Pore Pressure Regime and Fluid Flow Processes in the Shallow Nankai Trough Subduction Zone Based on Experimental and Modeling Results from IODP Site C0023

Abstract

Pore fluid pressure is a critical parameter controlling the dynamics of subduction zones and related geological hazards, but quantitative constraints on pore pressure are sparse. Here we provide robust estimates of pore pressure in the shallow Nankai Trough subduction zone based on consolidation experiments and numerical models. Our experimental data reveal excess pore pressures with nearly constant ratios of pore pressure to lithostatic stress for the accreted and underthrust sediments, implying the décollement is not a fluid barrier as previously inferred. Also, our research suggests repeated lithostatic pore pressure transients in the last 3 kyrs, probably associated with the propagation of the frontal thrust. The coupled pore pressure dissipation and chemical transport model results let us conclude the updip migration fluid originates from an overpressured horizon nearby, confirming the view of a heterogeneous plate boundary with overpressured and permeable patches along the fault surface that shift in time and space. The resulting high excess pore pressure leads to the shear strength of Nankai subduction thrust as low as 0.4–0.8 MPa. This provides quantitative constraints on the weakness of subduction thrusts at the toe of accretionary prisms and may explain why seismic slip can propagate all the way to the trench in some earthquakes (e.g., 2011 Tohoku event).