Coastal land change due to tectonic processes and implications for
relative sea-Level rise in the Samoan Islands
Abstract
Of the major coastal land change mechanisms responsible for relative
sea-level change, tectonic subsidence is generally quoted as ranging
from < mm/yr to 1 cm/yr. However, we documented coseismic and
ongoing post-earthquake surface displacements from continuous GPS and
tide gauge/altimetry data that indicated rapid subsidence on two of the
major Samoan Islands of 12 - 20 cm during and following the 8.1 2009
Tonga-Samoa earthquake. Earlier results and our modeling of
GRACE-derived gravimetric data provided a preliminary forecast of future
relative sea-level rise through rapid land subsidence [Han et al.,
2019]. Of course these numerical forecasts of time-dependent
deformation are only as good as our input observations and our assumed
rheological models. As part of our current NASA Earth Surface and
Interior study, we are obtaining a wider range of data to constrain and
test alternate models of ongoing postseismic deformation across American
Samoa and Upolu, Samoa: (1) times series of altimetry plus tide gauge
data processed to complement the cGPS data available to provide
high-temporal resolution, point measurements of uplift/subsidence, (2)
InSAR derived observations of surface deformation across the highly
vegetated Samoan Islands, (3) evaluating and using NASA satellite lidar
data (ICESat-I & ICESat-II, GEDI) for fusion with multi-source
topographic data sets and for estimating topographic change on the
decadal time scale. We are evaluating and using these new observations
to better understand and separate out local, island-wide, and
multi-island subsidence patterns and to evaluate the high impact of
rising sea-level in a tectonically active region.