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doi:10.2204/iodp.proc.314315316.216.2012

Introduction

Integrated Ocean Drilling Program (IODP) Expeditions 315 and 316 of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) complex drilling project were primarily dedicated to core sampling at multiple sites across the Kumano Basin region, offshore Kii Peninsula, Japan. Six relatively shallow sites (<1000 meters below seafloor [mbsf]) were successfully cored and sampled along a transect from the Kumano forearc basin (Site C0002) to the frontal thrust of the Nankai accretionary prism (Sites C0006 and C0007), including cores taken from the shallow portion of a major out-of-sequence thrust fault (megasplay) (Sites C0001, C0004, and C0008) (Fig. F1).

This report presents the results of a pilot study intended to evaluate the potential of using anisotropy of P-wave velocity (APV) as a complementary tool for deformation studies. This evaluation was performed through a sample-to-sample comparison between APV and the commonly used anisotropy of magnetic susceptibility (AMS) parameter.

Whereas AMS is sensitive to magnetic mineral fabric, P-wave velocity may illuminate the mechanical history of sedimentary rock by its sensitivity to pore space, grain alignment, the nature of intergranular contacts, and damage features such as microcracks. In particular, changes in clay platelet alignment and pore space anisotropy in response to tectonic stress, which have been shown to significantly influence electrical conductivity (Henry et al., 2003), are also likely to have an effect on P-wave velocity. Many studies have focused on microstructural control of APV (e.g., Lo et al., 1986; Kern, 1993; Johnston and Christensen, 1995; Hornby, 1998; Louis et al., 2003, 2004, 2005), including work related to IODP expeditions (Brückmann et al., 1993, 1997). However, its potential use as a structural tool has received limited attention (Hrouda et al., 1993; Siegesmund et al., 1993; Louis et al., 2006, 2008; Amrouch et al., 2010). The data presented here supplement the shipboard physical property measurements performed during Expeditions 315 and 316 (see site chapters in this volume).

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