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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 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 in a range of tectonic settings 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 slipped coseismically during large earthquakes or that have 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 Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) ocean drilling project (Tobin and Kinoshita, 2006a, 2006b).

NanTroSEIZE is a multiexpedition, multistage Integrated Ocean Drilling Program (IODP) drilling project focused on understanding the mechanics of seismogenesis along subduction plate boundary faults. The drilling program includes a coordinated effort to characterize, sample, and instrument the plate boundary system at several locations offshore the Kii Peninsula, culminating in drilling, sampling, and instrumenting the plate boundary fault system near the updip limit of inferred coseismic slip ~6–7 km below seafloor (Tobin and Kinoshita, 2006b) (Figs. F1, F2, F3). The main objectives are to understand:

  • The mechanisms controlling the updip aseismic–seismic transition along 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 overarching 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, including great earthquakes, and may influence tsunami generation. It remains locked during the interseismic period and accumulates strain.

Sedimented 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 (Figs. F1B, F2). Several lines of evidence indicate that the megasplay system is active and may accommodate a significant fraction of plate boundary motion (e.g., Moore et al., 2007; Strasser et al., 2009). 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. 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 (Fig. F3). Two of these sites were preparatory pilot holes for planned deeper riser drilling operations, and the others 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). Expedition 314 was dedicated to in situ measurement of physical properties and borehole imaging through logging while drilling (LWD) (Kinoshita et al., 2008). Expedition 315 was devoted to core sampling and downhole temperature measurements at a site in the megasplay region and one in the forearc basin (Ashi et al., 2008). Expedition 316 targeted the frontal thrust and megasplay fault in their shallow, aseismic portions (Ashi et al., 2008; Kimura et al., 2008).

NanTroSEIZE Stage 2 included 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. IODP Expedition 319 investigated the properties, structure, and state of stress within the hanging wall above the locked plate boundary at Site C0009 and across the shallow megasplay at Site C0010, and prepared boreholes for the future installation of observatories. IODP Expedition 322 sampled and characterized the properties of sediments on the subducting Philippine Sea plate. The initial results from Expedition 319, described in detail here, include data and operations for two sites: Site C0009, a riser drilling site in the Kumano Basin, and Site C0010, a riserless site into the shallow megasplay fault near its updip terminus.

In future IODP expeditions, long-term borehole observatory installations are planned for the two boreholes drilled during Expedition 319. The boreholes are located within and above regions of contrasting behavior of the megasplay fault zone and plate boundary as a whole (i.e., a site ~10 km above the locked seismogenic plate boundary [Site C0009] and a shallow site in the megasplay fault zone and footwall where slip is presumed to be aseismic [Site C0010]). These observatories have the potential of capturing seismic activity, slow slip behavior, hydraulic transients, and possibly interseismic strain accumulation on the megasplay fault and décollement. Currently, the planned observation system for the boreholes consists of an array of sensors designed to monitor slow crustal deformation (e.g., strain, tilt, and pore pressure as a proxy for strain), seismic events including very low frequency (VLF) earthquakes, hydrologic transients associated with strain events, ambient pore pressure, and temperature. These borehole observatories will be connected to the submarine cabled observation network Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) (, which will be constructed in and around the drilling target area.