I've been a great admirer of GRACE since the first time I 'met her'. That was at a meeting in Potsdam, Germany, a beautiful city just outside (on the 'wrong side' = old Eastern Germany) Berlin. I was attending a geodesy meeting, I do not even remember exactly which one, when we popped in to listen to Chris Reigber, one of the pioneers in space-based gravity measurements, giving a talk thanking the community for a reward he had just gotten for his work in this field.
GRACE is the brainchild of Chris Reigber and his group at Geoforschungzentrum in Potsdam, and NASA. One of the remarkable things about GRACE is that she over performs, all the time, again and again.
Shin-Chan Han works at NASA and has looked at GRACE looking at Tohoku. He found astonishing facts. GRACE can namely see the earthquake over a longer period of time so that we get the accumulated magnitude of the earthquake. The plates continue to move after the quake and that is added up in terms of energy and then magnitude.
Independently from seismic and GPS ‘displacement’ data analysis, GRACE finds the CMT source depth within the lower crust and further delineates a group of earthquake CMT parameters; increasing dipping angles (7 – 16° ± 2°) and, simultaneously, decreasing moment magnitudes (Mw 9.17 – 9.02 ± 0.04) with increasing source depths from 15 to 24 km within the lower crust.
GRACE CMT solutions indicate the averaged earthquake moment over a month including postseismic effects (~10% of coseimic moment).
Large negative anomalies (-5 to -20 microGal) were observed in the back-arc region of the thrust faults after the 2004 Sumatra-Andaman IS. (Mw9.2-9.3), 2010 Maule, Chile (Mw8.7-8.8), and 2011 Tohoku-Oki earthquake (Mw9.0-9.1). They consistently indicate dilatation in the crust after the ruptures. (ocean reduces offshore positive anomalies and is not the ‘cause’ of negative anomalies)
The enhanced instrumentation such as laser tracking (from GRACE FO, GRACE-II) would be helpful to explore the smaller and more frequent seismic events (~Mw 7.5-8).
