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doi:10.2204/iodp.sp.333.2010

Overview of the NanTroSEIZE complex drilling project

Subduction zones account for 90% of global seismic moment release, generating damaging earthquakes and tsunamis with potential disastrous effects on heavily populated areas (e.g., Lay et al., 2005). Understanding the processes that govern the strength, nature, and distribution of slip along these plate boundary fault systems is crucial for evaluating earthquake and tsunami hazards. Sediment-dominated subduction zones such as the Eastern Aleutian, Cascadia, Sumatra, and Nankai margins are characterized by repeated great earthquakes of magnitude M ~8.0+ (Ruff and Kanamori, 1983). Although the causative mechanisms are not well understood (e.g., Byrne et al., 1988; Moore and Saffer, 2001; Saffer and Marone, 2003), the updip limit of the seismogenic zones at these margins is thought to correlate with a topographic break, often associated with the outer rise of the forearc (e.g., Byrne et al., 1988; Wang and Hu, 2006). At Nankai, seismic reflection profiles across the forearc outer rise document an out-of-sequence thrust (OOST) fault system (the megasplay fault) that branches from the plate boundary décollement close to the updip limit of inferred coseismic rupture of the 1944 Tonankai M 8.2 earthquake (Moore et al., 2007) (Fig. F2).

The NanTroSEIZE Project is a complex drilling project (CDP): a multiexpedition, multistage component of IODP focused on understanding the mechanics of seismogenics and rupture propagation along subduction plate boundary faults. NanTroSEIZE is a coordinated effort to sample and instrument the plate boundary system at several locations offshore the Kii Peninsula, Japan (Tobin and Kinoshita, 2006b) (Fig. F1). The main objectives are to understand

• The mechanisms and properties governing the updip aseismic–seismic transition of the megathrust and plate interface fault systems;

• Processes of earthquake and tsunami generation, as well as strain accumulation and release; and

• The mechanical strength and hydrogeologic behavior of the plate boundary fault and megasplay.

The following hypotheses are paraphrased from the original IODP proposals and outlined in Tobin and Kinoshita (2006a, 2006b):

1. Systematic, progressive material and state changes control the onset of seismogenic behavior on subduction thrust faults.

2. Subduction megathrusts are weak faults.

3. Plate motion is accommodated primarily by coseismic slip in a concentrated zone (i.e., the fault is locked during the interseismic period).

4. Physical properties of the plate boundary system (including the fault system and its hanging wall and footwall) change with time during the earthquake cycle.

5. A significant, laterally extensive upper plate fault system (the megasplay) slips in discrete events that may include tsunamigenic slip during great earthquakes. It remains locked during the interseismic period and accumulates strain.

To address Hypothesis 1 above, it is essential to document the composition and geotechnical/frictional/hydrogeological properties of the subduction inputs (i.e., the initial conditions) before the rocks reach the deformation front and begin to change. As stated in Hypothesis 5 above, two of the first-order goals in characterizing the seismogenic zone along the Nankai Trough are to document the role of the megasplay fault in accommodating plate motion (both seismically and interseismically) and to characterize the fault's mechanical and hydrological behavior. This research bears on understanding both fault behavior and tsunami hazards.

Presently, the NanTroSEIZE CDP encompasses twelve sites along a transect that extends from the northwest edge of the Shikoku Basin across the frontal thrust region, the midslope megasplay region, and into the Kumano Basin forearc region (Figs. F1, F2). One of these sites (IODP Site C0002) currently includes a pilot hole for the planned deep riser drilling operations. The other sites targeted fault zones in the shallow, aseismic portions of the accretionary complex (Kinoshita et al., 2008b) and the subduction inputs (Underwood et al., 2009).

From late 2007 through early 2008, IODP Expeditions 314, 315, and 316 were carried out as a unified program known as NanTroSEIZE Stage 1. Expedition 314 was dedicated to downhole measurement of physical properties and borehole imaging through logging while drilling (LWD). Expedition 315 was devoted to core sampling and downhole temperature measurement at two sites in the hanging wall: IODP Site C0001 just seaward of the outer rise and Site C0002 in the Kumano Basin. Expedition 316 targeted the frontal thrust region and megasplay in their shallow aseismic portions: IODP Site C0004 near the surface expression of the megasplay in the Kumano Basin, IODP Sites C0006 and C0007 at the frontal thrust of the accretionary wedge, and IODP Site C0008 in a trench-slope basin seaward of the splay fault. For more details, see Kinoshita et al. (2008b), Ashi et al. (2008), and Kimura et al. (2008).

IODP Expeditions 319 and 322 followed in 2009, as NanTroSEIZE Stage 2. Expedition 319 prepared boreholes at IODP Sites C0009 and C0010 for future installation of long-term monitoring systems. At Site C0009, Expedition 319 also conducted the first riser operation in IODP history, as well as a walkaway vertical seismic profile experiment. Expedition 322 cored Sites C0011 and C0012 in the Shikoku Basin to document the composition and material properties of sediment and uppermost igneous basement that eventually enters the Nankai subduction zone.

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