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Slip-parallel grooves (striations) on fault surfaces are considered a robust indicator of fault slip direction, yet their potential for recording aspects of earthquake rupture dynamics has received little attention. During the 2016 Kaikōura earthquake (South Island, New Zealand), >10 m of dextral strike-slip on the steeply dipping Kekerengu fault exhumed >200 m2 of fresh fault exposure (free faces) where it crossed bedrock canyons. Inscribed upon these surfaces, we observed individual striae up to 6 m long, all of which had formed during the earthquake. These were typically curved. Using simulations of spontaneous dynamic rupture on a vertical strike-slip fault, we reproduce the curved morphology of striae on the Kekerengu fault. Assuming strike-slip pre-stress, our models demonstrate that vertical tractions induced by slip in the so-called cohesive zone result in transient changes in slip direction. We show that slip-path convexity is sensitive to the direction of rupture propagation. To match the convexity of striae formed in 2016 requires the rupture to have propagated in a northeast direction, a prediction that matches the known rupture direction of the Kaikōura earthquake. Our study highlights the potential for fault striae to record aspects of rupture dynamics, including the rupture direction of paleo strike-slip earthquakes.

How earthquake ruptures evolve and propagate are major outstanding questions in seismology. Our current understanding is limited to modern events captured by seismic networks, making it impossible to observe rupture propagation that occurred during earthquakes in the distant past. Here we propose a new method to discern the rupture propagation directions of past large earthquakes based on geological features preserved on fault slip planes. These features-called slickenlines-are striations formed during seismic slip and record dynamic fault movement during past surface-breaking earthquakes. We develop a theoretical framework that links slickenline curvature with rupture mode and rupture propagation direction for all faulting types and test our model using a global catalogue of historical surface-rupturing earthquakes with seismologically constrained rupture directions. Our results reveal that historical observations are consistent with theoretical predictions, thus providing a robust way to uncover the rupture directions of large earthquakes that lack instrumental data. Plain Language Summary How earthquake ruptures evolve and propagate are major outstanding questions in seismology. Currently, we are unable to observe the details of earthquakes that occurred in the distant past, which limits our understanding to events recorded by modern technology. Here we propose a new method to uncover the rupture propagation direction of past large earthquakes, using geological features preserved on faults scarps. These features-called slickenlines-are scratch marks that form when two sides of a fault move past one another during an earthquake. We develop a theoretical framework that links the geometry of slickenlines with rupture propagation direction for all types of faults and test our model using a global catalogue of surface-breaking earthquakes. Our results reveal a strong link between our model and the available data, providing a new way to uncover the rupture direction of large earthquakes that are not recorded by modern seismic instruments.

High-resolution, three-dimensional (3-D) measurements of surface displacements during earthquakes can provide constraints on fault geometry and near-surface slip and also quantify on-fault and off-fault deformation. However, measurements of surface displacements are often hampered by a lack of high-resolution preearthquake elevation data, such as lidar. For example, preearthquake lidar for the 2016 M W 7.8 Kaikōura, New Zealand, earthquake only covers ≲10% of~180 km of mapped surface ruptures.To overcome a lack of preearthquake lidar, we measure 3-D coseismic displacements during the Kaikōura earthquake using point clouds generated from aerial photographs. From these point clouds, which cover the whole area of the 2016 surface ruptures, it is possible to measure 3-D displacements to within ±0.2 m. We measure coseismic slip and estimate the geometries of faults in the steep, inaccessible Seaward Kaikōura mountains, where postearthquake field observations are very sparse. The Jordan Fault (previously the Jordan Thrust) ruptured in 2016 as a moderate-to-steeply dipping (~50-80°), predominantly strike-slip fault. Slip on this fault in 2016-which included a normal-sense component in some areas-contrasts with field observations that indicate significant longer-term shortening across the Jordan Fault during the Holocene. It is therefore likely that different earthquakes on the Jordan Fault have very different slip vectors and that the fault does not exhibit "characteristic" slip behavior. For faults like the Jordan Fault, long-term time-averaged estimates of slip rate may not be reliable indicators of the sense and magnitude of slip in individual earthquakes.

We provide a summary of the surface fault ruptures produced by the Mw7.8 14 November 2016 Kaikōura earthquake, including examples of damage to engineered structures, transportation networks and farming infrastructure produced by direct fault surface rupture displacement. We also provide an overview of the earthquake in the context of the earthquake source model and estimated ground motions from the current (2010) version of the National Seismic Hazard Model (NSHM) for New Zealand. A total of 21 faults ruptured along a c.180 km long zone during the earthquake, including some that were unknown prior to the event. The 2010 version of the NSHM had considered multi-fault ruptures in the Kaikōura area, but not to the degree observed in the earthquake. The number of faults involved a combination of known and unknown faults, a mix of complete and partial ruptures of the known faults, and the non-involvement of a major fault within the rupture zone (i.e. the Hope Fault) makes this rupture an unusually complex event by world standards. However, the strong ground motions of the earthquake are consistent with the high hazard of the Kaikōura area shown in maps produced from the NSHM.

Areas that experience permanent ground deformation in earthquakes (e.g., surface fault rupture, slope failure, and/or liquefaction) typically sustain greater damage and loss compared to areas that experience strong ground shaking alone. The 2016 Mw 7.8 Kaikōura earthquake generated ≥220 km of surface fault rupture. The amount and style of surface rupture deformation varied considerably, ranging from centimetre-scale distributed folding to metre-scale discrete rupture. About a dozen buildings – mainly residential (or residential-type) structures comprising single-storey timber-framed houses, barns and wool sheds with lightweight roofing material – were directly impacted by surface fault rupture with the severity of damage correlating with both local discrete fault displacement and local strain. However, none of these buildings collapsed. This included a house built directly atop a discrete rupture that experienced ~10 m of lateral offset. The foundation and flooring system of this structure allowed decoupling of much of the ground deformation from the superstructure thus preventing collapse. Nevertheless, buildings directly impacted by surface faulting suffered greater damage than comparable structures immediately outside the zone of surface rupture deformation. From a life-safety standpoint, all these buildings performed satisfactorily and provide insight into construction styles that could be employed to facilitate non-collapse performance resulting from surface fault rupture and, in certain instances, even post-event functionality.