Using InSAR and GNSS to estimate shore platform erosion of Kaikōura Peninsula, New Zealand, and Otway Coast, Australia

Quantifying shore platform evolution at appropriate spatial and temporal scales remains challenging due to the complex scaling between erosion at the instantaneous scale from waves and granular decay and the centennial to millennial age of platform systems. Unique multi-decadal records of in situ Micro-Erosion Meter (MEM) measurements, spanning more than 40 years, are available from shore platforms on the Kaikōura Peninsula, South Island, New Zealand, and the Otway Coast, southeastern Australia. MEM observations from Kaikōura indicate that, following 1 m of coseismic uplift during a Mw 7.8 earthquake in 2016, mean platform lowering rates have averaged ~2.30 mm/yr. In contrast, MEM measurements from the tectonically stable Otway shore platform between 2015 and 2024 indicate lower erosion rates of 0.264 mm/yr. While the MEM provides high-precision point-scale erosion rates, its spatial and temporal coverage is limited relative to the full extent and evolutionary timescales of shore platforms. To overcome these limitations, this study integrates Interferometric Synthetic Aperture Radar (InSAR) with GNSS observations to extend erosion and deformation assessments across entire shore-platform surfaces. Persistent Scatterer Interferometry (PSI) was applied using the Surface motioN mAPPING (SNAPPING) PSI Med and PSI Full services on Sentinel-1 imagery acquired between 2017 and 2024 and processed via the Geohazards Exploitation Platform. At Kaikōura, PSI Full analysis indicates an average uplift rate of 4.28 mm/yr, whereas PSI Med analysis yields an apparent subsidence rate of 1.76 mm/yr, highlighting scale- and processing-dependent variability in deformation estimates. Initial InSAR results from the Otway Coast reveal low-magnitude deformation, with velocities generally ranging between −2.0 and +2.0 mm/yr. GNSS observations located at the Kaikōura Peninsula provide an independent constraint on vertical motion, indicating net subsidence at Kaikōura of approximately 3.0 mm/yr between 2017 and 2024. GNSS stations near the Otway shore platforms record subsidence rates of ~3.75 mm/yr at Lorne (2022–2024) and ~1.54 mm/yr at Marengo (2017–2024). Further analysis is required to reconcile GNSS-derived vertical motion rates with MEM observations; however, initial results highlight the value of integrating MEM, InSAR, and GNSS to resolve shore-platform downwearing across multiple spatial scales.