State of Stress Across Major Faults in the Nankai Subduction Zone Estimated From Wellbore Breakouts

Quantifying the orientation and magnitude of stress at tectonically active margins and along major fault systems is integral to understanding the mechanics of faulting and earthquakes. Here we use data collected as part of the IODP (Integrated Ocean Drilling Program) Nankai Trough Seismogenic Zone Experiment to constrain in situ stress magnitudes within the Nankai accretionary prism from wellbore breakouts. During drilling, stresses are concentrated around the borehole wall and result in compressional failure in the form of breakouts. We place bounds on the minimum (S_hmin) and maximum (S_Hmax) horizontal stresses using a combination of breakout widths, together with estimates of rock strength derived from p-wave velocity. Our analysis focuses on two regions: (a) a major out of sequence thrust fault located ∼25 km landward of the trench, termed the megasplay (Sites C0004, C0010, and C0022), and (b) the décollement within a few kilometers of the trench (Sites C0006 and C0024). We find that the stress state along the megasplay lies in a thrusting regime (S_Hmax > S_hmin > S_v) with shear stresses consistent with slip on the fault, whereas at the toe of the prism stress state is near-isotropic, with small differential stresses. Our results suggest that the megasplay fault is near failure. In contrast, the décollement near the trench is far from failure, likely requiring a combination of a frictionally weak plate boundary and progressive interseismic loading that drives increasing horizontal stress over time.