InSAR illuminates Pohnpei island's subsidence, validating GNSS and late Holocene seal-level data

Relative sea level (RSL) reconstructions using radiocarbon-dated mangrove sediment from Pohnpei Island, Federated States of Micronesia, in Remote Oceania reveal that RSL rose at an average rate of 0.7 mm/yr since 5,700 years BP. This is contrary to the predictions of Glacial Isostatic Adjustment (GIA) models that suggest a RSL fall. This RSL rise was attributed to ongoing subsidence supported by GPS CORS (Continuously Operating Reference Station) on the coastal plain of the island directly measuring subsidence at 1.0 ± 0.2 mm/yr over 2003-2021 (Sefton et al., 2022, PNAS). The availability of Sentinel-1 SAR data for Pohnpei presents a unique opportunity to further investigate subsidence and independently evaluate the trends inferred from proxy RSL records and GPS CORS.

To create a robust vertical land motion (VLM) map of the island, we employed the coherence-based Small Baseline Subset (SBAS) method for InSAR analysis of Sentinel-1 data. This generated a high-resolution VLM map that provides detailed insights into deformation patterns across the island from 2016 to 2021. Application of corrections for tropical atmospheric condition and tidal loading significantly improved the results. We found an average subsidence rate of 0.89 mm/yr with a Root Mean Square Error (RMSE) of 2.47 mm/yr. The InSAR findings validate the previous results indicating island-scale subsidence of Pohnpei.