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

Large 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. Plain Language Summary It is important to understand the types and activity of faults in subduction zones, which can host large earthquakes that generate devastating tsunamis. Subsurface drill cores and seismic reflection imaging provide the most direct information on these faults, but these data sets are spatially limited and can be difficult to collect. Here, we identify and characterize faulting by developing a workflow of algorithms applied to seafloor elevation data, which is often more readily available and spatially continuous than subsurface data. Using data collected from the Kumano region of the Nankai Trough offshore of southeastern Japan, we test an approach to identify the topography related to faulting, folding, and slope failures, and interpret the relative rates of fault-related deformation. We create a detailed fault map and show how the fault activity varies across the study area. Lastly, we demonstrate the importance of a prominent strike-slip feature across the margin, which has implications for earthquake and tsunami potential in the region.

Automated summary

This study develops a new approach to identify and characterize geological structures in forearc regions using seafloor topographic data, which provides high-resolution maps of active structures and reveals variations in strain accumulation and out-of-sequence deformation. The study highlights the importance of a strike-slip fault system in the Kumano region of the Nankai Trough, which has implications for earthquake and tsunami potential.