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Topographic effects on coseismic gravity change for the 2011 Tohoku‐Oki earthquake and comparison with GRACE

We estimate gravity changes due to coseismic horizontal deformation and inclined bathymetry (referred to as topographic effects in this study) associated with the 2011 Tohoku‐Oki earthquake, using a layered half‐space dislocation model. As an approximation, we represent inclined topography with ramp‐body models in the near field of the earthquake and calculate topographic effects from both an inclined seafloor and Moho. The Moho effect partly compensates that from the seafloor. An additional contribution comes from seawater column variations as the sloped seafloor moves trenchward. Topographic effects are first‐order contributions to coseismic gravity changes for the 2011 Tohoku‐Oki earthquake. After 300 km Gaussian smoothing, comparable to the spatial resolution of the Gravity Recovery and Climate Experiment (GRACE), topographic effects create a dominantly negative gravity pattern with amplitudes up to 1.1 μGal for oceanic areas surrounding the epicenter. There are discrepancies between model predictions (−10.4 to +6.9 μGal) and GRACE observations (10.1 to +1.7 μGal). Errors in the adopted fault slip model and difficulties in separating coseismic and postseismic deformation may contribute to these, but topographic effects, currently neglected in dislocation models, are also responsible. Topographic effects ought to be included in coseismic fault slip and related gravity modeling for the 2011 Tohoku‐Oki earthquake and similar events in regions of steep topography.