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Background and objectives

Geological setting

The Nankai Trough is formed by subduction of the Philippine Sea plate to the northwest beneath the Eurasian plate at a rate of ~4.1–6.5 cm/y (Fig. F1) (Seno et al., 1993; Miyazaki and Heki, 2001). The convergence direction is slightly oblique to the trench, and Shikoku Basin sediment is actively accreting at the deformation front. The Nankai Trough has been one of the focus sites for studies of seismogenesis by both IODP and the U.S. MARGINS initiative, based on the wealth of geological and geophysical data available. A better understanding of seismic and tsunami behavior at margins such as Nankai is relevant to assessment of hazard to heavily populated coastal areas globally.

Subduction zones like the Nankai Trough, where most great earthquakes (Mw > 8.0) occur, are especially favorable for study because the entire downdip width of the seismogenic zone ruptures in major events, 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 Mw 8.2 and 1946 Nankai Mw 8.3 earthquakes (Fig. F1) (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). Land-based geodetic studies suggest that currently the plate boundary thrust 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., 2001) implies significant interseismic strain accumulation on the megathrust. However, recent observations of very low frequency earthquakes within or just below the accretionary prism in the drilling area (Obara and Ito, 2005; Sugioka et al., 2012) demonstrate that strain release along the megathrust is not restricted to great earthquakes. Slow slip phenomena, including episodic slow slip events and nonvolcanic tremor (e.g., Schwartz and Rokosky, 2007), are also known to occur near the downdip edge of the great earthquake rupture zone (Ito et al., 2007). In the subducting Philippine Sea plate below the rupture zone, weak seismicity is observed (Obana et al., 2005). Seaward of the subduction zone, deformation of the incoming oceanic crust is suggested by microearthquakes as documented by ocean-bottom seismometer 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 1944 Mw 8.2 Tonankai 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 only in this region did past coseismic rupture clearly extend shallow enough for drilling (Ichinose et al., 2003; Baba and Cummins, 2005), and an updip zone of large slip has been identified and targeted. The reflector previously identified as the megasplay is now believed to be the main plate boundary fault, with the splay fault branching point up-dip of this location (Fig. F2, F5). Therefore, there may not be a décollement at a deeper position. The so-called megasplay is therefore the primary plate boundary drilling target. 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). Finally, the drilling targets are within the operational limits of riser drilling by the D/V Chikyu (i.e., maximum of 2500 m water depth and 7000 m subseafloor penetration).

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 2-D seismic reflection (e.g., Park et al., 2002), wide-angle refraction (Nakanishi et al., 2002), passive seismicity (e.g., Obana et al., 2001, 2005), heat flow (Yamano et al., 2003), side-scan sonar, swath bathymetry, and submersible and remotely operated vehicle (ROV) dive studies (Ashi et al., 2002). In 2006, Japan and the United States conducted a joint 3-D seismic reflection survey over an ~11 km × 55 km area, acquired by Petroleum GeoServices (Moore et al., 2009). 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

  1. Refine selection of drill sites and targets in the complex megasplay fault region,
  2. Define the 3-D regional structure and seismic stratigraphy,
  3. Analyze physical properties of the subsurface through seismic attribute studies, and
  4. Assess drilling safety (Moore et al., 2007, 2009).

These high-resolution 3-D data are being used in conjunction with petrophysical and geophysical data obtained from core analyses and both wireline logging and LWD to allow extensive and high-resolution integration of core, logs, and seismic data (e.g., Bangs et al., 2009; Kitajima and Saffer, 2012).