The Gravitational Signature of the December 26, 2004 Sumatran Earthquake

Besides generating seismic waves, which eventually dissipate, an earthquake also generates a static displacement field everywhere within the Earth. This global displacement field rearranges the Earth's mass thereby causing the Earth's gravitational field to change. The size of this change depends upon the size and focal mechanism of the earthquake. The Sumatran earthquake of December 26, 2004 is the largest earthquake to have occurred since the 1960 Chilean earthquake. Using a spherical, layered Earth model, the coseismic effect of the Sumatran earthquake upon the Earth's gravitational field is computed to spherical harmonic degree and order 100. Modeling the earthquake as consisting of 5 subevents having a total moment magnitude Mw of 9.3, it is found that the Earth's oblateness J2 should have decreased by 2.37×10-11 and the Earth's pear-shapedness J3 should have decreased by 0.63×10-11. The predicted change in oblateness is perhaps detectable if other effects, such as those of the atmosphere, oceans, and continental water storage, can be adequately removed from the observations. The predicted change at higher degrees is compared to GRACE measurements acquired before and after the earthquake from which atmospheric and oceanic effects have been removed.