Geological setting

The Nankai Trough is a plate convergent margin where the Philippine Sea plate subducts to the northwest beneath the Eurasian plate at a rate of ~4 km/m.y. (Seno et al., 1993). The convergence direction is approximately normal to the trench, and Shikoku Basin sediments are actively accreting at the deformation front (Fig. F1). The Nankai Trough is among the most extensively studied subduction zones in the world, and great earthquakes during the past 3000 y are well documented in historical and archeological records (e.g., Ando, 1975). The Nankai Trough has been selected as a focus site for studies of seismogenesis by both the Integrated Ocean Drilling Program (IODP) and the U.S. MARGINS initiative because of the wealth of geological and geophysical data available, the long historical record of great (M > 8.0) earthquakes, and the direct societal relevance of understanding the generation and impact of tsunamis and earthquakes on the heavily populated coastal region.

The region offshore the Kii Peninsula has been identified as the best location for seismogenic zone drilling for several reasons. First, the rupture area of the most recent great earthquake, the 1944 M 8.1 Tonankai event, is well constrained by recent seismic and tsunami waveform inversions (e.g., Tanioka and Satake, 2001; Ichinose et al., 2003; Kikuchi et al., 2003). A horizon of significant coseismic slip is reachable by drilling with the D/V Chikyu. Second, the region offshore the Kii Peninsula is generally typical of the Nankai margin in terms of heat flow and sediment on the incoming plate, in contrast to the area offshore Cape Muroto where previous Deep Sea Drilling Program and Ocean Drilling Program (ODP) drilling has focused and where local stratigraphy associated with both basement topography and anomalously high heat flow has been documented (Moore et al., 2001). Third, ocean bottom seismometer campaigns and on-land high-resolution geodetic studies (though of short duration) indicate significant interseismic strain accumulation (e.g., Miyazaki and Heki, 2001; Obana et al., 2004).

A large out-of-sequence thrust branches from the master décollement ~50 km landward of the trench along the drilling transect and forms the trenchward boundary of the Kumano Basin (Figs. F2, F3). Swath-bathymetric and multichannel seismic data show a pronounced, continuous outer ridge of topography extending >120 km along strike, which may be related to the splay fault slip. Remotely operated vehicle (ROV) and submersible surveys along this feature have revealed very steep slopes on either side of the ridge, suggesting recent activity (Ashi et al., 2002, unpubl. data). This fault has been termed a "megasplay" and differs markedly from other out-of-sequence thrusts in several respects:

    • The megasplay is continuous along strike, is associated with a significant break in the seafloor slope, and is a strong seismic reflector, suggesting that it is a first-order structural element of the margin.

    • Significant long-term slip is documented by sequence boundaries and progressive landward tilting of strata in the Kumano Basin is observed in seismic reflection data.

    • The megasplay separates rocks with significantly higher seismic velocity on its landward side from rocks of lower seismic velocity toward the trench, suggesting that it represents a major mechanical discontinuity (Nakanishi et al., 2002).

    • The megasplay is geographically coincident with the updip termination of slip during the 1944 M 8.1 Tonankai event, as inferred from tsunami (Tanioka and Satake, 2001) and seismic (Kikuchi et al., 2003) waveform inversions, and recent structural studies indicate that it may have experienced coseismic slip (e.g., Park et al., 2002).

Mechanical arguments further suggest that the megasplay is the primary coseismic plate boundary near the updip terminus of slip (e.g., Kame et al., 2003; Wang and Hu, 2006).

Seismic studies and site survey data

A significant volume of site survey data has been collected in the drilling area over many years, including multiple generations of two-dimensional (2-D) seismic reflection (e.g., Park et al., 2002), wide-angle refraction (Nakanishi et al., 2002), passive seismicity (e.g., Obana et al., 2004), heat flow (Kinoshita et al., 2003), side-scan sonar, and swath bathymetry data. In 2006, Japan and the United States conducted a joint three-dimensional (3-D) seismic reflection survey over a ~11 km x 55 km area, acquired by PGS Geophysical, an industry service company. This 3-D data volume was used to refine selection of drill sites and targets in the complex megasplay fault region, to define regional structures and seismic stratigraphy, to analyze subsurface physical properties through seismic attribute studies, to expand findings in boreholes to wider areas, and to assess drilling safety (Moore et al., 2007) (Fig. F3).

Since 2001, Shinkai 6500 dives have revealed a general distribution of cold seeps, surface sediment pore fluid chemistries, thermal structures, and geological structures. Cold seeps are distributed at active faults on the prism slope (Ashi et al., 2002; Toki et al., 2004) and mud volcanoes in the forearc basin, the "Kumano Trough" (Kuramoto et al., 2001). The densest chemosynthetic biological communities are observed along the fault scarp base of the megasplay 30 km southwest of proposed Site NT2-03B. This cold seep site is characterized by high heat flow based on 1 y of monitoring (Goto et al., 2003) and low chlorinity in pore fluid chemistry (Toki et al., 2004), suggesting updip migration of fluids probably through the fault zone from the deep prism. Seafloor observations were also conducted near proposed Site NT2-03B using the submersible Shinkai 6500, the JAMSTEC deep-tow video camera 4K, and the ROV Kaiko. The gentle slope around proposed Site NT2-03B is completely covered by hemipelagic sediment and shows no indications of any cold seep activity. In contrast to the southern slope of the outer ridge, which was formed by recurrent slip events of the splay fault system, bacterial mats and a carbonate chimney were observed on the landward flank of the outer ridge (Toki et al., 2004). A northeast–southwest elongated depression has developed between the outer ridge and the forearc basin. The deep-towed side-scan sonar system Wadatsumi revealed a strong east-northeast–west-southwest lineament on the basin floor of the depression and a swarm of normal faults at the southern margin of the forearc basin. Bacterial mats, tubeworms, and carbonate crusts were also observed on the landward slopes of the depression where the forearc basin strata are partly exposed (Lallemant, unpubl. data).