Changes in shore platform wetting and drying cycles following the 2016 Kaikōura earthquake: Implications for incipient marine terrace evolution

Abstract

Co-seismic uplift of Kaik oura Peninsula in 2016 has substantially reduced the number of wetting and drying cycles that occur on the shore platforms and the newly uplifted incipient marine terraces. A simple empirical model incorporating field and laboratory measurements was used to determine the number and frequency of wetting and drying cycles. The mudstone supratidal terraces are vulnerable to material disintegration and slaking through sustained drying, and occasional sweeping by storm waves.Overall, wetting and drying cycles have decreased on six of eight field transects, between À8% and À148%, resulting in prolonged drying of the supratidal terraces following uplift (upwards of a 29% increase in annual drying hours). We conclude that accelerated rates of denudation due to enhanced drying post-uplift are likely to return sections of the incipient mudstone terraces to their former intertidal pre-uplift state, potentially removing evidence of the co-seismic uplift event. Terrace preservation, however, will likely be highly variable between locations depending on its inherited morphology, lithological vulnerability, and the timing of any future tectonism.

Storm overtopping of a marine terrace at the penultimate stage of evolution

“…The lithofacies interpretation is that the high tide and storm swash deposits overlay the former shore platform surface and that environmental conditions following the uplift event were such that tide and storm deposits could accumulate. The elevation of the T4 shore platform during its incipient marine terrace stage is unknown since the period immediately following uplift the mudstone platform likely experienced rapid rock disaggregation as has been observed in the post co‐seismic uplift environment at Kaikōura, New Zealand (Dickson et al, 2022; Horton, Stephenson, & Dickson, 2022; Omidiji et al, 2022). Notwithstanding these uncertainties, the elevation of the platform surface is within the range of annual storm wave action, thus, storms may erode the marine terrace immediately following co‐seismic uplift and deposit storm swash material as cover deposits.…”

“…This stochastic readjustment is near‐instantaneous and forces a redistribution of contemporary shore platform erosion processes (Stephenson et al, 2017). However, late Holocene marine terraces may occur at low elevations relative to the present sea‐level, exposing them to the effects of wave erosion and rock breakdown (Clark et al, 2019; Kennedy & Beban, 2005; Matsumoto et al, 2021) and can potentially be destroyed by contemporaneous processes (Dickson et al, 2022; Horton, Stephenson, & Dickson, 2022). Records of seismicity are therefore dependent on an uninterrupted chronostratigraphic interpretation of marine terrace sequences that assumes complete preservation of each marine terrace that has been superimposed on the landscape from co‐seismic uplift.…”

Testing the equations for wave attenuation over shore platforms

“…Although complicated by effects of glacial-interglacial sea-level variation, these models typically show that a negative feedback develops, in which coastline retreat leads to a wider shallow rock platform, which attenuates waves and thus slows down further retreat (Sunamura, 1978;Trenhaile, 2000Trenhaile, , 2001Quartau et al, 2010). Long-term relative sea-level change, such as due to tectonics, can potentially further complicate the effects of attenuation (Trenhaile, 2002;Matsumoto et al, 2021;Horton et al, 2022;Dickson et al, 2022).…”