Abstract
The 2016 Mw 7.8 earthquake uplifted the Kaikōura shore platforms ~1 m transforming the intertidal wave and weathering regime, creating new marine terraces. This thesis observed post-uplift processes at Mudstone Bay, Kaikōura to reinterpret the post-seismic development of the youngest marine terrace at Kahutara Point, Māhia. Onshore waves were measured at Mudstone Bay and Kahutara Point using pressure transducers under calm and storm conditions and used to develop models of storm surge inundation of the marine terraces at each location. Inundation was calculated using platform morphology (RTK surveys, LiDAR and UAV) combined with offshore wave heights, air pressure, and tidal data. Sediment movement onshore was quantified using time integrated mass-sediment samplers and characterised using laser particle size analysis, SEM, and XRF to interrogate sediment provenance. Rock samples were collected to determine rock porosity and wetting and drying rates to build an empirical model of wetting and drying, combining inundation and bioprotection. Comparing pre-uplift and post-uplift wave data from Mudstone Bay showed a 40% decline in incident onshore energy. Uplift caused a 50% reduction in intertidal width and gravity wave energy attenuation. Pre-uplift, the inner platform received 28% of total energy as swell and wind spectra, but post-uplift it was 49%. Post-uplift, Mudstone Bay has ~ 90 m of a supratidal incipient marine terrace that can be partially inundated. Storm waves when congruent with high tides inundated the proximal edge of the incipient marine terrace ~13% of tides, but full inundation occurred less than 0.05% of the tides. Sediment samplers collected 2 – 33 g over a 9-days at Mudstone Bay and the net bedload flux was greatest in a seaward direction and autochthonous to the platform. Co-seismic uplift reduced wetting and drying cycles by 8% and 148%, resulting in protracted drying of incipient marine terraces. Wave data from Kahutara Point showed that infragravity spectra was >90% of total energy at the terrace riser. Significant wave heights for gravity (Hm0H) and infragravity (Hm0L) spectra showed platform width was effective at reducing Hm0H and Hm0L, with 50–80% of Hm0H and 20–50% of Hm0L dissipation. The northern flank of Kahutara Point is vulnerable to wave inundation with 24 storms overtopping the youngest marine terrace between 1980–2020. Since 2013 terrace inundation on the eastern flank has increased and may offset decreases in wave energy attenuation from increased platform width. Kahutara Point is vulnerable to marine terrace destruction through backwearing, whereas downwearing dominates incipient terrace removal at Kaikōura. Co-seismic uplift reduces wave inundation, but these terraces may still be vulnerable to inundation from storm. Storms sweep the incipient marine terraces of sediment and contribute to accelerated rates of denudation from enhanced drying, potentially removing evidence of co-seismic uplift. Terrace preservation, however, is highly variable between locations depending on inherited morphology, lithological vulnerability, and the timing of any future tectonism.