After the occurrence of a huge subduction-zone earthquake, a characteristic crustal movement called aftereffect change is observed.Accurately grasping the spatial change and time evolution of this fluctuation is extremely important for estimating the scale of the next major earthquake that can occur around the epicenter and for evaluating the possibility of occurrence.

　The group established a wide-area seafloor crustal movement observation network after the 2011 Tohoku-oki Earthquake.After observing for about 4 years, we succeeded in grasping the actual condition of the aftereffect fluctuation. The observation point continued to move westward off Miyagi prefecture, where a huge fault slip occurred during the 2011 earthquake, while the opposite eastward movement was observed off Fukushima prefecture to the south.The movement off the coast of Miyagi prefecture is explained by the "viscoelastic relaxation" caused by the slip during a huge earthquake, and the movement off the coast of Fukushima prefecture is explained by the "after-effect slip" where the plate boundary continues to slide slowly after 2011. It was shown that the deformation mechanism is simultaneously progressing under the seabed off the coast of Tohoku.

　This result provided an important clue for predicting future seismic activity in the Tohoku region, as well as improving the understanding of the mechanism of the Tohoku-oki earthquake six years ago.The progress of such research is expected to contribute to improving the accuracy of prediction of the occurrence of large subduction-zone earthquakes off the coast of Tohoku in the future.

Paper information: [Science Advances] Along-trench variation in seafloor displacements after the 2011 Tohoku earthquake