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doi:10.2204/iodp.sp.315.2007

Introduction

Subduction zones like the Nankai Trough, a region of strong earthquakes (M 8), are especially favorable for study because the entire width (dip extent) of the seismogenic zone ruptures in each great event, so that future rupture areas are perhaps more predictable than for smaller earthquakes (Fig. F1A). The Nankai Trough region is among the best-studied subduction zones in the world. It has a 1300 y historical record of recurring, and typically tsunamigenic, great earthquakes, including the 1944 Tonankai M 8.2 and 1946 Nankaido M 8.3 earthquakes (Ando, 1975; Hori et al., 2004). The rupture area and zone of tsunami generation for the 1944 event are now reasonably well understood (Ichinose et al., 2003; Baba and Cummins, 2005). Land-based geodetic studies suggest that the plate boundary thrust here is strongly locked (Miyazaki and Heki, 2001). Similarly, the relatively low level of microseismicity near the updip limits of the 1940s earthquakes (Obana et al., 2004) implies significant interseismic strain accumulation on the megathrust; however, recent observations of very low frequency (VLF) earthquake event swarms apparently taking place within the accretionary prism in the drilling area (Obara and Ito, 2005) demonstrate that interseismic strain is not confined to slow elastic strain accumulation.

Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a multistage, multiyear project. A total of 10 drill sites, including 2 riser drilling sites, are planned (Fig. F1B). During Stage 1 of this project, three expeditions are planned. During Expedition 315, we will drill one site, proposed Site NT2-03, which is the first phase of a two-part strategy. The ultimate objective is to perform riser drilling to ~3500 meters below seafloor (mbsf) during NanTroSEIZE Stage 2, across the megasplay fault at depth, and establish a deep borehole long-term observatory. To achieve the depth objective using riser-based drilling involves setting multiple casing strings, the depth of each depending on the least principal stress, the fracture strength of the formation, and the pore fluid pressure gradient. The key part of this casing plan is the "top hole" portion, where tolerances on mud weight are tight. Planning the casing program requires detailed information of the physical properties existing in the uppermost 1000 mbsf. Our strategy, therefore, is to drill and core a riserless pilot hole in this section during Stage 1, and then return for the deeper portion in Stage 2. These pilot studies are essential for well designing for the future planned riser drilling but are also important for science, as described in the following sections. The second half of this expedition may consist of only engineering and operations to set the riser seafloor structure and uppermost casing in preparation for Stage 2 riser operations.

Background 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 ~4.1 cm/y (Seno et al., 1993). The convergence direction is approximately normal to the trench, and sediments of the Shikoku Basin are actively accreting at the deformation front. 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 IODP and the U.S. MARGINS initiative, based on the wealth of geological and geophysical data available, the long historical record of great (M > 8.0) earthquakes, and 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 Tonankai M 8.2 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 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 (DSDP) and Ocean Drilling Program (ODP) drilling has focused and where both local stratigraphy associated with basement topography and anomalously high heat flow have been documented (Moore, Taira, Klaus, et al., 2001). Third, ocean-bottom seismometer (OBS) campaigns and shore-based high-resolution geodetic studies (though of short duration) indicate significant interseismic strain accumulation (e.g., Miyazaki and Heki, 2001; Obana et al., 2004).

As noted above, a large out-of-sequence thrust (OOST) branches from the master décollement ~50 km landward of the trench along the drilling transect and forms the trenchward boundary of the Kumano Basin (Fig. F2). Swath-bathymetric and multichannel seismic (MCS) 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; Toki et al., 2004). This fault has been termed a "megasplay" and differs markedly from other OOSTs in the following ways:

• It 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).
• It is geographically coincident with the updip termination of slip during the 1944 Tonankai M 8.2 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/site survey data

A significant volume of site survey data have 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. 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, the first deep-penetration, fully 3-D marine survey ever acquired for basic research purposes, is being used to refine selection of drill sites and targets in the complex megasplay fault region, to define the regional structure and seismic stratigraphy, to analyze physical properties of the subsurface through seismic attribute studies, to expand findings in the boreholes to wider areas, and to assess drilling safety.

Since 2001, Shinkai 6500 dives have revealed a general distribution of cold seeps, pore fluid chemistries of surface sediments, thermal structure, 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 Kumano Basin (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 is characterized by high heat flow based on 1 y of monitoring (Goto et al., 2003) and low chlorinity of 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 Japan Agency for Marine-Earth Science and Technology (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 slips 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 north-northeast–south-southwest lineament on the basin floor of the depression and a swarm of normal faults at the southern margin of the forearc basin (Fig. F3). Bacterial mats, tubeworms, and carbonate crusts were also observed on the landward slopes of the depression where the forearc basin strata are partly exposed (Fig. F3). The supporting site survey data for the NanTroSEIZE Stage 1 expeditions are archived at the IODP-MI Site Survey Data Bank (ssdb.iodp.org).

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