Transient evidence of shallow coseismic submarine landslides shaping canyon head geomorphology: Insights from the 2016 Kaikōura earthquake, New Zealand

The impact of recurrent landslides on submarine canyon morphology, sediment dynamics, and tsunami hazard is poorly understood. This study presents the first detailed inventory of earthquake-induced submarine landslides, using high-resolution (2 m) pre- and postevent multibeam bathymetry of the upper Kaikōura Canyon, New Zealand, following the Mw 7.8, 2016 Kaikōura earthquake. Most failures initiated as small (∼1950 m2), shallow disaggregating slides (<10 m) in postglacial sediments draped over bedrock at the canyon headwall. Despite their size, ∼11.2 Mm3 of material failed from the upper canyon, nearly 100 times the volume loss per unit area observed on land, resulting in up to 210 m of headwall retreat and erasure of small gully systems. These shallow slides transitioned into flows, contributing to the 935 Mm3 gravity flow deeper in the canyon. Corrected digital elevation model (DEM) differencing shows submarine landslide volumes were 1.5−3 times larger than postevent estimates would predict. The sediment mobilized by shallow landsliding from the Kaikōura Canyon headwall provides valuable insights that could improve sediment gravity flow and submarine landslide hazard models. Despite the large sediment volume, no tsunami was generated, likely due to failure occurring as numerous small disaggregating slides. These findings indicate that landslide-driven tsunami hazard in active margin settings may be lower than expected.