Abstract
Marine terraces have long been a subject of paleoseismology, revealing the rupture history of megathrust earthquakes. However, the crustal deformation mechanisms responsible for their formation remain inadequately explained by conventional kinematic models. A major challenge lies in the tendency of seismically uplifted shorelines to subside back to sea level during interseismic periods. This study focuses on the residual, permanent vertical deformation produced by repeated megathrust earthquakes. We investigate the effects of irregularities in the plate interface, particularly subducted seamounts. To address this, we introduce a mechanical subducting plate model (MSPM) that incorporates more realistic boundary conditions and three-dimensional geometry of the plate interface and subducting slab, using stress-boundary conditions. As a result, subducted seamounts significantly affect surface deformation, resulting in concentrated permanent uplift directly above them. We apply the MSPM to the geometry of the Sagami Trough, central Japan, and compare the simulation outcomes with the observations of marine terraces. The modeled earthquake sequences demonstrate that coseismic uplift can persist over time and contribute to terrace formation. These findings suggest that geological observations of both coseismic and long-term deformations can be explained by the influence of a subducted seamount, previously identified in seismic surveys.
Keywords
crustal deformation, Marine terraces, Plate subduction model, Sagami Trough, Subducted seamount, subduction zone