Pressure and Stress Prediction in the Nankai Accretionary Prism: A Critical State Soil Mechanics Porosity-Based Approach

We predict pressure and stress from porosity in the Nankai accretionary prism with a critical state soil model that describes porosity as a function of mean stress and maximum shear stress, and assumes Coulomb failure within the wedge and uniaxial burial beneath it. At Ocean Drilling Program Sites 1174 and 808, we find that pore pressure in the prism supports 70% to 90% of the overburden ((u)=0.7 to 0.9), for a range of assumed friction angles (5-30 degrees). The prism pore pressure is equal to or greater than that in the underthrust sediments even though the porosity is lower within the prism. The high pore pressures lead to a mechanically weak wedge that supports low maximum shear stress, and this in turn requires very low basal traction to remain consistent with the observed narrowly tapered wedge geometry. We estimate the decollement friction coefficient ((b)) to be similar to 0.08-0.38 ((b)=4.6 degrees-21 degrees). Our approach defines a pathway to predict pressure in a wide range of environments from readily observed quantities (e.g., porosity and seismic velocity). Pressure and stress control the form of the Earth's collisional continental margins and play a key role in its greatest earthquakes. However, heretofore, there has been no systematic approach to relate material state (e.g., porosity), pore pressure, and stress in these systems. Plain Language Summary Pressure and stress control the form of the Earth's collisional continental margins and play a key role in its greatest earthquakes. Geoscientists often describe the rock at collisional continental margins as analogous to the dirt piled in front of a bulldozer or the snow in front of a snow plow: the dirt or snow piles up to a critical angle that is a function of the strength of the material, the friction along the base of the pile, and the fluid pressure within the pile. We show how to use the material's porosity (the volume of pore space between the grains) to predict the pore pressure and stress magnitudes. The approach allows us to predict pressure and stress in collisional continental margins. We show that there are high pressures in the Nankai prism (offshore Japan), one of the great collisional zones in the world.