Sea level rise is a major challenge facing coastlines worldwide and can be strongly exacerbated by land subsidence. However, detailed characterization of vertical land motion (VLM) is limited for many tectonically active islands, as many remote sensing methods are hindered by dense vegetation and thick cloud cover. In American Samoa, strong post-seismic deformation from the 2009 Samoa-Tonga earthquake has increased flooding, but large uncertainties remain in hazard forecasting as only point measurements of VLM have been available. Here, we present novel VLM results over Tutuila, the largest and most populated island in American Samoa, using interferometric synthetic aperture radar, GPS, tide gauge, and satellite altimetry data. Measurements cover populated areas, with subsidence rates of 6–9 mm/yr and uncertainties of <1 mm/yr; the highest rates lie along the coastlines. We find differences in rate changes across the island, suggesting that local processes need to be well-constrained for effective flood forecasting efforts.
NASA ESI Program, Grant 19-ESI19-0027, “Coastal land change due to earthquakes and implications for sea-level rise in the Samoan Islands”