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Authors: Cannelli, V.* 
Melini, D.* 
Piersanti, A.* 
Boschi, E.* 
Title: Postseismic signature of the 2004 Sumatra earthquake on low degree gravity harmonics
Issue Date: 2008
Series/Report no.: /113 (2008)
DOI: 10.1029/2007JB005296
Keywords: low degree gravity harmonics
postseismic relaxation
Sumatra earthquake
Subject Classification04. Solid Earth::04.03. Geodesy::04.03.99. General or miscellaneous 
04. Solid Earth::04.07. Tectonophysics::04.07.99. General or miscellaneous 
Abstract: We perform an extensive analysis of the low degree gravitt 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 ($J_l$) in correspondence of the 2004 Sumatra earthquake, and a similar perturbation, but with weak associated statistical significance, also in the non-zonal 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 astenospheric viscosities is provided. As a result, an evident signature of the Sumatra earthquake on $\dot{J}_l$ time series is expected for asthenospheric viscosity values below $10^{18}$ Pa s. Therefore, long term $\dot{J}_l$ time histories from SLR geodetic missions 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.
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