The consequences of uplift on wave transformation across a shore platform, Kaikōura, New Zealand

Abstract

Co-seismic uplift of the Kaikōura Peninsula in 2016 provided a rare opportunity to examine changes in wave hydrodynamics on a tectonically active rock coast comprised of a microtidal shore platform and marine terraces. We compared pre-uplift wave conditions measured in February 2007 with post-uplift conditions at the same site in December-2019 following +0.95 m of vertical displacement. Shore platform gradient was not affected by uplift, but the earthquake raised the seafloor, resulting in a nearly 40% decline in incident onshore energy under comparable offshore conditions. Uplift also resulted in a 50% reduction in the intertidal width of the shore platform, which decreased the amount of gravity wave (>0.05 Hz) energy attenuation. In pre-uplift conditions, energy in the swell (0.05–0.125 Hz) and wind (0.125–0.33) wave spectral frequencies at the most landward sensor contributed 28% of total energy, whereas after uplift the most landward sensor received 49% of total energy from these frequencies. Most of the remaining energy detected on the innermost sensors was from infragravity spectral frequencies, with 45% post-uplift compared to 71% pre-uplift. Co-seismic uplift has reorganised the shore platform and marine terrace sequence, and the associated wave hydrodynamics: about half of the pre-uplift shore platform is now disconnected from wave erosion under calm swell conditions. Uplift has reduced wave inundation on the shore platform and when combined with a reduction in incipient wave energy, suggests less vulnerability to wave erosion so that the uplifted surface may be preserved as a new marine terrace.