Quantification of in situ stress and pore pressure in the Nankai subduction zone: Effects of lithology and loading path

Abstract:

In subduction systems, sediments are deformed under complicated loading conditions in time and space. Although quantification of in situ stress and pore pressure at depth is crucial for understanding the absolute strength and the range of slip behaviors along megathrust faults, direct measurements of these quantities are very limited. Our recent work has used empirical relations between sonic velocity, porosity, and stress defined through a suite of laboratory deformation experiments on sediment cores to estimate in situ stress from seismic survey data along the megathrust plate boundary in the Nankai Trough [Kitajima and Saffer, 2012]. Here, we extend this analysis to examine the effects of loading path and lithology in detail, and to quantify in-situ pore pressure and stress state throughout the subduction complex - including the underthrust section, the accretionary prism, and overlying Kumano basin.

To first order, different loading paths do not affect the relationship between P-wave velocity and porosity for a given sediment, but they do affect the relationship between porosity and effective mean stress: at the same effective mean stress, sediments are more compacted with increasing differential stress. The relations between P-wave velocity, porosity, and effective mean stress are also different for different lithologies. In particular, the empirical relations defined by our laboratory experiments are markedly different for sands and mudstones. Based on an estimation of in situ stress and pore pressure for a range of possible scenarios with different lithologies and loading paths, we suggest that the Kumano basin is loaded in a uniaxial stress condition, whereas the prism and underthrust section are most likely loaded along a near critical stress condition with lateral compression.