A source model for the great 2011 Tohoku earthquake (Mw=9.1) from inversion of GRACE gravity data

The co-seismic slip distribution of the 2011 Tohoku megathrust earthquake is constrained from GeoForschungsZentrum (GFZ) Gravity Recovery and Climate Experiment (GRACE) Level 2 data time series and our self-gravitating, compressible 1-D Earth model. After spatial localization of space gravity data in the surrounding of the U.S. Geological Survey (USGS) epicenter by means of orthogonal Slepian functions, we estimate the long-wavelength co-seismic gravity signature. The pattern is bipolar: the positive pole off-shore in the Pacific ocean (+3.61 mu Gal) and the negative pole in the northern Japan and Japan sea (-8.6 mu Gal). Inversion of the GRACE data resolves average features of finite fault models: the total seismic moment (5.3 +/- 1 x 10(22) Nm, corresponding to M-w = 9.1 in agreement with the centroid-moment-tensor solution), the rake angle (87 degrees +/- 9 degrees), and the mean position of the slip distribution on the fault plane. We obtain that the mean depth of the rupture is 17.1 +/- 5 kin, just below the Moho discontinuity, although we cannot exclude that the rupture also extended to shallower crustal layers and deeper within the lithospheric mantle due to the poor resolution of the along-dip dimension of the fault. (C) 2012 Elsevier B.V. All rights reserved.