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The Nankai Trough is formed by subduction of the Philippine Sea plate to the northwest beneath the Eurasian plate at a rate of ~40 mm/y (Seno et al., 1993). 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.
Subduction zones like the Nankai Trough, at which great earthquakes (M > 8.0) occur, are especially favorable for study because the entire downdip width of the seismogenic zone ruptures in each event, suggesting that the zone of coseismic rupture in future large earthquakes may be more predictable than for smaller earthquakes. The Nankai Trough region has a 1300 y historical record of recurring great earthquakes that are typically tsunamigenic, 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 and includes Stage 1 and Stage 2 hanging wall drill sites (Ichinose et al., 2003; Baba et al., 2005). 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 implies significant interseismic strain accumulation on the megathrust (Obana et al., 2001). However, recent observations of very low frequency earthquakes within or just below the accretionary prism in the drilling area demonstrate that interseismic strain is not confined to slow elastic strain accumulation (Obara and Ito, 2005). 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). Weak seismicity is also observed in the mantle of the subducting Philippine Sea plate and below the rupture zone (Obana et al., 2005). Seaward of the subduction zone, deformation of the incoming ocean crust is suggested by microearthquakes as documented by ocean bottom seismographic studies (Obana et al., 2005).
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., Tanioka and Satake, 2001; Kikuchi et al., 2003). Slip inversion studies suggest that past coseismic rupture events in this region have clearly extended to shallow enough depth to be within the reach of current drilling technologies (Ichinose et al., 2003; Baba and Cummins, 2005), and an updip zone of large slip has been identified and targeted (Fig. F1B, F2A). Notably, coseismic slip during events like the 1944 Tonankai earthquake may have occurred on the megasplay fault in addition to the plate boundary décollement (Ichinose et al., 2003; Baba et al., 2006). The megasplay fault is therefore a primary drilling target, equal in importance to the basal décollement. Second, ocean bottom seismometer 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, Taira, Klaus, et al., 2001; Moore et al., 2005). Finally, the drilling targets are within the operational limits of riser drilling by 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 ~6000 m beneath the seafloor (Nakanishi et al., 2002).
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, and swath bathymetry and submersible and ROV dive studies (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. This 3-D data volume is the first deep-penetration, fully 3-D marine survey ever acquired for basic research purposes and 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 through seismic attribute studies, and assess drilling safety (Moore et al., 2007; Moore et al., 2009). These high-resolution, 3-D data will be used in conjunction with physical properties and geophysical data obtained from core analyses and both wireline and LWD logging to allow extensive and high-resolution integration of core, logs, and seismic data.
In future IODP expeditions, a series of long-term borehole observatories will be installed into the three holes potentially drilled during Expedition 319 at proposed Sites NT2-11B and NT2-01J and IODP Site C0002 (contingency site). The three boreholes are located within and above regions of contrasting behavior of the megasplay fault zone and plate boundary as a whole (i.e., a site ~6–7 km above the "locked" seismogenic plate boundary [proposed Site NT2-11B], a site above the updip edge of the locked zone [Site C0002], and a shallow site in the megasplay fault zone and footwall where slip is presumed to be aseismic [proposed Site NT2-01J]). These observatories have the potential of capturing seismic activity, slow slip behavior, and possibly interseismic strain accumulation on the plate boundary and megasplay faults across a range of pressure, temperature, and kinematic conditions.
Currently, the planned observation system for the boreholes consists of an array of sensors designed to monitor slow crustal deformation (e.g., strain, tilt, and pore pressure as a proxy for strain), seismic events including very low frequency earthquakes, hydrologic transients associated with strain events, ambient pore pressure, and temperature. To ensure the long-term and continuous monitoring necessary to capture events occurring over a wide range of timescales, these borehole observatories will be connected to submarine cabled observation network called Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) (www.jamstec.go.jp/jamstec-e/maritec/donet), which will be constructed in and around the drilling target area.