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doi:10.2204/iodp.proc.348.101.2015

Introduction

The Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a multiexpedition, multistage 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 of the Kii Peninsula (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009) (Figs. F1, F2). The main objectives are to understand:

• The mechanisms and processes controlling the updip aseismic–seismic transition of the megathrust fault system,
• The processes of earthquake and tsunami generation,
• The mechanics of strain accumulation and release,
• The absolute mechanical and elastic strength of the plate boundary fault and surrounding upper and lower plate crustal material, and
• The potential role of a major upper plate fault system (termed the “megasplay” fault) in seismogenesis and tsunamigenesis.

Along the Nankai margin, high-resolution seismic reflection imaging across the inner and outer wedge tectonic domains of the accretionary prism shows that, above ~10 km depth, the plate boundary (megathrust) fault separates the wedge from a zone of underthrust sediments (Moore et al., 2007, 2009; Bangs et al., 2009; Strasser, Dugan, Kanagawa, Moore, Toczko, Maeda, and the Expedition 338 Scientists, 2014). Furthermore, a large out-of-sequence splay thrust system described as the “megasplay fault” in Tobin and Kinoshita (2007) (Fig. F2), branches from the plate boundary décollement close to the updip limit of inferred coseismic rupture in the 1944 Tonankai Mw 8.2 earthquake (Fig. F1) into a fanning system of thrusts that reach the near-surface (Park et al., 2002). Based on recent 3-D reflection and wide-angle refraction seismic interpretation (Bangs et al., 2009; Kamei et al., 2013), the high-amplitude reflector beneath Site C0002 previously identified as the megasplay fault is now believed to be the main plate boundary fault (Figs. F2), with the fault splay point up-dip of this location. Several lines of evidence indicate that faults of this megasplay system are geologically active (Kimura et al., 2011) and may participate in coseismic slip (e.g., Sakaguchi et al., 2011). However, the partitioning of strain between the outer décollement zone and the megasplay system (Fig. F2) and the nature and mechanisms of fault slip as a function of depth and time on the megasplay and the plate boundary fault are not understood. One of the main objectives of the NanTroSEIZE project is to document the role of the megasplay and deeper plate boundary faults in accommodating plate motion (both seismically and interseismically) and to characterize the mechanical and hydrologic behavior of the margin.

NanTroSEIZE Stage 1 comprised IODP Expeditions 314, 315, and 316, which were carried out as a unified drilling program in 2007–2008 (Tobin et al., 2009a). Eight sites were selected for riserless drilling to target the frontal thrust region, the midslope shallow portion of the megasplay fault region, and the Kumano forearc basin (Figs. F1, F2). Holes C0002A–C0002D (Figs. F3, F4) were the pilot LWD and coring holes for planned deeper riser drilling and operations, whereas the other sites 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) (Tobin et al., 2009b). Expedition 315 was devoted to core sampling and downhole temperature measurements at sites in the megasplay region and in the forearc basin (Ashi et al., 2009). Expedition 316 targeted the shallow frontal thrust and megasplay fault (Screaton et al., 2009).

Stage 2 of NanTroSEIZE comprised four IODP expeditions (319, 322, 332, and 333), with the aims of building on the results of Stage 1, characterizing the subduction inputs from the Philippine Sea plate, and preparing for later observatory installations for long-term monitoring of deformation at the updip limit of the seismogenic zone (Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010; Underwood et al., 2010; Kopf, Araki, Toczko, and the Expedition 332 Scientists, 2011; Henry, Kanamatsu, Moe, and the Expedition 333 Scientists, 2012).

NanTroSEIZE Stage 3 started with IODP Expedition 326, during which a 20 inch casing string was installed in Hole C0002F to 860.3 mbsf (Expedition 326 Scientists, 2011) and continued with Expedition 338 (Strasser, Dugan, Kanagawa, Moore, Toczko, Maeda, and the Expedition 338 Scientists, 2014), which was originally planned to deepen Hole C0002F to 3600 mbsf. However, operations were aborted because of mechanical and weather-related events after drilling to ~2010 mbsf. Additional casing was not installed during the expedition, and Hole C0002F was suspended with drilling mud left in the open hole below the 20 inch casing shoe at 860.3 mbsf. The original objective of Expedition 338 was to investigate the properties, structure, and state of stress within the hanging wall above the locked plate boundary at Site C0002 and advance the hole for further deepening during a later expedition. These remained the primary goals of Expedition 348. During Expedition 348, the hole was sidetracked below the 20 inch casing shoe, and the interval to 2300 mbsf (renamed Hole C0002N) was drilled with measurement-while-drilling (MWD) tools and cased. It was then sidetracked again (creating Hole C0002P) and drilled with limited coring and LWD/MWD to 3058.5 mbsf. An 11¾ inch liner was emplaced to 2922.5 mbsf and cemented in to conclude the Expedition 348 operations.

Site C0002 is the 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 of repeated clusters of very low frequency earthquakes (Ito and Obara, 2006; Sugioka et al., 2012) and of the first tectonic tremor recorded in an accretionary prism setting (Obana and Kodaira, 2009). To access, sample, and monitor these zones along the plate interface, Hole C0002F/N/P was deepened with the ultimate goal of penetrating the plate boundary fault and for the future installation of a long-term observatory (Fig. F5).

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