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doi:10.2204/iodp.sp.326.2010

Introduction

CDEX is implementing three Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) expeditions during 2010: Expedition 326 (NanTroSEIZE Stage 3: plate boundary deep riser: top hole engineering), Expedition 332 (NanTroSEIZE Stage 2: riserless observatory), and Expedition 333 (NanTroSEIZE Stage 2: subduction inputs 2 and heat flow). The expedition schedule and planned operations are subject to changes based on final budget and operational time decisions, as well as the Kuroshio Current and weather conditions at the proposed drill sites.

The ultimate objective for IODP Site C0002 is to drill to a total depth (TD) of ~7000 meters below seafloor (mbsf) and thereby penetrate the entire plate boundary fault zone to the subducting oceanic crust. For this expedition, the plan is to jet in 36 inch conductor casing to ~60 mbsf, and then drill down to ~800 mbsf and set 20 inch casing in the borehole. The borehole will then be suspended with a cement plug and a cap, awaiting further drilling during a subsequent expedition. Since the upper 1000 m at Site C0002 was already logged with logging while drilling (LWD) and cored during NanTroSEIZE Stage 1 (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009), only riserless drilling and casing will be performed at riser Site C0002 during Expedition 326, and no coring, logging, or other scientific operations are planned. The TD of the 20 inch casing installation may be extended to 830 mbsf or deeper, depending on hole conditions and available casing hardware.

Overview of the NanTroSEIZE complex drilling project

Subduction zones account for 90% of global seismic moment release, generating damaging earthquakes and tsunamis with potentially 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 a crucial step toward evaluating earthquake and tsunami hazards. More generally, characterizing fault slip behavior and mechanical state at all plate boundary types through direct sampling, near-field geophysical observations, and measurement of in situ conditions is a fundamental and societally relevant goal of modern earth science.

Several recent and ongoing drilling programs have targeted portions of active plate boundary faults that have either slipped coseismically during large earthquakes or nucleated smaller events. These efforts include the San Andreas Fault Observatory at Depth (Hickman et al., 2004), the Taiwan-Chelungpu Drilling Project (Ma, 2005), and IODP NanTroSEIZE (Tobin and Kinoshita, 2006a, 2006b).

The NanTroSEIZE project is a complex drilling project (CDP): a multiexpedition, multistage IODP drilling program focused on understanding the mechanics of seismogenics and rupture propagation along subduction plate boundary faults. NanTroSEIZE includes 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 processes governing the updip aseismic–seismic transition of the megathrust fault system;

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

• The absolute mechanical strength of the plate boundary fault, and

• The potential role of a major upper fault system (termed the "megasplay" fault) in seismogenesis and tsunamigenesis.

The drilling program will evaluate a set of core hypotheses through a combination of riser and riserless drilling, long-term observatories, and associated geophysical, laboratory, and numerical modeling efforts. 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 fault; Park et al., 2002) slips in discrete events that may include tsunamigenic slip during great earthquakes. It remains locked during the interseismic period and accumulates strain.

Sediment-dominated subduction zones such as the East 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 (e.g., Byrne et al., 1988; Wang and Hu, 2006). At Nankai, high-resolution seismic reflection profiles across the outer rise clearly document a large out-of-sequence thrust fault system (the megasplay fault). This system branches from the plate boundary décollement close to the updip limit of inferred coseismic rupture in the 1944 Tonankai M 8.2 earthquake (Fig. F2).

Several lines of evidence indicate that the megasplay system as well as the nature and mechanics of fault slip as a function of depth and time are not well understood. As stated in the fifth hypothesis above, one of the first-order goals in characterizing the seismogenic zone along the Nankai Trough is to document the role of the megasplay fault in accommodating plate motion (both seismically and interseismically) and to characterize its mechanical and hydrological behavior. This bears both on understanding subduction zone megathrust behavior and on defining tsunami hazards.

Presently, the overall CDP encompasses eight sites along a transect across the frontal thrust region, the midslope megasplay region, and the Kumano forearc basin region (Fig. F1). Two of these already occupied sites are preparatory pilot holes for planned deep riser drilling operations, including Site C0002. The other sites primarily targeted fault zones in the shallow, presumed aseismic portions of the accretionary complex (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009).

In late 2007 through early 2008, IODP Expeditions 314, 315, and 316 were carried out as a unified program of oceanic drilling collectively known as NanTroSEIZE Stage 1. Expedition 314 was dedicated to downhole measurement of physical properties and borehole imaging through LWD in holes drilled specifically for that purpose, including one at Site C0002 that reached 1401 mbsf. Expedition 315 was devoted to core sampling and downhole temperature measurements at two sites in the hanging wall: IODP Site C0001 just seaward of the outer rise into indurated thrust sheet of the megasplay fault and to a depth of 1057 mbsf at Site C0002 in the Kumano Basin. Expedition 316 targeted the frontal thrust and megasplay in their shallow, aseismic portion: IODP Site C0004 near the surface expression of the megasplay, IODP Sites C0006 and C0007 at the main frontal thrust at the seaward edge of the accretionary wedge, and IODP Site C0008 in the slope basin seaward of the megasplay fault. For more details, see Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists (2009).

IODP Expeditions 319 and 322 followed this first stage in 2009 as NanTroSEIZE Stage 2. Expedition 319 was dedicated to preparing boreholes at IODP Sites C0009 and C0010 for future installation of long-term borehole monitoring systems. Site C0009 was drilled to ~1600 mbsf as the first IODP riser drilling operation and included a walkaway vertical seismic profile (VSP) experiment. The hole was cased and left ready for future observatory installation work. During Expedition 322, IODP Sites C0011 and C0012 in the Shikoku Basin, seaward of the trench axis and deformation front, were cored and logged to evaluate the composition and properties of sediment and fluids entering the subduction zone.

Previous Site C0002 drilling achievements

Site C0002 was previously drilled during Expedition 314, as Hole C0002A, in which 1401 m of Kumano forearc basin and accretionary prism sediments were successfully drilled and logged with a full suite of LWD and measurement-while-drilling tools. Despite strong Kuroshio Current conditions, the expedition retrieved an excellent series of logs and zero-offset VSP data. They drilled and logged four units, separated by unconformities (Fig. F3):

• Logging Unit I: slope basin deposits;

• Logging Unit II: basin fill comprised of repeating turbidite deposits (contains two potentially gas-bearing sandy intervals);

• Logging Unit III: basin fill consisting of homogeneous clay-rich mudstone; an angular unconformity cuts Unit III, but with no discernible lithological changes across the boundary; and

• Logging Unit IV: accretionary prism sediment with very variable responses in the downhole logs.

Expedition 315 cored the 475–1057 mbsf interval (middle of logging Unit I to top of logging Unit IV), confirming the boundaries of the logging units and adding lithologic detail through core description as well as preliminary nannofossil-based biostratigraphy. Unit II is of Pleistocene age, Unit III is from the Pliocene, and Unit IV is of late Miocene age. Paleomagnetic measurements support this age model by placing the Bruhnes/Matayuama border at 850 m core depth below seafloor. Unit IV has a strongly varied dip and azimuth, confirming the presence of highly deformed strata as suggested by the reflection profiles.

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