In Situ Permeability and Scale Dependence of an Active Accretionary Prism Determined From Cross-Borehole Experiments

Permeability controls fluid flow in the Earth's crust and affects a wide range of processes including advective transport and pore pressure generation. However, in situ measurements of permeability are few, especially in active tectonic settings or at scales relevant to regional flow. We analyze formation fluid pressure records from oceanic boreholes in the Nankai accretionary prism offshore southwest Japan, focusing on unexpected responses to drilling operations conducted at boreholes ~100 m to the northeast. We develop a 2-D numerical model of transient fluid flow and conduct a parametric grid search to define hydraulic diffusivity. A value of 0.19–0.46 m²/s (corresponding to a permeability of 9.8 × 10⁻¹³ to 2.4 × 10⁻¹² m²) yields the best fit to observed pressure responses. Together with laboratory measurements on core samples and drillstrem tests reported in previous studies, our analysis indicates a strong scale dependence of permeability, likely reflecting the presence of permeable faults and fractures.