Overview of the NanTroSEIZE drilling project

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 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. To this end, 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 (SAFOD) (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, multistage IODP drilling program focused on understanding the mechanics of seismogenesis and rupture propagation along subduction plate boundary faults. The drilling program includes a coordinated effort to sample and instrument the plate boundary system at several locations offshore the Kii Peninsula (Tobin and Kinoshita, 2006a) (Figs. F1, F2). The main objectives are to understand

  • The mechanisms and processes controlling the updip aseismic–seismic transition of the megathrust fault system,

  • Processes of earthquake and tsunami generation and strain accumulation and release,

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

  • The potential role of a major upper plate 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 frictional 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 ~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, after Park et al., 2002) that branches from the plate boundary décollement close to the updip limit of inferred coseismic rupture in 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 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 both on understanding subduction zone megathrust behavior globally and on defining tsunami hazards—is to document the role of the megasplay fault in accommodating plate motion (both seismically and interseismically) and to characterize its mechanical and hydrologic behavior.

In late 2007 through early 2008, IODP Expeditions 314, 315, and 316 were carried out as a unified program of drilling collectively known as NanTroSEIZE Stage 1. A transect of eight sites was selected for riserless drilling to target the frontal thrust region, the midslope megasplay fault region, and the Kumano forearc basin region (Figs. F1, F2). Two of these sites are preparatory pilot holes for planned deeper riser drilling operations, whereas the other sites primarily targeted fault zones in the shallow, presumed aseismic portions of the accretionary complex (Kinoshita et al., 2008). Expedition 314 was dedicated to in situ measurement of physical properties and borehole imaging through logging while drilling (LWD) in holes drilled specifically for that purpose. Expedition 315 was devoted to core sampling and downhole temperature measurements at two sites in the hanging wall of the megasplay system: one in the Kumano Basin and the second just seaward of the outer rise. Expedition 316 targeted the frontal thrust and megasplay fault in their shallow, aseismic portions (for more details see Kinoshita et al., 2008; Ashi et al., 2008; Kimura et al., 2008)

NanTroSEIZE Stage 2 is composed of two Expeditions (319 and 322), with the aims of building on the results of Stage 1 and preparing for later observatory installations for long-term monitoring of deformation at the updip limit of the seismogenic zone. Expedition 319 will investigate the properties, structure, and state of stress within the hanging wall above the locked plate boundary at proposed Site NT2-11B and across the shallow megasplay at proposed Site NT2-01J and prepare the boreholes for future installation of observatories. IODP Expedition 322 will sample and characterize the properties of sediments on the subducting Philippine Sea plate ("input sites") as a critical part of investigating the progressive changes in material properties and state hypothesized to control the upper transition from aseismic to seismic slip (hypothesis 1 above; Moore and Saffer, 2001). Expedition 319, described in detail in this prospectus, includes a coordinated riser/riserless drilling plan to drill two sites: a riser site in the Kumano Basin above the portion of the plate boundary thrust that slips coseismically (proposed Site NT2-11B) and a riserless site into the shallow megasplay fault near its updip terminus (proposed Site NT2-01J). Riser drilling and casing at proposed Site NT2-11B are planned for the first part of Stage 2 followed by riserless drilling and casing at proposed Site NT2-01J. Both sites have also been selected for future installation of long-term observatories. A contingency site (C0002) may be drilled and cased in preparation for a second riserless observatory, time permitting. The second Stage 2 expedition, Expedition 322, will complete drilling and coring at the subduction input sites (Saito et al., in press).