Erosion Rates on Uplift Marine Terraces Following the 2016 Kaikōura Magnitude 7.8 (Mw) Earthquake

Since 1973 micro-erosion meters (MEM) have been used at Kaikōura

Peninsula to determine lowering rates on inter-tidal shore platforms. Rates measured over two, two year periods (1973-1975 and 1994-1996) and at decadal scales (20-43 years) demonstrate that platform surface lowering is on average 1.1 mm/yr. The 14 November 2016 Kaikōura magnitude 7.8 (Mw) earthquake caused an instantaneous uplift of 0.8-1.0 m of the peninsula. The uplift offers the rare opportunity to examine how such an event alters processes and rates of erosion on these shore platforms, since these are now partially marine terraces as the inner margins of some platforms are not above high tidal levels (but perhaps not storm surge). Since the earthquake, 42 MEM sites have been measured 12 times at 3 monthly intervals. Most recently in October 2019. MEM sites show widely varying responses to uplift. Erosion rates are at some MEM sites are three times the earthquake rates while other sites show significant amounts of rock swelling (3-4 mm in 6 months), or aggradation as weathered rock fragments are no longer removed by wave action. Consequently soil development is now underway on the uplift surfaces. The coseismic uplift has fundamentally changed the process regime operating on the still inter-tidal shore platforms. Zones of maximum wetting and drying have migrated seaward across the platforms causing previously slow eroding (< 1 mm/yr) MEM sites to accelerate to twice the pre-earthquake rates. Erosion rates demonstrate rapid adjustment of the platform surface to this disturbance and illustrate how uplifted marine terraces initially weather rapidly despite being above sea level. The preservation of the new marine terrace is probably dependent on further uplift within the next 300-400 years, otherwise erosion by lowering and backwear of the riser will likely remove the new surface. This scenario has significant implications for marine terrace preservation and the recording of coseismic events in the landscape.