Postseismic signature of the 2004 Sumatra earthquake on low-degree gravity harmonics

We perform an extensive analysis of the low‐degree gravity field harmonics measured by the GRACE mission, in order to find a signature of the postseismic relaxation following the 2004 Sumatra earthquake. We find a statistically significant perturbation in the secular trend of low‐degree zonal coefficients (Jl) in correspondence of the 2004 Sumatra earthquake and a similar perturbation, but with weak associated statistical significance, also in the nonzonal coefficients. Technical features and results of such analysis are discussed. The time‐dependent postseismic evolution of harmonic coefficients is modeled for various asthenosphere viscosity values, using a theoretical model of global postseismic deformation. The observed change in secular trend is found to be consistent with our modeling results but it cannot be used to discriminate between viscosities. A forward modeling of the perturbations to time‐dependent zonal variation rates following the Sumatra earthquake for various asthenospheric viscosities is provided. As a result, an evident signature of the Sumatra earthquake on l time series is expected for asthenospheric viscosity values below 1018 Pa s. Therefore, long term l time histories from satellite laser ranging will be able to put constraints on the asthenosphere viscosity, if such a signature is evidenced from data or, at least, put lower limits if no significant perturbation will be observed.