Co-seismic Gravity Changes Computed for a Spherical Earth Model Applicable to GRACE Data

Dislocation theories were developed conventionally for a deformed earth surface because most traditional gravity measurements are performed on the terrain surface. However, through development of space geodetic techniques such as the satellite gravity missions, co-seismic gravity changes can be detected from space. In this case, the conventional dislocation theory cannot be applied directly to the observed data because the data do not include surface crustal deformation (the free air gravity change). Correspondingly, the contribution by the vertical displacement part must be removed from the traditional theory. This study presents the corresponding expressions applicable to space observations. In addition, a smoothing technique is necessary to damp the high-frequency contribution so that the theory can be applied reasonably. As examples, the Sumatra earthquakes (2004, 2007) are considered and discussed.