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doi:10.2204/iodp.sp.316.2007

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 cm/y. (Seno et al., 1993). The convergence direction is approximately normal 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 direct societal relevance of understanding tsunamis and earthquakes that have had, and will have, great impact on nearby heavily populated coastal areas.

The Kumano Basin region, off the Kii Peninsula (Fig. F1A, F1B), was chosen based on three criteria: (1) the updip end of the seismogenic zone must be definable based on slip in past great earthquakes, (2) seismic imaging must present clear drilling targets, and (3) deep targets must be within the operational limits of riser drilling by the Chikyu (i.e., maximum of 2500 m water depth and 7000 mbsf). In the Kumano Basin, the seismogenic zone lies at ~6000 mbsf (Nakanishi et al., 2002). Slip inversion studies suggest that only here 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 (asperity) has been identified and targeted (Fig. F1B). Coseismic slip during events like the 1944 Tonankai M 8.2 earthquake likely occurred on the megasplay fault rather than on the décollement beneath it, though slip on either plane is permissible given the available data. The megasplay fault, therefore, is a primary drilling target equal in importance to the basal décollement zone.

The region offshore the Kii Peninsula on Honshu Island has been identified as the best location 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). A horizon of significant coseismic slip is reachable by drilling with the Chikyu. Second, 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 where previous Deep Sea Drilling Project and Ocean Drilling Program (ODP) drilling has focused and where both local stratigraphy associated with basement topography and anomalously high heat flow have been documented (Moore, Taira, Klaus, et al., 2001; Moore et al., 2005). Third, 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).

As noted above, the megasplay, a large OOST, branches from the master décollement ~50 km landward of the trench along the drilling transect and forms the trenchward boundary of the Kumano Basin, a forearc basin filled by turbiditic sediments with a maximum thickness of ~2000 m. A pronounced continuous ridge of topography extending >120 km along strike is evident in swath-bathymetric and multichannel seismic data and is likely related to splay fault slip. Remotely operated vehicle (ROV) and submersible surveys reveal a very steep slope on both sides of the ridge, suggesting recent activity (Ashi et al., 2002, unpubl. data). This fault has been termed a megasplay because it differs markedly from other OOSTs in several respects:

• It is continuous along strike, is associated with a significant break in seafloor slope, and is a strong seismic reflector, suggesting that it is a first-order structural element of the margin.
• Significant long-term slip is documented by sequence boundaries and progressive landward tilting of strata in the Kumano Basin as observed in seismic reflection data (Fig. F2).
• The megasplay separates rocks with significantly higher seismic velocity on its landward side from rocks of lower seismic velocity toward the trench, suggesting that it represents a major mechanical discontinuity (Nakanishi et al., 2002).
• It is geographically coincident with the updip termination of slip during the 1944 Tonankai M 8.2 event, as inferred from tsunami (Tanioka and Satake, 2001) and seismic (Kikuchi et al., 2003) waveform inversions, and recent structural studies indicate that it may have experienced coseismic slip (e.g., Park et al., 2002).

Mechanical arguments further suggest that the megasplay is the primary coseismic plate boundary near the updip terminus of slip (e.g., Kame et al., 2003; Wang and Hu, 2006).

Subduction zones like the Nankai Trough, on which great earthquakes (M > 8.0) occur, are especially favorable for study because the entire width (dip extent) of the seismogenic zone ruptures in each great event, so the future rupture area is perhaps more predictable than for smaller earthquakes. The Nankai Trough region is among the best-studied subduction zones in the world. It has a 1300 y historical record of recurring and typically 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). Land-based geodetic studies suggest that the plate boundary thrust here 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 (VLF) earthquake event swarms apparently taking place within the accretionary prism in the drilling area (Obara and Ito, 2005) demonstrate that interseismic strain is not confined to slow elastic strain accumulation.

The outer accretionary wedge seaward of the 1944 earthquake seismogenic zone is characterized by strongly deformed fold-and-thrust belts above the aseismic plate boundary. Similar spatial and geometric relationships are commonly observed at other accretionary prisms (Plafker, 1972). The mechanical relationship between great earthquake slip in seismogenic zones and slip and deformation in the outer wedge is not well understood. However, Wang and Hu (2006) have proposed that the forearc region can be described as a "dynamic critical wedge," and recent discoveries of low-frequency events within the accretionary wedge (Ito and Obara, 2006) suggest that deformation and slip may occur episodically as both high-frequency postseismic events or as much lower frequency, "slower" interseismic events. These factors contribute to both the evolution of the margin architecture and to the behavior of the system today (e.g., Park et al., 2002) (Figs. F2, F7, F8).

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 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 regional structure and seismic stratigraphy, analyze physical properties of the subsurface through seismic attribute studies, and assess drilling safety. Further, these data will be used in conjunction with physical properties 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. The supporting site survey data for the NanTroSEIZE Stage 1 expeditions are archived at the IODP-MI Site Survey Data Bank (ssdb.iodp.org).

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