Lacustrine mass movements in active tectonic settings: Lake tsunami sources in New Zealand's South Island

Lake tsunamis generated from subaerial and subaqueous mass movements can pose a significant hazard to lakeshore communities. However, current knowledge of lake tsunamis is derived from a limited number of studies, primarily in low-seismicity settings such as Switzerland and Norway. Underrepresented are studies from active tectonic settings, characterised by high relief, high sediment yields, and frequent seismic shaking that is likely to produce episodic subaerial and subaqueous mass movements with the potential to produce lake tsunamis. This study seeks to address this gap in our understanding through an examination of four lakes in the active tectonic region of New Zealand's South Island (Lake Rotoiti, Lake Rotoroa, Lake Brunner, and Lake Mapourika). High-resolution multibeam bathymetry and high-resolution seismic reflection data were used to identify and characterize the landforms and sediment associations of large and potentially tsunamigenic mass movements. A qualitative evaluation of the tsunamigenic capacity of the identified mass movements was undertaken using established empirical relationships between mass movement volumes and run-up heights. The bathymetric and seismic surveys have revealed 16 previously undocumented mass movements capable of generating lake tsunami. Fluvial deltas and bedrock slopes proximal to active faults were the most prevalent mass movement source areas, with the largest mass movements identified being a 49 Mm3 delta collapse and a 110 Mm3 subaerial mass movement in Lake Rotoroa. The findings suggest the origin and characteristics of the potentially tsunamigenic mass movements identified were strongly influenced by the high rates of delta progradation, frequent seismic activity, and the preconditioning and displacement by active faults, typical of active tectonic settings. Our results provide new insights into the heterogeneity of lacustrine mass movements and the drivers of lake tsunami hazards in underrepresented high-seismicity regions. The findings illustrate the importance of developing regionally specific insights into lake tsunami susceptibility and may inform the direction of more detailed lake tsunami hazard assessment in New Zealand.