120˚) showed significant rotation of material, including cross-faults, within the shear zone. The amount of off-fault deformation and shear zone width depends on the presence of pre-existing weaknesses (even if they had low slip during reactivation) and the persistence of fault irregularities that arose from the early reactivation of pre-existing faults. Tracking fault geometry and off-fault deformation during experiments of strike-slip fault evolution provides insight that can guide interpretations of crustal faults.