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doi:10.2204/iodp.proc.338.101.2014

Background

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 highly relevant to heavily populated coastal areas.

Subduction zones like the Nankai Trough, where most of the 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 year 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 (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 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 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 known to occur in the downdip part of the 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 (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 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. 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, 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 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 drilling), where 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 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 <6000 m beneath the seafloor (Nakanishi et al., 2002).

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 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 will be used in conjunction with petrophysical and geophysical data obtained from core analyses, wireline logging, and LWD to allow extensive and high-resolution integration of cores, logs, and seismic data.

Long-term observatories

During future IODP expeditions, a series of long-term borehole observatories will be installed at IODP Sites C0002, C0006 or C0007, and C0010. The three sites are located within and above regions of contrasting behavior of the megasplay fault zone and plate boundary as a whole (i.e., a site in the toe of the accretionary prism [Sites C0006 and C0007], 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 [Site C0010]). 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 seismogenic settings. These temporal and spatial observations are necessary to understand how each part of the plate boundary functions through the seismic cycle of megathrust earthquakes.

Currently, the planned observation system for the observatory 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 VLF 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 a submarine cabled observation network called Dense Oceanfloor Network System of Earthquakes and Tsunamis (DONET) (www.jamstec.go.jp/donet/e/), which will be constructed in and around the drilling target area.

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