Marine terraces have long been a subject of paleoseismology to reveal the rupture history of megathrust earthquakes. However, the mechanisms underlying their formation, in relation to crustal deformation, have not been adequately explained by kinematic models. A key challenge has been that the uplifted shoreline resulting from a megathrust earthquake tends to subside back to sea level during subsequent interseismic periods. This study focuses on the remaining permanent vertical deformation resulting from steady plate subduction and examines it quantitatively using three plate subduction models. Specifically, we pay attention to the effects of irregular geometries in the plate interface, such as subducted seamounts. Besides a simplified model examination, this study employs the plate geometry around the Sagami trough, central Japan, to compare with surface deformation observation. The subduction models employed are the kinematic subducting plate model, the elastic/viscoelastic fault model, and the mechanical subducting plate model (MSPM). The MSPM, introduced in this study, allows for more realistic simulations of crustal displacements by imposing net zero shear stress change on the plate boundary. Notably, the presence of a subducted seamount exerts a significant influence on surface deformation, resulting in a concentrated permanent uplift above it. The simulation of earthquake sequence demonstrates that coseismic uplifts can persist over time and contribute to the formation of marine terraces. The results demonstrated that the geological observations of coseismic and long-term deformations can be explained by the influence of a subducted seamount, previously identified in seismic surveys.

We developed a mechanical subducting plate model and re-examined the crustal deformation history in the Sagami Trough subduction zone, central Japan, the northernmost convergence boundary of the Philippine Sea Plate. The elevation distributions and formation ages of the Holocene marine terraces, representing past coseismic and long-term coastal uplifts, have been thoroughly investigated in this region. However, no physically consistent formation scenario to explain them has been demonstrated. Surface deformations within subduction zones are typically calculated using kinematic elastic dislocation models, such as the back-slip model. However, these models cannot explain permanent deformation after an earthquake sequence. This study develops a mechanical subducting plate model that balances the slips of interplate shear stress and can produce permanent deformations caused by a local bump geometry. We modeled earthquake recurrences by shear stress accumulation and coupling patches. As a result, we successfully reproduced the averaged uplift rate distribution estimated from the Holocene marine terraces. The findings suggest that the subducted seamount significantly affects long-term deformation patterns. In addition, the discrepancy between the elevation distributions and formation ages of Holocene marine terraces, which previous geological studies have indicated, can be interpreted by the rupture delay of coupling patches. This study also demonstrates that the traditional assumption of the back-slip model on the plate boundary for long-term subduction possibly results in an oversimplified model.