Investigation of the Co-seismic Gravity Field Variations Caused by the 2004 Sumatra-Andaman Earthquake Using Monthly GRACE Data

Gravity Recovery and Climate Experiment (GRACE) observations have been used to detect the co-seismic and post-seismic gravity field variations due to the M_w=9.3 Sumatra-Andaman earthquake that occurred on December 26, 2004. This article focuses on investigating some gravitational effects caused by this huge earthquake. We computed the geoid height changes, the equivalent water height (EWH) changes, and the gravity changes using the GRACE Level-2 monthly spherical harmonic (SH) solutions released by University of Texas Center for Space Research (UTCSR). The GRACE results agree well with the prediction by a dislocation model and are consistent with the results obtained by some previous scholars. In particular, we calculated the three components of the gravity gradient variations and found that they can recover the seismic-related signature more sensitively due to a certain degree of amplification of the signals. A positive-negative-positive mode predominates in the spatial distribution of the horizontal components of the gravity gradient variations, which is possibly attributed to the anomalies in the crustal density distribution caused by the uplift-subduction effect of the dip-slip earthquake. Moreover, the latitude components of the gravity gradient changes show strong suppression of the north-south stripes, which is due to the along-orbit measurements of the two GRACE satellites. We conclude that the positive-negative-positive mode in latitude gravity gradient changes would be a more sensitive feature to detect the deformations of some major dip-slip earthquakes by GRACE data.