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doi:10.2204/iodp.sp.348.2013

Introduction

Overview of the NanTroSEIZE drilling project

Subduction zones account for 90% of the global seismic moment release and generate damaging earthquakes and tsunamis with potentially disastrous effects on heavily populated coastal areas (e.g., Lay et al., 2005; Moreno et al., 2010; Simons et al., 2011). 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, measurement of in situ conditions, and shore-based laboratory experiments 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), IODP NanTroSEIZE drilling (Tobin and Kinoshita, 2006a, 2006b), and the Japan Trench Fast Drilling Project (Mori et al., 2012).

NanTroSEIZE is a multiexpedition, multistage IODP drilling project 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, including fault processes associated with very low frequency (VLF) earthquakes and tremor (e.g., Sugioka et al., 2012);

• Processes of earthquake and tsunami generation;

• Mechanics of 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 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.

An additional hypothesis has been developed following the emergence of observations of tremor and VLF earthquake modes of fault activity in the outer forearc:

• Shallow tremor, VLF earthquakes, and/or slow-slip events occur by faulting under conditional frictional stability conditions facilitated by elevated pore fluid pressure.

Sediment-dominated subduction zones such as the East Aleutian, Cascadia, Sumatra, and Nankai margins are characterized by the repeated occurrence of great earthquakes of Mw ~8.0+ (Ruff and Kanamori, 1983). Although the causal mechanisms are not well understood (e.g., Byrne et al., 1988; Moore and Saffer, 2001; Saffer and Marone, 2003) and great earthquakes are also known to occur within sediment-starved subduction zones such as the Japan Trench, 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). Along the Nankai margin, 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; Tobin and Kinoshita, 2006a) that branches from the plate boundary décollement close to the updip limit of inferred coseismic rupture in the 1944 Tonankai Mw 8.2 earthquake (Fig. F2). Several lines of evidence indicate that the megasplay system is active; 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 fully understood (e.g., Strasser et al., 2009; Kimura et al., 2011). Therefore, documenting the role of the megasplay fault in accommodating plate motion (both seismically and interseismically) and characterizing its mechanical and hydrologic behavior constitute an important goal for NanTroSEIZE and, more generally, toward understanding subduction zone megathrust behavior globally.

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 (Tobin et al., 2009). 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 (Tobin et al., 2009). 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, including Site C0002 (Expedition 314 Scientists, 2009). Expedition 315 was devoted to core sampling and downhole temperature measurements at a site in the megasplay region and Site C0002 in the forearc basin (Expedition 315 Scientists, 2009b). Expedition 316 targeted the frontal thrust and megasplay fault in their shallow, aseismic portions (Screaton et al., 2009).

Stage 2 of NanTroSEIZE comprised four IODP expeditions (319, 322, 332, and 333), with the aims of defining the state of stress, composition, and mechanical properties of the inner accretionary wedge at Site C0009, characterizing the subduction inputs on the Philippine Sea plate, and preparing for later observatory installations for long-term monitoring of deformation, seismicity, and hydrological processes (Saito, Underwood, Kubo, and the Expedition 322 Scientists, 2010; Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010).

Stage 3 operations in Hole C0002F, the main NanTroSEIZE riser hole, began with the installation of a riser wellhead and shallow casing string to 860 mbsf during IODP Expedition 326 (Kinoshita et al., 2012). IODP Expedition 338 deepened the hole to ~2005 mbsf in 2012, collecting a complete set of logs and cuttings (Moore et al., 2013). Expedition 338 also drilled and cored several riserless holes at Site C0002 near Hole C0002F. The primary objective of Expedition 348 is to log, core, and case this hole to 3600 (or 4400) mbsf, as conditions permit. Riser expeditions after 2015 will focus on deepening this hole to penetrate the megasplay fault at ~5200 mbsf and installing a long-term observatory at the fault zone (Figs. F2, F3).

Site C0002 is the centerpiece of the NanTroSEIZE project, and Hole C0002F will access the plate interface at a zone where the fault system is believed to be capable of interseismic locking (and therefore coseismic slip) and to have slipped in the 1944 Tonankai earthquake (e.g., Ichinose et al., 2003). This zone also coincides broadly with the locations of VLF earthquake swarms observed in 2004–2005 (Ito and Obara, 2006) and 2009 (Sugioka et al., 2012) and with tectonic tremors in the outer accretionary prism (Obana and Kodaira, 2009).

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