Present‐day stress states underneath the Kumano basin to 2 km below seafloor based on borehole wall failures at IODP site C0002, Nankai accretionary wedge

Abstract

We constrain the state of stress to 2 km below seafloor in the Nankai accretionary prism at the Integrated Ocean Drilling Program (IODP) site C0002F, southwest Japan, based on borehole wall failures and rock strengths. The logging‐while‐drilling resistivity images from 872.5 to 2005.5 m below seafloor show that drilling‐mud control in riser drilling worked properly to minimize borehole wall failures. Available breakouts indicate a consistent maximum compression orientation subparallel to the subducting plate margin. Breakout analysis with drill logs suggests that breakouts occurred only when borehole pressure was slightly lowered and time lag between hole cutting and image logging was several hours. This indicates that the observed breakouts are not immediate stress‐induced failure but brought up into shape gradually with time due to other mechanisms. Laboratory investigations on deformation and failure of the cores suggest that the time‐delayed breakout might be a result of progressive rock spall‐out in borehole wall damage zones that occur at a stress level close to failure condition. We constrain stress magnitudes assuming that the stress state is sufficient to bring about the damage zones at the borehole wall. An integrated method utilizing breakouts, drilling‐induced tensile fractures, and a leak‐off test suggests that the stress states are on the boundary between strike‐slip faulting and normal faulting stress regimes, and somewhat variable depending on depth. The stress magnitudes in the accretionary wedge appear to be controlled by frictional strength of the rock, such that the differential stresses are constrained by the laboratory determined frictional coefficients.

Strength Profile of the Inner Nankai Accretionary Prism at IODP Site C0002

“…S hmin and S Hmax gradually increase from 33.2 and 49.4 MPa at 975 mbsf to 57.1 and 139.6 MPa at 3,005 mbsf under hydrostatic conditions, respectively. Comparing with previous in situ stress estimation (Chang & Song, ; Kitajima et al, ), we find that their results fall between our estimated values of S hmin and S Hmax , confirming that our method provides realistic constraints on S hmin and S Hmax .…”

“…Black dashed line is overburden stress S v (Tobin et al, ). S Hmax and S hmin estimated in previous studies are also shown (Chang & Song, ; Kitajima et al, ).…”

In Situ Stress and Pore Pressure in the Deep Interior of the Nankai Accretionary Prism, Integrated Ocean Drilling Program Site C0002

“…Our analysis suggests that at Site C0002, S H is smaller than S v (normal faulting regime) in the Kumano Basin and becomes close to or greater than S v within the prism. Our estimation of stresses is generally consistent with borehole breakout analyses above 2,000 mbsf at Site C0002 (Chang et al, ; Chang & Song, ; Huffman et al, ; Song & Chang, ), although our estimated S H magnitudes below 1,350 mbsf are somewhat greater than those from the breakout analyses. One potential explanation for this difference is that as mentioned above, the assumption of an in situ CSL condition yields upper bound estimates of both S H and P p .…”

In situ stress conditions at IODP Site C0002 reflecting the tectonic evolution of the sedimentary system near the seaward edge of the Kumano basin, offshore from SW Japan

“…Here we suggest that sedimentary materials in the Quaternary basin are possibly poorly consolidated, so that analysis using the Mohr‐Coulomb failure criterion is valid, whereas the modified Wiebols and Cook criterion may be more applicable to the relatively more consolidated rock mass of the accretionary prism. Present‐day stresses underneath the Kumano basin, as constrained by empirical rock strength values and borehole failures during and after drilling, are also consistent with the results of our stochastic analysis based on the true‐triaxial strength criterion [ Chang and Song , ]. After the major change in σ h min during the initial formation of the megasplay, cyclical variations of σ h min may not have been large enough to change the principal stress orientation during every coseismic event.…”

“…In the Kumano basin, the occurrence of many normal faults around C0002 (Figure ) supports the interpretation that the least horizontal stress trends parallel to the seismic profile, which is nominally inconsistent with the observed plate convergence along the same direction [ Sacks et al ., ]. Present‐day stress states at this location beneath the Kumano basin, down to 2 km below the seafloor, appear to be controlled by the frictional strength of the rocks [ Chang and Song , ]. Vertical variations of in situ stress at this site are pivotal to understanding the tectonic evolution of the sedimentary system in relation to frictional activity along the seismogenic zone [ Bilek and Lay , ; Park et al ., ; Moore et al ., ].…”