InSAR Measurement of Vertical Land Motion in New Zealand Cities, and Implications for Sea-level Rise ProjectionsInSAR Measurement of Vertical Land Motion in New Zealand Cities, and Implications for Sea-level Rise Projections

Densely populated urban coastal strips are most at risk from the effects of relative sea-level rise. At the same time, anthropogenic activities associated with urbanisation, such as groundwater withdrawal, land reclamation and cut and fill operations can lead to local land subsidence (LLS), further exacerbating the risk to urban infrastructure. We generate the first high-resolution urban maps (10 m) of LLS using Sentinel-1 InSAR data between 2018 and 2021. This analysis reveals 77% of urban coastlines are subsiding at rates of −0.5 mm/yr or greater, and 10% are subsiding faster than −3.0 mm/yr. This analysis documents highly-localised hotspots of LLS, with subsidence rates exceeding −10.0 mm/yr in some cases. Highest subsidence rates occur on land that was reclaimed during the twentieth century, areas occupying Holocene sediment, or locations that were recently cut and filled. Time-series analysis of LLS at sites of reclaimed land shows both linear and non-linear rates of deformation over time periods of up to 6–8 years. This study reveals the spatial variability of exposure to sea-level rise hazard within New Zealand cities, and demonstrates that in many cases current rates of VLM should be expected to continue for the next few decades.