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We quantify gravity changes after great earthquakes present within the 10 year long time series of monthly Gravity Recovery and Climate Experiment (GRACE) gravity fields. Using spherical harmonic normal‐mode formulation, the respective source parameters of moment tensor and double‐couple were estimated. For the 2004 Sumatra‐Andaman earthquake, the gravity data indicate a composite moment of 1.2 × 10 23 N m with a dip of 10°, in agreement with the estimate obtained at ultralong seismic periods. For the 2010 Maule earthquake, the GRACE solutions range from 2.0 to 2.7 × 10 22 N m for dips of 12°–24° and centroid depths within the lower crust. For the 2011 Tohoku‐Oki earthquake, the estimated scalar moments range from 4.1 to 6.1 × 10 22 N m, with dips of 9°–19° and centroid depths within the lower crust. For the 2012 Indian Ocean strike‐slip earthquakes, the gravity data delineate a composite moment of 1.9 × 10 22 N m regardless of the centroid depth, comparing favorably with the total moment of the main ruptures and aftershocks. The smallest event we successfully analyzed with GRACE was the 2007 Bengkulu earthquake with M 0 ~ 5.0 × 10 21 N m. We found that the gravity data constrain the focal mechanism with the centroid only within the upper and lower crustal layers for thrust events. Deeper sources (i.e., in the upper mantle) could not reproduce the gravity observation as the larger rigidity and bulk modulus at mantle depths inhibit the interior from changing its volume, thus reducing the negative gravity component. Focal mechanisms and seismic moments obtained in this study represent the behavior of the sources on temporal and spatial scales exceeding the seismic and geodetic spectrum.
Han et al. (Fri,) studied this question.