The Samoan islands are an archipelago hosting a quarter million people mostly residing in
three major islands, Savai'i and Upolu (Samoa), and Tutuila (American Samoa). The islands have
experienced sea level rise by 2–3 mm/year during the last half century. The rate, however, has dramatically
increased following the Mw 8.1 Samoa‐Tonga earthquake doublet (megathrust + normal faulting) in
September 2009. Since the earthquake, we found large‐scale gravity increase (0.5 μGal/year) around the
islands and ongoing subsidence (8–16 mm/year) of the islands from our analysis of Gravity Recovery And
Climate Experiment gravity and GPS displacement data. The postseismic horizontal displacement is faster in
Samoa, while the postseismic subsidence rate is considerably larger in American Samoa. The analysis of
local tide gauge records and satellite altimeter data also identified that the relative sea level rise becomes
faster by 7–9 mm/year in American Samoa than Samoa. A simple viscoelastic model with a Maxwell
viscosity of 2–3×1018 Pa s for the asthenosphere explained postseismic deformation at nearby GPS sites as
well as Gravity Recovery And Climate Experiment gravity change. It is found that the constructive
interference of viscoelastic relaxation from both megathrust and normal faulting has intensified the
postseismic subsidence at American Samoa, causing ~5 times faster sea level rise than the global average.
Our model indicates that this trend is likely to continue for decades and result in sea level rise of 30–40 cm,
which is independent of and in addition to anticipated climate‐related sea level rise. It will worsen coastal
flooding on the islands leading to regular nuisance flooding.