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.

This poster introduces a new method of analyzing gravity change associated with the solid Earth deformation by earthquakes. The vertical deformation and density change after earthquakes result in changes in the Earth’s gravity field that are detectible by GRACE and GRACE Follow-On (GRACE-FO) spacecraft. Our approach exploits instantaneous gravity perturbation measured by the intersatellite ranging systems between two GRACE Follow-On satellites for early detection, with 1-3 days of latency after the event. This method can be particularly useful for assessing and distinguishing between early models of earthquake fault slip. The Mw 8.2 Chignik earthquake is near the coseismic detection threshold estimated during the earlier GRACE (2002-2017) and the current GRACE-FO (2018-present) gravity and mass change satellite missions. The GRACE-FO mass change data include the higher-precision Laser Ranging Interferometer in addition to the microwave (K-/Ka-band) instrument. A particular challenge for the Chignik event is that the Gulf of Alaska is poorly modeled with existing ocean correction models such as Atmosphere and Ocean De-aliasing (AOD) model currently used by the GRACE and GRACE-FO project. Two other subduction zone sequence of earthquakes of similar magnitude, the 2006-2007 Kuril events (Mw 8.3 & 8.1) and the 2009 Tonga-Samoa (Mw 8.1) complex event, exhibited large, long-wavelength post-seismic mass changes that were detectable by the GRACE and GRACE-FO data. Both cases produced on-going gravity changes that can be accounted for by viscoelastic relaxation. In fact, the cumulative gravity change over several years exceeded the coseismic gravity change. It is, therefore, anticipated that the Mw 8.2 Chignik event will likely yield significant postseismic gravity perturbation as well, depending primarily on the elastic lithosphere thickness and viscosity of the asthenosphere. Post-seismic relaxation following the earlier, nearby 2020 (M 7.8 & 7.6) earthquakes may contribute to the gravimetric signal as well. We will present our early results of gravity changes after the Mw 8.2 Chignik earthquake. Additionally, we will discuss what can be improved for timely detection of the gravity change signature and how we can use gravimetric data for a unique perspective on the subduction zone process.