ABSTRACT
An IODP (Integrated Ocean Drilling Program) scientific deep drilling project, NanTroSEIZE (Nankai Trough Seismogenic Zone Experiments) is undergoing in the southwest Japan subduction zone to understand the physics of an active fault (Kinoshita et al., 2006). Determination of current in-situ stress is one of the main scientific objectives. In the first stage of NanTroSEIZE, several vertical boreholes were drilled by the deep drilling vessel CHIKYU. In two drilling sites C0002 and C0006, principal horizontal stress orientations were independently determined by two methods. The first method is a core-based three-dimensional method called anelastic strain recovery (ASR) method; and the other one is a traditional method using borehole wall image to analyze drilling induced borehole wall compressive failures (breakouts) and tensile fractures developed in oil industry which yields orientations of the principal horizontal stresses (Zoback et al., 2003). The highlights of these stress measurement results
and its interpretations have been reported by Byrne et al. (2009) and Chang et al. (2009). Hereafter, we focus on the comparison between the orientations of the in-situ stresses determined by the ASR measurements and borehole breakout analyses, and give a stress orientation summary in the region.
