Subduction zones account for 90% of global seismic moment release, generating damaging earthquakes and tsunamis with potentially disastrous effects on heavily populated coastal 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 behavior at all plate boundary types through direct sampling, near-field geophysical observations, and measurement of in situ conditions at depths of coseismic slip is a fundamental and societally relevant goal of modern earth science.

To this end, several recent and ongoing drilling programs have targeted portions of active plate boundary faults either that have slipped coseismically during large earthquakes or that nucleate 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 drilling (Tobin and Kinoshita, 2006a, 2006b).

The NanTroSEIZE project is a multiexpedition, multiplatform, multistage IODP complex drilling program focused on understanding the mechanics of seismogenesis and rupture propagation along subduction plate boundary faults. The drilling program outlines a coordinated effort to sample and instrument the plate boundary system at several locations offshore the Kii Peninsula (Fig. F1, F2) with the main objectives of understanding (Tobin and Kinoshita, 2006a, 2006b)

  • The aseismic–seismic transition of the megathrust fault system,
  • Processes of earthquake and tsunami generation, and
  • The hydrologic behavior of the plate boundary.

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 proposals and outlined in Tobin and Kinoshita (2006a, 2006b):

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

  2. Subduction megathrusts are weak faults.

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

  4. Physical properties of the plate boundary system change with time during the earthquake cycle.

  5. A significant, laterally extensive 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, and Nankai margins are characterized by repeated occurrences of great earthquakes of ~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 along the outer rise (e.g., Byrne et al., 1988; Wang and Hu, 2006). Accretionary prisms in these subduction zones are separated into two parts by this outer rise (Wang and Hu, 2006; Kimura et al., 2007), and the inner and outer wedges are located above the seismogenic plate boundary and aseismic décollement, respectively.

At Nankai, high-resolution images of the outer rise from seismic reflection profiles clearly document a large out-of-sequence thrust (OOST) fault system (the "megasplay" fault after Park et al., 2002) that branches from the plate boundary décollement within the coseismic rupture zone of the 1944 Tonankai M 8.2 earthquake (Fig. F2A). Several lines of evidence indicate that the megasplay system is active and may accommodate an appreciable component of plate boundary motion. However, the partitioning of strain between the lower plate interface (the décollement zone) and the megasplay system and the nature and mechanisms of fault slip as a function of depth and time on the megasplay are not understood. As stated in the fifth hypothesis above, one of the first-order goals in characterizing the seismogenic zone along the Nankai Trough—and which bears on understanding subduction zone megathrust behavior globally—is to document the role of the megasplay fault in accommodating plate motion and to characterize its mechanical and hydrologic behavior.

Stage 1 of the NanTroSEIZE program includes three coordinated riserless drilling expeditions to drill several sites across the continental slope and rise offshore the Kii Peninsula, within the inferred coseismic slip region of the 1944 Tonankai M 8.2 earthquake (Figs. F1B, F2B) (Tobin and Kinoshita, 2006a, 2006b). The first of these will be a LWD expedition that will serve as a geophysical baseline for all of the Stage 1 drilling sites (Expedition 314: LWD Transect; Chikyu) to define physical properties, lithostratigraphy, and structural information in advance of coring operations. This will be followed by a coring expedition (Expedition 315: Megasplay Riser Pilot; Chikyu) aimed at sampling the materials and characterizing in situ conditions within the accretionary wedge to 1 km below seafloor at proposed Site NT2-03. This site will also serve as a pilot hole for later Stage 2 riser drilling targeting the megasplay fault at ~3–3.5 km below seafloor. These expeditions and their scheduled operations are summarized in Figure F2.

Expedition 316 (Shallow Megasplay and Frontal Thrusts) on the Chikyu targets two shallow fault zones: (1) the frontal thrust near the trench (proposed Site NT1-03B) and (2) the older accretionary prism and megasplay fault (proposed Site NT2-01B). These operations are aimed at sampling two major thrust fault systems at relatively shallow depths where they are accessible to riserless drilling. The first is the main frontal thrust at the seaward edge of the accretionary wedge (Figs. F3, F4). Based on seismic data and submersible studies, this thrust is thought to have placed moderately consolidated, ~2 Ma clastic rocks over weak and unlithified late Quaternary trench section clastic sediments (Ashi et al., 2002). The scientific objectives of drilling at proposed Site NT1-03B are to clarify the following:

  • The function of the frontal thrust with respect to large earthquakes,
    • Do great earthquakes trigger slip along this fault plane, and if so, are they tsunamigenic?
    • Does the frontal thrust generate low-frequency events or does it creep during the interseismic period?
  • The relationship between fluid behavior and slip and deformation, and
  • The evolution of the frontal thrust from its birth to death.

These objectives will be accomplished by complete coring and a suite of downhole measurements of the 950 m of planned borehole along with the LWD logs from Expedition 314. The investigations undertaken onboard and postcruise will include the following:

  • Comprehensive characterization of deformation at macro, meso, and microscopic scales;
  • Evaluation of the geophysically inferred depth of detachment;
  • Structural partitioning at the frontal thrust;
  • Physical properties of the fault zone and surrounding wall rocks; and
  • Diagenetic, chemical, and other evidence for fluid flow on the frontal thrust.

Later stages of drilling may target deeper intervals at this site, depending on what is discovered in Stage 1.

Operations at proposed Site NT2-01B will target the shallow portion of the megasplay fault system, just seaward of the break in slope marking the boundary between the inner and outer accretionary wedge (Figs. F5, F6). The scientific objectives of drilling at proposed Site NT2-01B are as follows:

  • To clarify the character and behavior of the shallow portion of the megasplay;
    • Is it an active blind fault or an inactive fault?
    • Is there evidence for past seismogenic slip, supporting contention that the megasplay is the primary candidate for the source of great earthquakes and tsunamis?
  • To clarify the slip and deformation mechanisms in the stable region above the (inferred) unstable seismogenic fault;
  • To clarify the relationship between fluid behavior, slip, and deformation along the megathrust; and
  • To clarify the evolutionary development of the splay fault.

Coring and downhole measurement will be carried out in similar fashion as that at proposed Site NT1-03B and will be used to obtain samples of the fault rock and wall rock and characterize them as completely as possible in conjunction with logging data (from Expedition 314 and possibly from Expedition 316). The drilling will penetrate numerous subfaults and zones of deformation before reaching the splay fault. Drilling is planned to penetrate 200 m below the splay fault to characterize deformation in the footwall. Thus, operations are planned based on a total depth (TD) of 1000 mbsf. If the targeted splay fault is at greater depth than expected and time allows, drilling and coring will proceed to a maximum of 1200 mbsf. Work on this expedition, including the installation and cementation of casing, will lead to subsequent installation of a monitoring system in later stages of the NanTroSEIZE complex drilling project.