Constraints on paleo-stress magnitude from triaxial test results and stress inversion analysis of Nankai accretionry prism sediments

The stress magnitude in an accretionary wedge is significant for the understanding of wedge architecture and seismogenic behavior. In this study, we constrained the stress magnitude in the Nankai accretionary wedge using stress orientations from micro-fault inversion and critical state lines of sediments from laboratory experiments. The micro-faults were observed in core samples of the IODP project `NanTroSEIZE' (Nankai Trough Seismogenic Zone Experiment). At Site C0001 (upper slope site), normal faults and reverse faults were identified in cover sediments and accretionary prism, respectively. At Site C0002 (forearc basin site), reverse, normal and strike-slip faults were observed. We conducted micro-fault inversion to obtain stress orientations and stress ratio for these faults. Triaxial deformation tests were carried out to determine critical state lines. During compaction confining pressure was increased stepwise to 400KPa, 700KPa and 1000KPa at constant fluid pressure of 300KPa. At each pressure level undrained deformation tests were carried out at an axial displacement rate of 0.01mm/s. Slopes of the critical state lines varied from 1.1 to 1.6, corresponding to a deviatoric stress vs. effective mean stress (friction coefficient) between 0.55 and 0.80. The stress polygon was used to constrain stress magnitude with the friction coefficient of the sediments from the experiments. A linear relationship of the stress polygon was calculated from stress orientations and stress ratio. The stress magnitude was smaller for normal faults than for reverse faults suggesting changing fault regimes due to stress built-up in the inter-seismic stage and stress drop after the seismic stage.