The Japan Trench Fast Drilling Project (JFAST) drilled three boreholes at Site C0019 to investigate the large shallow slip during the 2011 Tohoku-Oki, Japan earthquake (Mw 9.0). Borehole C0019B, dedicated to logging-while-drilling (LWD), penetrated ~850 meters below seafloor (mbsf) at a water depth of approximately 6890 m, and penetrated through the plate boundary fault zone between the subducting Pacific Plate and the overriding North American Plate. The plate boundary is located at ~820 mbsf and was the likely slip zone during the Tohoku-Oki earthquake. Borehole wall resistivity images obtained from LWD show that drilling-induced borehole breakouts are common in a wide depth range above the fault but are not recorded below the fault. Breakouts are reliable indicators of the orientations of current maximum and minimum horizontal stresses and can also be used to constrain horizontal stress magnitudes. The results of breakout analyses indicate the maximum horizontal stress orientation is highly variable in the upper part of the borehole, but has a clear preferred orientation in a northwest–southeast direction in the deeper part just above the fault. This preferred orientation adjacent to and above the fault is approximately parallel to the direction of the Pacific Plate subduction. Stress magnitude constraint calculations incorporating core data, indicate that the post-earthquake stress states in the hanging wall are either in or close to the normal faulting stress regime.

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A method for measurement of stress change is suggested to monitor rock stress using a borehole. The two-dimensional state of stress change in a plane perpendicular to the axis of a borehole drilled within a rock mass can be measured by this method, which is named the Cross-sectional Borehole Deformation Method (CBDM). In this paper, the theory of the CBDM is firstly described, as well as the prototype instrument and theoretical procedure for correcting eccentric positioning of the instrument in the borehole.Then the estimation of the stress change is demonstrated in a laboratory experiment, using a granite plate with a borehole. Finally, applying the CBDM to measured stress change around an underground cavern during and after excavation, the changes of rock stress distribution around the cavern is shown. It makes clear that stress change in the immediate rock mass of the cavern can be estimated by the CBDM and that the CBDM is available for measuring rock stress change.

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