Bathymetric Signatures of Submarine Forearc Deformation: A Case Study in the Nankai Accretionary Prism

Abstract

AbstractLarge earthquakes and tsunamis in subduction zone forearcs occur via slip on the shallow plate boundary and upper plate faults, but the locations, geometries, and slip histories of these faults can be difficult to constrain in regions with minimal subsurface geophysical and stratigraphic data. Here, we test a new approach to quantify the submarine seafloor geomorphic response to forearc deformation in order to identify structures that contribute to active deformation, to interpret their geometry and kinematics, and to evaluate their relative rates, magnitudes, and timing of deformation. We develop a workflow that uses filtered bathymetric digital elevation models, where long wavelength topography has been removed, to isolate the slope, relief, curvature, ridgelines, and trough lines associated with faults, fault‐related folds, and slope failures. We apply these methods to the Kumano region of the Nankai accretionary prism, southeastern Japan, where existing constraints on fault geometry, kinematics, and deformation history allow us to both evaluate the efficacy of our approach and to identify the lateral continuity of deformation processes. Our bathymetric analyses yield a high‐resolution tectono‐geomorphic map of active structures and reveal along strike variations in strain accumulation and out‐of‐sequence deformation. These metrics also demonstrate the importance of a strike‐slip fault system at the seaward edge of the Kumano Basin as a primary structure that accommodates deformation and partitions strain in the Nankai forearc. These results show the utility of using a submarine tectono‐geomorphic approach to evaluate active deformation in forearcs, particularly in regions with limited geophysical and core data.