Gravity changes due to the Sumatra-Andaman and Nias earthquakes as detected by the GRACE satellites: a reexamination

P>In this paper, we study the geoid changes caused by the Sumatra-Andaman earthquake (2004 December 26) and the Nias earthquake (2005 March 28) using data from the gravity recovery and climate experiment (GRACE) satellites. New is that we explore the possibility to separate the effects of the Sumatra-Andaman and Nias earthquakes. Based on correlation analysis we investigate the separability of the Sumatra-Andaman and Nias signals, and the feasibility of separating the non-seismic linear trend from the time-dependent post-seismic relaxation effect. As input data, we use 76 months of global geopotential coefficients from the GFZ release 4 (RL04) of GRACE data. To suppress unphysical artefacts in GRACE data ('striping'), the GRACE coefficients are smoothed with an anisotropic decorrelation filter. Model parameters are estimated through a Bayesian approach. We compare our results with geophysical models of both earthquakes, calculated based on the viscoelastic-gravitational dislocation theory. The slip models used are inverted from GPS measurements. Our findings indicate that, the inversion of GRACE data for the Sumatra-Andaman effect will contain significant signal from the Nias earthquake, and will benefit from removing the modelled Nias effect from the data. Comparing GRACE measurements to geophysical models reveals significant differences. We suggest, how the GPS-calibrated models may be improved to better fit the GRACE data. In the first months after the earthquake, we also find evidence for a fast process in the Andaman sea, possibly due to aseismic slip. Similar phenomena were observed in previous studies.