Reconstructing Lacustrine Tsunami Hazard in High-Seismicity Regions of New Zealand’s South Island

<p><strong>Lake tsunami are a significant and underrepresented hazard globally. The current understanding of this hazard is derived from only 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, likely to cause more frequent subaerial and subaqueous mass movements with the potential to produce lake tsunami. Despite this vulnerability, the drivers and spatiotemporal distribution of the lacustrine tsunami hazard in active tectonic settings remained largely unquantified, due in part to methodological limitations for establishing long records of magnitude and frequency.</strong></p><p>Using the case study of fault-contact and fault-proximal lakes in New Zealand’s South Island, this thesis explored how the active tectonic setting controls the mechanisms and spatiotemporal distribution of lacustrine tsunami hazard by reconstructing past occurrences of lacustrine tsunami from evidence preserved in the geological record. To achieve this, the thesis developed new approaches to producing long records of magnitude frequency using the morphological and sedimentological signatures of the lacustrine tsunami. The approach of this study was threefold. Firstly, high-resolution multibeam bathymetry and seismic reflection data were collected for four lakes, Lake Rotoiti, Lake Rotoroa, Lake Brunner, and Lake Mapourika, to establish the spatial distribution and characteristics of past lacustrine mass movements. 16 previously undocumented mass movements capable of generating lake tsunami were discovered, with the most prevalent source areas being fluvial deltas and bedrock slopes proximal to active faults. The origin and characteristics of the potentially tsunamigenic mass movements indicate that the high rates of delta progradation, frequent seismic activity, and the preconditioning and displacement by active faults strongly influenced the spatial distribution of lake tsunami hazard. However, composite deposits formed by the repeated failure of deltas are challenging to deconvolve into a time series of discrete events where the timing, mass and tsunamigenic potential are resolvable.</p><p>The largest mass movement identified across the four study lakes was a 6.1 km2 block field proximal to the fault-contact delta complex in Lake Rotoroa. This deposit was the focus of the second phase of research, which developed an innovative multiproxy approach to deconvolving the timing, volume, and tsunamigenic capacity of individual events with complex deltaic mass movement deposits. Using the case study of Lake Rotoroa, this approach was employed to investigate the conditions that lead to catastrophic delta collapse, specifically how fault-displacement influences the failure dynamics and tsunamigenic capacity of fault-contact delta collapse. The integrated analysis of evidence revealed that a catastrophic collapse of the Sabine and D’Urville deltas occurred between 800 CE to 976 CE and generated a tsunami in Lake Rotoroa with maximum run-up heights of 22 to 41 m. Comparisons to the regional palaeoseismic record and reconstruction of emplacement dynamics indicate that deep-seated and catastrophic delta collapse was initiated by permanent displacement on the Northern Alpine Fault that intersects the delta. The case studies highlight the significant tsunami hazard associated with fault-contact deltas, particularly in active tectonic settings where tsunami hazard potential is rapidly recharged over the seismic cycle. Given the potentially high temporal frequency of tsunamigenic delta failures, it was critical to establish a method for accurately reconstructing records of past lacustrine tsunami over a long-time scale.</p><p>The potential of using lake sediment to reconstruct long records of magnitude and frequency was then investigated. An in-depth multiproxy analysis of sediment cores retrieved throughout Lake Rotoroa demonstrated that the 800 – 976 CE lacustrine tsunami left diagnostic imprints on the sedimentary record distinct from deposits produced by non-tsunamigenic mass-wasting. The findings corroborate the genetic link between lacustrine tsunami and megaturbidite formation and refined this association by demonstrating the interrelationship between the sedimentological characteristics of the event deposit and the spatial and temporal distribution of tsunami current velocity. Comparison to historic event deposits in Lake Rotoroa and globally demonstrated that tsunami deposits can be differentiated from coseismic mass-wasting turbidites and other mass transport deposits. The research highlights the potential of using sediment records to reconstruct the frequency of lake tsunami over long time scales, which is essential to advancing hazard management practices. The outcomes of this thesis demonstrate that it is of great societal importance for lacustrine tsunami to be represented in future hazard resilience and management frameworks. The novel quantitative evidence and reconstruction methodologies generated in this thesis make this goal achievable.</p>