The Gravity Recovery and Climate Experiment (GRACE) Level-2 products, i.e., monthly gravity field models expressed in spheric harmonic (SH) coefficients, have been released in several versions since 2002. In this study, we analyze the extraction of co-seismic signals associated with the 2004 Sumatra-Andaman M_W9.1 earthquake, using GRACE data from four versions, namely the RL01, RL04, RL05, and RL06, provided by the Center for Space Research (CSR), University of Texas at Austin. Results indicate that the co-seismic signals extracted from later versions of GRACE data are less affected by noise. Moreover, the earliest RL01 vision can also reflect the co-seismic gravity-change signals of the 2004 Sumatra-Andaman earthquake. In particular, we find that the spatial pattern of the co-seismic signals extracted from the RL04 version of GRACE data exhibits significant distortion, which is probably due to the large errors introduced in the processing of the atmosphere and ocean models in the gravity field inversion. Comparison between the dislocation model prediction and GRACE observation suggests that the GRACE results from later versions are more consistent with the model prediction. In addition, the dislocation models with stronger constraints from near-field measurements during the fault-slip inversion provide a more consistent prediction with the GRACE observation. The uncertainty estimation of GRACE data over the global oceanic region reveals that the noise levels gradually decrease from the earlier to later versions, with the RMSs of 7.1, 4.9, 2.4, and 2.1 μGal (with 300 km spatial smoothing) for the RL01, RL04, RL05, and RL06 versions, respectively.

On January 20, 2025, an M_W6.0 earthquake occurred in Chiayi County, southwestern Taiwan, China. In this study, we utilize Sentinel-1 data to investigate the coseismic ground displacement and fault slip model of this event. Here, we present the geodetic fault model of the 2025 Chiayi earthquake. While both east-dipping and west-dipping fault models can adequately reproduce the InSAR observations, the east-dipping model was found to better align with the quasi-east-west and quasi-vertical deformation fields derived from decomposing Interferometric Synthetic Aperture Radar (InSAR) ascending and descending orbits, the characteristics of interseismic deformation, and the tectonic features of the seismic region. The best-fitting east-dipping uniform-slip model shows that the seismogenic fault is characterized as a reverse slip striking 336.2°, dipping northeastward. The fault's upper boundary is buried at a depth of 8.0 km, with a dip angle of 27.6° and a slip angle of 61.6°. Calculations of Coulomb stress changes reveal that regions experiencing positive changes are primarily concentrated at the northeast and southwest extremities of the Chiayi earthquake rupture, as well as in the northwest and southeast lobes flanking the rupture zone.