![]() The arid setting and thorough documentation 28 of the 2019 M7.1 Ridgecrest earthquake rupture make it an ideal target for investigating the effects of geologic inheritance and fault geometry on earthquake slip distributions. Despite the ubiquity of fault nonplanarity, however, the relative sensitivity of slip at Earth’s surface to rupture geometry versus other factors (e.g., lithology and structural complexity) remains unknown. Similarly, secondary ruptures associated with the 1905 M~8 Bulnay, Mongolia, earthquake likely followed pre-existing (unspecified) structures, whereas the damage distribution appeared to be lithologically-controlled 27. For instance, the southernmost section of the 2013 M7.7 Balochistan, Pakistan, earthquake rupture produced a zigzag pattern, where alternating kilometer-scale segments apparently exploited a penetrative fabric that accommodates regional shortening at the plate margin 26. ![]() Even during a single event, an earthquake rupture may activate pre-existing structures leading to unexpected propagation paths. ![]() Many crustal faults develop along pre-existing structures 21 (e.g., joints 22 and dikes 23), which often leads to nonplanarity during progressive slip, growth, and linkage of non-coplanar segments 24, 25. Yet even in a homogeneous continuum, mechanical models have shown that nonelliptical slip distributions can result from fault nonplanarity 15, 16, which also can control dynamic rupture propagation and arrest 17, 18, 19, 20. Recent investigations of fault slip distributions at Earth’s surface have focused on the effects of off-fault plasticity 10, 11, 12, structural complexity (e.g., steps and multiple fault strands) 13, fault zone maturity 5, and along-strike lithologic heterogeneities 14. For instance, coseismic slip distributions often are asymmetric 4 and/or characterized by significant short-length-scale variations 9. In an idealized mechanical model-a planar fault in a homogeneous, isotropic, linear elastic material with uniform driving stress – the along-strike slip distribution is elliptical and smoothly varying 8, which is atypical for slip distributions observed in nature. Thus, many insights gained from analyzing slip distributions depend implicitly on the physics of surface rupture, and its relation to slip along deeper portions of faults where most of the seismic moment is released, two factors that remain poorly understood. The vast majority of slip measurements along active faults are made at Earth’s surface. Improved knowledge of fault slip distributions also advances seismic risk mitigation efforts, including the design of fault-crossing infrastructure (e.g., Trans-Alaska oil pipeline 6) and potentially reducing uncertainty in ground-motion models 7. Similar content being viewed by othersįault slip distributions are a fundamental metric in earthquake science, illuminating the physical laws and conditions governing deformation 1, 2, as well as the processes that allow fault systems to grow and interact over geologic time 3, 4, 5. These results motivate revisiting the development of other large-magnitude earthquake ruptures (1992 M7.3 Landers, 1999 M7.1 Hector Mine) and tectonic provinces within the IDS. ![]() Mechanical models testing a range of fault geometries and stress fields indicate that the inherited rupture geometry strongly controlled the M7.1 earthquake slip distribution. Common geometries suggest the fault system evolved through reactivation of structures within the surrounding Independence dike swarm (IDS). Here we use remote sensing data and field observations to investigate the origin of the 2019 M7.1 Ridgecrest, California, earthquake rupture geometry and test its impact on the slip distribution observed at Earth’s surface. Competition between fault geometry and other factors (e.g., lithology) to control slip at Earth’s surface is an open question that is central to our knowledge of fault processes and seismic hazards. Faults often form through reactivation of pre-existing structures, developing geometries and mechanical properties specific to the system’s geologic inheritance.
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