Abstract-On the 11th March 2011, a megathrust event, called the Tohoku-oki earthquake, occurred at the North American-Pacific plate interface off northeast Japan. Transient crustal movements following this earthquake were clearly observed by a dense GPS network (GEONET) on land and a sparse GPS/Acoustic positioning network on seafloor. The observed crustal movements are in accordance with ordinary expectations on land, but not on seafloor; that is, slowly decaying landward movements above the main rupture area and rapidly decaying trench-ward movements in its southern extension. To reveal the cause of such curious offshore crustal movements, we analyzed the coseismic and postseismic GPS array data on land with a sequential stepwise inversion method considering viscoelastic stress relaxation in the asthenosphere, and obtained the following results: The afterslip of the Tohoku-oki earthquake rapidly proceeds for the first 1 year on a high-angle downdip extension of the main rupture, which occurred on the low-angle offshore plate interface. The theoretical patterns of seafloor horizontal movements due to the afterslip and the viscoelastic relaxation of coseismic stress changes in the asthenosphere are essentially different both in space and time; inshore trench-ward movements and offshore landward movements for the afterslip, while overall landward movements for the viscoelastic stress relaxation. General agreement between the computed horizontal movements and the GPS/Acoustic observations demonstrates that the postseismic curious offshore crustal movements can be ascribed to the combined effect of afterslip on a high-angle downdip extension of the main rupture and viscoelastic stress relaxation in the asthenosphere.

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A conventional “recipe” for strong ground motion prediction has been applied to the seismic fault (deep fault; located within seismogenic layer). In order to perform assessments of strong ground motions and permanent displacements at sites very close to the fault trace, we proposed the method of modeling that takes the entire ruptured fault from the ground surface to the seismic fault into account. Our approach was validated by the simulation of observed records obtained at stations very close to the fault trace of the mainshock of the 2016 Kumamoto Japan, earthquake (Mw7.1). Also, through the ground motion assessment performed for a hypothetical strike-slip fault with a 90[Formula: see text] dip angle, we found that adding the shallow fault had virtually no effect on acceleration time history, but it had a clear effect on the fault-parallel component of velocity and displacement time histories in the area close to the fault trace.