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Geological setting

The Nankai Trough is formed by northwestward subduction of the Philippine Sea plate beneath the Eurasian plate at a rate of ~40–65 mm/y (Seno et al., 1993; Miyazaki and Heki, 2001). The convergence direction is slightly oblique 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 or more years 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, a long historical record of great (M > 8.0) earthquakes, and the direct societal relevance of understanding tsunamis and earthquakes that have had, and will have, great impact on nearby heavily populated coastal areas.

The Nankai Trough region has a historical record of recurring 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 et al., 2005) (Fig. F1B). Land-based geodetic studies suggest that the plate boundary thrust is currently strongly locked (Miyazaki and Heki, 2001), and the relatively low level of microseismicity near the updip limits of the 1940s earthquakes (Obana et al., 2001) implies significant interseismic strain accumulation on the megathrust. However, recent observations of VLF earthquakes within or just below the accretionary prism in the drilling area (Obara and Ito, 2005) demonstrate that some strain release does occur during interseismic periods (Fig. F1B). Slow slip phenomena including episodic slow slip events and nonvolcanic tremor are also widely known to occur in the downdip part of the rupture zone (Ito et al., 2007).

The region offshore the Kii Peninsula on Honshu Island was selected 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., Kikuchi et al., 2003; Tanioka and Satake, 2001). Slip inversion studies suggest that only in this region did past coseismic rupture clearly extend to sufficiently shallow depths to be reached by drilling (Ichinose et al., 2003; Baba and Cummins, 2005), and an updip zone of large slip has been identified and targeted (Fig. F1B). Notably, coseismic slip during events like the 1944 Tonankai earthquake may have occurred on the megasplay fault (Ichinose et al., 2003; Baba et al., 2006). The megasplay fault is therefore one of the primary drilling targets. Second, ocean-bottom seismometer (OBS) campaigns and onshore high-resolution geodetic studies (though of short duration) indicate significant interseismic strain accumulation (e.g., Miyazaki and Heki, 2001; Obana et al., 2001). Third, the region offshore the Kii Peninsula is generally typical of the Nankai margin in terms of heat flow and sediment on the incoming plate. This is in contrast to the area offshore Cape Muroto (the location of previous Deep Sea Drilling Project and Ocean Drilling Program [ODP] drilling) where both local stratigraphic variation associated with basement topography and anomalously high heat flow have been documented (Moore et al., 2001; Moore, Taira, Klaus, et al., 2001). Finally, the drilling targets are within the operational limits of riser drilling by D/V Chikyu (i.e., maximum of 2500 m water depth and 7000 m subseafloor penetration). In the seaward portions of the Kumano Basin, the seismogenic zone lies ~6–7 km beneath the seafloor (Nakanishi et al., 2002).

Seismic studies/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 seismic reflection (e.g., Park et al., 2002), wide-angle refraction (Nakanishi et al., 2002), passive seismicity (e.g., Obara et al., 2004), heat flow (Yamano et al., 2003), side-scan sonar, swath bathymetry, and submersible and remotely operated vehicle (ROV) dive data (Ashi et al., 2002). 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 (Fig. F3). This 3-D data volume has been used to refine selection of drill sites and targets in the complex megasplay fault region, define the 3-D regional structure and seismic stratigraphy, analyze physical properties of the subsurface, and assess drilling safety (Moore et al., 2007, 2009).