Constraining Stress States in Nankai Accretionary Wedge, Southwest Japan, to 2 Km Below Seafloor

We constrain the state of tectonic stress in the Nankai accretionary wedge at the Integrated Ocean Drilling Program (IODP) site C0002, southwest Japan, based on borehole wall failures such as breakouts and drilling-induced tensile fractures (DITFs) observed during IODP Expedition 338. The borehole drilled is 2005.5 meter below seafloor (mbsf) deep and exhibits a limited number of borehole wall failures because of optimal drilling-mud pressure control under riser drilling. The breakouts occurred only when the borehole pressure was slightly lowered and the time lag between hole cutting and image logging was at least several hours due to drill-pipe extension, which suggests that the observed breakouts are not entirely stress-induced as typically assumed, but rather brought up into shape with time due to other mechanisms (e.g., hydraulic erosion). However, based on the consistent breakout azimuths throughout the drilled depths, it is inferred that the breakouts are stress-dependent such that the stress-induced premature rock damage zones are spalled out progressively with time. Thus, breakout azimuths can be used to estimate stress orientation, but breakout geometry (width) should not be used to estimate stress magnitudes. The orientation of the maximum horizontal principal stress (S_Hmax) is determined to be NE-SW to -2000 mbsf, subparallel to the subduction margin between Philippine Sea plate and Eurasian plate. We attempt to estimate stress magnitudes assuming that the stress state is sufficient to bring about rock damage zones at the borehole wall, and that it is on the verge of the creating borehole wall failures based on laboratory triaxial compression experiments in cores. An integrated method that utilizes breakouts and DITFs as well as the result from a leak-off test yields the S_Hmax values slightly higher than the vertical stress (S_v) and the minimum horizontal principal stress (S_hmin) possibly lower than S_v, suggesting that the stress state is predominantly in favor of strikeslip faulting.