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The time-variable features of interseismic deformation in subduction zones are poorly understood and commonly ignored. Here, by incorporating century-long leveling data and contemporary global navigation satellite systems (GNSS) velocities, we investigate the temporal evolution of interseismic deformation in southwest Japan using two-dimensional viscoelastic earthquake-cycle models. We find that a steady-state continental mantle viscosity of 10(19) Pa.s is required to explain these two geodetic data sets. Our preferred model predicts significant variations in the surface velocity field throughout the entire interseismic period due to viscous mantle flow driven by ongoing megathrust locking, indicating that the multiyear GNSS-derived velocity field represents a snapshot of time-varying interseismic deformation. The locking depth and locking time exert significant influences on the horizontal and vertical deformation patterns. Our findings highlight the importance of the interseismically relaxing mantle and the necessity of considering such an effect in determining the current locking state along the Nankai subduction zone.

Plain Language Summary Interseismic viscoelastic relaxation due to megathrust fault locking is explored recently using a viscoelastic Earth model for the late-stage interseismic period in the Cascadia and Peru-North Chile subduction zones, where the last great earthquake occurred centuries ago, and the next event is expected in the near future. In southwest Japan, the last great earthquake occurred only several decades ago, providing a unique opportunity to study the viscoelastic relaxation in early-stage interseismic period. We use historical measurements of uplift rates that were acquired before and after the last great earthquake and modern geodetic measurements of the horizontal and vertical surface velocities to constrain our numerical models and investigate how and why the surface deformation varies throughout the long interseismic period. We find that the horizontal and vertical velocity field gradually evolves into a long-wavelength pattern and a rather flat pattern, respectively, before the next event, which is consistent with both the historical and recent observations. Therefore, the recent and short time geodetic measurements in southwest Japan represent a snapshot of the time-variable signal because the weak Earth mantle is flowing viscously, driven by persistent megathrust fault locking, and the flow has not achieved a steady state.