Flights of Holocene marine terraces are useful for reconstructing past earthquakes, but coastal erosion can remove terraces from the landscape, potentially leading to incorrect estimates of earthquake magnitude and frequency. Relatively little effort has been afforded to studying terrace erosion processes, and this paper presents the first field evidence that we are aware of documenting terrace erosion rates. Two case studies from New Zealand provide a unique opportunity to observe the beginning and end phases of terrace development. We present downwear and backwear erosion measurements, showing that both sets of processes are important. Microerosion meter measurements from Kaik oura Peninsula, South Island, confirm that downwear processes are modifying new marine terraces that were created when the

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<p>Recent erosion rates from tectonically active coasts provide evidence of rapid rock breakdown following coseismic uplift. These rates are needed to solve puzzles about 'missing' marine terraces that prevent accurate reconstruction of past sea levels, earthquakes and rock coast evolution. Yet little scientific effort has been put into investigating the impact of tectonism on shore platform development. It is also presently unclear if erosion records from one coast can be extrapolated to other regions with similar tectonic, geologic, or geomorphic characteristics. We present shore platform downwearing rates and processes measured using the traversing micro-erosion meter (TMEM) and Structure-from-Motion photogrammetry from an inter-tidal shore platform uplifted by 3.1 m between 100 to 300 years ago. The site is a mudstone platform at Kahutara Point, Māhia Peninsula, North Island, New Zealand. Over 1.43 years, the mean annual downwearing rate was 1.94 mm/yr, while the total erosion at individual TMEM stations ranged from 0.29 to 8.31 mm (equivalent to mean erosion rates of 0.07 to 5.82 mm/yr). We found a lack of any spatial pattern in erosion rates cross-shore, suggesting the equal efficiency of waves and weathering processes. Orthophotographs of the eroded rock surfaces support the combined role of marine processes (waves and tides), sub-aerial weathering processes, salt weathering and biological activity in the erosion of the mudstone platforms at Kahutara Point. The mean erosion rate of 1.94 mm/yr from the Māhia Peninsula is similar to the mean post-uplift rate of 2.25 mm/yr reported for the Kaikōura Peninsula, New Zealand, where platforms were uplifted by ~1 m in 2016. This result suggests a comparable pattern of erosion response at both sites following co-seismic uplift and provides the first field evidence to support the comparison of the Kaikōura and Māhia shore platforms, thus helping to inform marine terrace development and destruction.</p>

<p>On tectonically active rock coasts, there is a dearth of erosion data documenting how rocks adjust (either fast or slow) in response to marine and subaerial processes immediately after coseismic uplift. Here we report erosion rates and evidence of reshaping of shore platform morphology on intertidal- and previously subtidal- rocks at Kaikōura Peninsula, South Island New Zealand. As a result of the November 2016 Kaikōura 7.8 (Mw) earthquake, platforms around the peninsula were uplifted by ~1.01 m, extended in width, and a 43-year active erosion monitoring campaign was abruptly halted but an opportunity to record how rocks respond to sudden environmental change like tectonics was presented. High-resolution topographic data obtained from quarterly surveys over four years using the micro-erosion meters (MEM) and Structure-from-Motion Multi View Stereo (SfM-MVS) surveys have provided accurate quantitative rates of erosion and visual representation of surface morphologies. MEM erosion data revealed variations in erosion, weathering and deposition rates across lithology, seasons, tidal positions, and platform elevation after the uplift. Four-years post-uplift erosion data shows a resetting of erosion rates and faster rock breakdown on both mudstone and limestone lithologies compared to pre-uplift rates. Over the 4-year period, surface downwearing rates for all platforms was 2.19 mm/yr, a 99.9% increase from a pre-uplift rate of 1.10 mm/yr. Average lowering rates on limestone, hard mudstone and soft mudstone platforms are 1.31 mm/yr, 2.13 mm/yr and 3.60 mm/yr, respectively. Seasonal trends in erosion rates remain unchanged as higher rates are still experienced during summer than winter seasons due to greater periods of higher temperatures and increased wetting and drying cycles. A year after uplift, previously reported across shore variations where erosion rates decreased from inner/landward margins of the platform to the outer/seaward sections disappeared with higher erosion rates fluctuating across all platform sections. Increased lowering rates on limestone rocks at the inner and outer sections were attributed to greater periods of wetting and drying, and loss of biological cover. These initially rapid rates decreased on the seaward sections after 3 years as a result of bioprotection and increased tidal wetting.  On one of the harder mudstone rocks, a dramatic increase from a pre-uplift erosion rate of 0.43 mm/yr to 19.23 mm/yr (1-year after uplift) and subsequent decline to 1.54 mm/yr after four years is suggestive of isolated incidents of block detachment and erosion. For the first time, we complement MEM data with available SfM-MVS derived orthomosaics to provide evidence of changing rock morphology and processes such as intense granular disintegration, flaking, algal growth, and boring. On tectonically active rock coasts, the strong fluctuations in erosion rates and platform morphological expressions indicate the actions of not only waves, tides, and weathering processes but also tectonics in shore platform development.</p>