Megathrust Coupling in Southwest Japan Inferred from Viscoelastic Modeling

In Southwest Japan, interseismic deformation exhibits distinct patterns, particularly across Kyushu Island, where its magnitude decreases significantly from north to south. Various mechanisms, including plate motions, fault slip on onshore fault systems, dilatational sources, and variable interplate coupling along the Nankai and Ryukyu subduction zones, have been proposed to explain these features. While previous studies have effectively modeled horizontal deformation and attributed the rotational pattern in southern Kyushu primarily to plate motion, they often neglect or inconsistently predict vertical deformation, underscoring the need for further investigation.

In this study, we employ a three-dimensional (3D) viscoelastic finite element model (FEM) to analyze interseismic deformation in Southwest Japan, spanning the transition from the Nankai to the Ryukyu subduction zone. To focus on megathrust coupling, we exclude block motion and consider other factors as secondary influences. Our goal is to reconcile horizontal and vertical geodetic observations and provide a first-order estimate of megathrust coupling in this margin through a viscoelastic model, offering a direct comparison with previously published elastic models.