Spatiotemporal Patterns of Subsidence and Sea Level Rise in the Samoan Islands 15 Years After the 2009 Samoa-Tonga Earthquake

Abstract

Fifteen years after the 2009 Samoa-Tonga earthquake, rates of subsidence on the Samoan Islands remain elevated compared with pre-earthquake levels. Coastal flooding has become a regular occurrence, increasing coastal erosion, risk of saltwater intrusion in freshwater aquifers, and threats to critical infrastructure. There is an urgent need to characterize ongoing trends in local and regional subsidence and constrain future behavior to inform the development of effective coastal resilience measures. Here, we have leveraged a multi-sensor, multi-frequency remote sensing suite to track changes in subsidence rates on the islands of Upolu (Samoa) and Tutuila (American Samoa). Our updated GPS/GNSS and tide gauge/altimetry records elucidate subsidence relaxation trends since the earthquake, and our analysis of high-resolution InSAR data from the Sentinel-1 mission—overcoming difficulties presented by vegetated terrain and small landmass sizes—reveal an unprecedented view of local subsidence in the Samoan Islands. These local signals need to be accounted for in coastal planning, including for the development of updated flooding thresholds that are relevant to the Samoan Islands and that account for spatial heterogeneities in subsidence. Overall, we find that subsidence on Upolu has nearly returned to pre-earthquake levels; meanwhile, subsidence on Tutuila will likely continue for a few more decades but ease more quickly than previously predicted. Both of these trends should alleviate previously anticipated pressures associated with high subsidence coupled with sea level rise.