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.

Most conceptual models for how fluids and sediment influence slip behavior and uplift along subduction margins are poorly constrained by geophysical observations. Given the complexity of subduction systems, overcoming this gap in knowledge will require a systems-level approach which uses high quality geophysical constraints. We present wide-angle, onshore-offshore seismic data collected along the northern Hikurangi margin, New Zealand, from which P-wave velocities were calculated using active- and passive-sources. A gravity model and reflection profiles were also assembled to create a complete, ~400 km long transect which images the incoming plate, down going slab, overthrusting forearc, and backarc rift. Velocities and gravity modelling help to constrain the lithology of the forearc basement to ~20 km depth. Upper plate lower crustal velocities and reflectivity point to the presence of underplated sediments immediately above the lithospheric mantle nose, suggesting that underplated sediments are driving uplift of the forearc. Comparing these results to geophysical images from the southern Hikurangi margin, we suggest that the backarc rift influences along-strike changes in the compressional stresses experienced by the forearc, driving changes in bending stresses within the subducting slab.

Most conceptual models for how fluids and sediment influence slip behavior and uplift along subduction margins are poorly constrained by geophysical observations. Given the complexity of subduction systems, overcoming this gap in knowledge will require a systems-level approach which uses high quality geophysical constraints. We present wide-angle, onshore-offshore seismic data collected along the northern Hikurangi margin, New Zealand, from which P-wave velocities were calculated using active- and passive-sources. A gravity model and reflection profiles were also assembled to create a complete, ~400 km long transect which images the incoming plate, down going slab, overthrusting forearc, and backarc rift. Velocities and gravity modelling help to constrain the lithology of the forearc basement to ~20 km depth. Upper plate lower crustal velocities and reflectivity point to the presence of underplated sediments immediately above the lithospheric mantle nose, suggesting that underplated sediments are driving uplift of the forearc. Comparing these results to geophysical images from the southern Hikurangi margin, we suggest that the backarc rift influences along-strike changes in the compressional stresses experienced by the forearc, driving changes in bending stresses within the subducting slab.

Seismic velocity structures have been estimated using ambient noise records observed by distributed acoustic sensing (DAS) technology. Previous studies have obtained S-wave velocity (Vs) structures along submarine cables using surface waves; however P-wave velocity (Vp) structure have seldom been constructed because ambient noise primarily contains surface waves. Here, we show P and Rayleigh wave extractions from ambient seafloor noise observed by DAS, and also estimate the Vp and Vs structures along a submarine cable deployed off Cape Muroto in the Nankai subduction zone, Japan. To extract the P waves, we calculated the cross-correlation functions (CCFs) of the ambient noise and applied a frequency-wavenumber filter to the CCFs to remove the effects of slower surface waves. The results indicate that similar features can be obtained in the Vp and Vs profiles, and that the P wave extractions can be performed on days with poor weather conditions.

Seismic wave extractions have been performed using ambient noise records observed by distributed acoustic sensing (DAS) technology. Extractions of microseisms can be investigated at a local scale using such DAS records observed in the ocean. Here, we show P and Scholte wave extractions from ambient seafloor noise observed by DAS along a submarine cable deployed off Cape Muroto in the Nankai subduction zone, Japan. The P waves can be observed at a frequency band of 0.1–0.3 Hz and up to a distance of 6–7 km. The distance in which the P waves can be observed is controlled by the P incident angle and the DAS sensitivity to the observable apparent velocity. The effective extractions of P and Scholte waves are performed in correspondence to large significant wave heights of the sea surface, which indicate that these waves are originated from fluid distturbances at the sea surface.

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.