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doi:10.2204/iodp.pr.322.2009

Introduction

Integrated Ocean Drilling Program (IODP) Expedition 322 represents just one part of a multistage project known as the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). The fundamental goal of NanTroSEIZE is to create a distributed observatory spanning the updip limit of seismogenic and tsunamigenic behavior along a subduction boundary, positioned at a location where great earthquakes are known to occur (Tobin and Kinoshita, 2006a, 2006b). Once completed, the network of boreholes and observatories will allow scientists to monitor the hydrogeologic behavior of subduction megathrusts, as well as the aseismic–seismic transition of the megathrust system. To accomplish this ambitious goal, several key components of the plate-boundary system must be investigated starting with the presubduction inputs of sediment and oceanic basement, moving landward into the shallow plate interface, and finally drilling to depths where earthquakes occur.

The Nankai Trough region of southeast Japan is among the most extensively studied subduction zones in the world (Fig. F1). The region has a nicely documented 1300 y history of great earthquakes, many of which have generated tsunamis. Recent events of this type include the 1944 Tonankai M 8.2 and 1946 Nankaido M 8.3 earthquakes (Ando, 1975; Hori et al., 2004; Ichinose et al., 2003; Baba and Cummins, 2005). The results of land-based geodetic studies indicate that the plate boundary fault is currently locked (Miyazaki and Heki, 2001). The relatively low level of microseismicity near the updip limits of the 1944 and 1946 great earthquakes (Obana et al., 2004) is consistent with the notion of significant interseismic strain accumulation on the megathrust. Very low frequency earthquake swarms have also been recorded recently, and their apparent locations are within the accretionary prism near the NanTroSEIZE drilling area (Obara and Ito, 2005; Ito and Obara, 2006). This seismic activity demonstrates that interseismic strain is not confined to slow elastic strain accumulation.

Given the backdrop of diverse seismic activity summarized above, our transect across the Kumano Basin region of the Nankai Trough (Fig. F2) was chosen among several candidates to implement NanTroSEIZE. The selection was based on satisfying three generic criteria:

1. The updip end of the seismogenic zone is definable based on slip during great earthquakes of the recent past.
2. Seismic imaging presents clear drilling targets.
3. Deep targets are within the operational limits of riser drilling by D/V Chikyu (i.e., maximum of 2500 m water depth and 7000 m subseafloor penetration).

According to studies of slip inversion, past coseismic ruptures within the transect area clearly extended to depths shallow enough for drilling (Tanioka and Satake, 2001; Ichinose et al., 2003; Baba and Cummins, 2005). Coseismic slip during the 1944 Tonankai earthquake may have occurred on the megasplay fault rather than on the décollement beneath it, but both faults represent primary drilling targets of equal importance (Tobin and Kinoshita, 2006a).

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