Runout characteristics of landslides triggered by the 2016 Kaikoura Earthquake

Estimating the potential runout distance of landslides and their associated impacted areas is a critical component of landslide hazard and risk analysis. Traditionally, back-analysis of past landslides has been employed to predict the runout behaviour of potential future events. To refine landslide runout models and characterize co-seismic landslide dynamics, we conducted an in-depth analysis of a subset of landslides triggered by the Mw 7.8 Kaikōura earthquake in New Zealand (14 November 2016), focusing on the Kowhai Valley in Kaikōura.First, we mapped polylines connecting landslide sources to their corresponding deposits. Given that all landslides were triggered during the same seismic event within steep upland catchments, source areas did not consistently correspond directly to mapped debris trails. Second, we attributed these polylines with information on confinement, substrate type, connectivity, geometry, and physiographic attributes, analysing their relationships with travel length and fall height to identify controls on runout distance. Third, we applied three regional-scale runout modelling approaches—1) a Fahrböschung angle method, 2) the Gravitational Path Process Model, and 3) Flow-R—to evaluate their effectiveness in predicting travel distances and patterns of co-seismic landslide runout.Our mapping identified 3,535 landslide polylines linking 3,105 source areas to 2,652 debris trails. Approximately two-thirds of the landslides exhibited a one-to-one relationship between source and deposit, while the remainder displayed more complex linkages, including multiple deposits from a single source, single deposits from multiple sources, or interactions involving multiple sources and deposits. Statistical analysis revealed significant relationships between runout distance and factors such as substrate type, confinement, coupling, and geometry, although no significant relationship was observed with landslide volume.Model accuracy assessments, using goodness of fit metrics, showed that most approaches either displayed weak accuracy or overestimated landslide runout areas. The best fit models indicated that the landslides triggered in the Kaikōura earthquake travelled a shorter distance than expected from the international literature. Further analysis revealed considerable variability in model accuracy for individual landslides, with larger landslides showing better goodness-of-fit metrics than smaller ones. Landslides located in the lower reaches of the Kowhai Valley also demonstrated higher model accuracy, potentially as a function of landscape relief. These findings underscore the complex controls influencing co-seismic landslide runout and highlight the importance of accounting for uncertainties in regional-scale landslide runout models.