Structural restoration and application of dynamic Coulomb wedge theory to the Nankai Trough accretionary wedge toe

Abstract

A three-dimensional (3-D) prestack depth migrated seismic reflection data volume acquired off Shikoku Island, Japan, (Muroto Transect) images the seaward portion of the accretionary prism where the Philippine Sea plate subducts beneath the Eurasian Plate. We use structural restoration software (LithoTect®), in combination with velocity-porosity transforms derived from seismic interval velocities, to calculate and interpret total horizontal shortening lengths along four cross-sectional profiles through the volume. The results reveal that 63-82% of the total horizontal shortening results from strain. Evaluation of conjugate thrust pairs indicate a relatively low coefficient of internal friction (µ≈0.2-0.3) and basal friction (µb"≈0.025) along the wedge, suggesting that sediments deform internally yet still maintain low sediment strength during the initial stages of accretion. Application of the dynamic Coulomb theory demonstrates that the current state of the Muroto Transect outer wedge toe is in a near-critical stable state and may become critical upon small increases in either basal friction or fluid pressure observed during seismic rupture.