Abstract
Coseismic landslides are a major driver of landscape evolution and geohazards in active mountain belts, reshaping topography and influencing drainage divide migration. The debris they generate initiates sediment cascades, producing transient changes in sediment flux and river incision. However, accurately quantifying coseismic landslide erosion and deposition remains challenging. Following the 2016 Mw 7.8 Kaikōura earthquake, we developed a 3D method to volumetrically map landslide erosion and debris distribution across hillslopes using a 2 m resolution vertical difference model over 6875 km2. Our results show that large coseismic landslides preferentially eroded rock mass from upper slopes, with most debris retained on-slope. Only 14% of total landslide volume was deposited off slope, with ≥4% within channels. In contrast, traditional 2D connectivity approaches estimated that up to 36% of landslides were channel-connected, with 89% of the total landslide volume available for fluvial transport. This new volumetric mapping demonstrates that the transfer of landslide debris from hillslopes to river systems is primarily governed by post earthquake remobilisation processes rather than immediate delivery following the earthquake.