Small-scale stress fluctuations in borehole breakouts and their implication in identifying potential active faults around the seismogenic megasplay fault, Nankai Trough, SW Japan

Borehole breakouts are enlargements and elongation of a borehole in a particular direction, caused by failure of the borehole wall rock due to concentration of stresses around the borehole, and thus, have been widely used to determine the in situ stress orientation. We used electrical borehole wall images obtained during offshore scientific drilling (IODP) that penetrated through a seismogenic megasplay fault in the Nankai Trough, off SW Japan, and extracted a number of borehole breakouts. Most of the breakouts show directions that can be explained by the regional convergence, but some are obviously rotated by faults and fractures in the megasplay fault zone and in its hanging wall. Stress magnitudes estimated from the width of the breakouts also show some decrease in the horizontal stresses, suggesting that slip along the faults and fractures release shear stress affecting these surfaces. Since such surfaces may have the capability to reactivate where the stresses affecting the surface are geometrically appropriate, the method presented in this paper may contribute to identifying active fault surfaces. This knowledge allows us to identify which surfaces need to be examined in detail to assess their potential for future activity.