Seafloor geodesy delineates fast megathrust relocking after the giant 2011 Tohoku earthquake

How and when the subduction megathrust recovers its locking state after a great earthquake is important to understanding earthquake-cycle physics and future hazards. Here we report the delineation of the relocking process after the 2011 magnitude 9 Tohoku earthquake with 14 years of postseismic seafloor geodetic observations. These data, combined with onshore geodetic observations, analyses of repeating earthquakes, and numerical modeling, reveal how the recovery of locking is interrupted by afterslip on the megathrust and accompanied by stress relaxation in the viscoelastic upper mantle. The results indicate that the megathrust has already arrived at a locking state today similar to that before the earthquake despite the ongoing stress relaxation. The acquired knowledge aids understanding of megathrust earthquake cycles at other subduction zones.