Co- and postseismic slip behaviors extracted from decadal seafloor geodesy after the 2011 Tohoku-oki earthquake

Abstract Investigations of the co- and postseismic processes of the 2011 Tohoku-oki earthquake provide essential information on the seismic cycle in the Japan Trench. Although almost all of the source region lies beneath the seafloor, recent seafloor geophysical instruments have enabled to detect the near-field signals of both the coseismic rupture and the postseismic stress relaxation phenomena. Annual-scale seafloor geodesy contributed to refining the postseismic deformation models, specifically to the incorporation of viscoelastic effects. However, because of the insufficiency in the spatial coverage and observation period of seafloor geodetic observations, no consensus on crustal deformation models has been reached, especially on the along-strike extent of the main rupture, even for the coseismic process. To decompose the postseismic transient processes in and around the source region, i.e., viscoelastic relaxation and afterslip, long-term postseismic geodetic observations on the seafloor play an essential role. Here, from decadal seafloor geodetic data, we provide empirical evidence for offshore aseismic afterslip on the rupture edges that had almost decayed within 2–3 year. The afterslip regions are considered to have stopped the north–south rupture propagation due to their velocity strengthening frictional properties. In the southern source region (~ 37° N), despite not being resolved by coseismic geodetic data, shallow tsunamigenic slip near the trench is inferred from postseismic seafloor geodesy as a subsequent viscoelastic deformation causing persistent seafloor subsidence at a geodetic site off-Fukushima. After a decade from the earthquake, the long-term viscoelastic relaxation process in the oceanic asthenosphere is currently in progress and is still dominant not only in the rupture area, but also in the off-Fukushima region.