Pre-Stack Depth Imaging and Pore-Fluid Pressure Estimation Along the Nankai Trough Subduction Zone Off the Kii Peninsula, SW Japan

The behavior of the plate-boundary thrust fault (décollement), which is the source of the devastating Nankai megathrust earthquakes, is strongly influenced by pore-fluid pressure, a key factor in controlling fault strength. To better understand these controls, the composition of sediments in the Shikoku Basin has been extensively studied through samples obtained by ODP and IODP. These studies reveal that turbidite sediments are widespread throughout the Shikoku Basin and are also present beneath the décollement, suggesting a potential link between sediment composition and fault mechanics. To investigate the crustal structure of the Nankai Trough subduction zone offshore Cape Shiono, we applied 2D pre-stack depth migration imaging by using multi-channel seismic reflection data and estimated physical properties of the décollement from 0 to 30 km toward the trench. Results show significantly lower vertical effective stress than expected, implying high pore-fluid pressure. While previous research has identified pore-fluid pressure as the primary factor influencing shallow very low frequency earthquakes (SVLFEs), our comparison between Cape Shiono and Cape Muroto shows no significant difference in pore-fluid pressure ratios beyond 13 km. However, the accretionary wedge off Cape Shiono is roughly twice as thick as that off Cape Muroto, suggesting that wedge thickness—and the associated shear stress differences—may be the key control on SVLFE activity. We also reviewed studies from the Japan Trench, Alaska, and Ecuador, estimating overpressure ratios to compare seismicity. These results align with our findings, supporting the conclusion that physical properties, and wedge thickness, play a critical role in SVLFE activity.