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 ~40–60 mm/y (Seno et al., 1993; Miyazaki and Heki, 2001). The convergence direction is 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 1300 or more years are well documented in historical and archeological records (e.g., Ando, 1975). 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 highly relevant to heavily populated coastal areas.

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 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 Mw 8.2 and 1946 Nankaido Mw 8.3 earthquakes (Ando, 1975; Hori et al., 2004). The rupture area and zone of tsunami generation for the 1944 event (within which Site C0002 is located) 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. However, recent observations of VLF earthquakes within or just below the accretionary prism in the drilling area (Obara and Ito, 2005; Sugioka et al., 2012) demonstrate that interseismic strain is not confined to slow elastic strain accumulation. Slow slip phenomena, referred to as episodic tremor and slip, including episodic slow slip events and nonvolcanic tremor (Schwartz and Rokosky, 2007), are also widely known to occur in the downdip part of the rupture zone (Ito et al., 2007). In the subducting Philippine Sea plate mantle below the rupture zone, weak seismicity is observed (Obana et al., 2005). Seaward of the subduction zone, deformation of the incoming ocean crust is suggested by microearthquakes as documented by ocean-bottom seismometer (OBS) 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 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 (Figs. F2, F3). Second, 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, previously drilled during the Deep Sea Drilling Project and the Ocean Drilling Program (ODP), where both local stratigraphic variation associated with basement topography and anomalously high heat flow have been documented (Moore et al., 2001, 2005). Finally, the drilling targets are within the operational limits of riser drilling by the 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).

The position of the plate boundary décollement fault or faults (sometimes called the “plate interface”) is the subject of ongoing debate. Most previous NanTroSEIZE publications (e.g., Tobin and Kinoshita, 2006a; Moore et al., 2007; and many others) have followed the general interpretation of Park et al. (2002), showing a branching point with the prominent megasplay reflector above a deeper décollement horizon that continues seaward as the outer wedge décollement. However, recent imaging, including the 3-D seismic volume and wide-angle OBS inversion (Kamei et al. 2013), have led to an alternative hypothesis that at Site C0002 the megasplay is the main plate boundary reflector and shallows into the outer décollement. In this scenario, the splay fault branching occurs seaward of and shallower than the reflector at ~5000 mbsf at Site C0002 (Figs. F2, F3), and the section beneath the reflector is composed primarily of subducting sediments underlain by down-going plate basement. It seems to be clear in the 3-D seismic imaging that the top of ocean crust is not likely to be the décollement at this location (Bangs et al., 2009), but a décollement could lie somewhere between 5000 and 7000 mbsf. In either interpretation of the décollement geometry, coseismic plate boundary slip during events like the 1944 Tonankai earthquake may have occurred on the megasplay fault reflector in addition to any deeper décollement (Ichinose et al., 2003; Baba et al., 2006). That reflector is therefore a primary drilling target.

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 2-D seismic reflection (e.g., Park et al., 2002), wide-angle refraction (Nakanishi et al., 2002; Kamei et al., 2013), passive seismicity (e.g., Obara and Ito, 2005; Sugioka et al., 2012), heat flow (Yamano et al., 2003), side-scan sonar, swath bathymetry, and submersible and remotely operated vehicle dive studies (Ashi et al., 2002). In 2006, Japan and the United States conducted a joint 3-D seismic reflection survey over a ~11 km × 55 km area, acquired by PGS Geophysical, an industry service company (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; Kitajima and Saffer, 2012). These high-resolution, 3-D data will be used in conjunction with physical properties, petrophysical, 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.

The supporting site survey data for Expedition 348 are archived at the IODP Site Survey Data Bank.