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doi:10.2204/iodp.sp.338.2012

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 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 Japan Trench Fast Earthquake 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, 2006b) (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,

• 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 multiexpedition 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.

Sediment-dominated subduction zones such as the East Aleutian, Cascadia, Sumatra, and Nankai margins are characterized by repeated great earthquakes of magnitude 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) 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 and that it 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, which bears on both understanding subduction zone megathrust behavior globally and 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 (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 deep 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 IODP 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) (Expedition 319 Scientists, 2010; Underwood et al., 2010; Kopf et al., 2011; Expedition 333 Scientists, 2011), with the aims of building on the results of Stage 1, characterizing the subduction inputs on the Philippine Sea plate, and preparing for later observatory installations for long-term monitoring of deformation at the updip limit of the seismogenic zone. IODP Expedition 326 started Stage 3 by installing the first casing string in Hole C0002F to 860 meters below seafloor (mbsf) (Expedition 326 Scientists, 2011). Expedition 338 will deepen the hole to investigate the properties, structure, and state of stress within the hanging wall above the locked plate boundary at Site C0002. The borehole will be further deepened later in 2013, with the ultimate goal of penetrating the megasplay fault and for future installation of a long-term observatory (Fig. F3).

Site C0002 is the deep centerpiece of the NanTroSEIZE project, as it is planned to access the plate interface fault system at a location where the fault system is believed to be capable of seismogenic locking and slip and to have slipped coseismically in the 1944 Tonankai earthquake (e.g., Ichinose et al., 2003). This zone also coincides with the location where a cluster of very low frequency (VLF) seismic events occurred in 2004–2005 (Ito and Obara, 2006) and the first tectonic tremor recorded in an accretionary prism setting has been found (Obana and Kodaira, 2009). The primary targets for Site C0002 include both the basal décollement and the reflector known as the megasplay fault (Tobin and Kinoshita, 2006b). The megasplay fault reflection lies at an estimated depth of 5200 mbsf, and the top of subducting basement is estimated to lie at ~6800 mbsf (Fig. F2). The planned ultimate target depth for this site is 7000 mbsf, to be reached during future operations.

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