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doi:10.2204/iodp.pr.338.2013

Introduction

The Integrated Ocean Drilling Program (IODP) drilling phase of the 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 the Kii Peninsula (Tobin and Kinoshita, 2006) (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 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.

Along the Nankai margin, high-resolution seismic reflection profiles across the outer wedge of the accretionary prism clearly document a large out-of-sequence-thrust fault system (the megasplay fault, after Park et al., 2002) (Fig. F2) 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. F1). Several lines of evidence indicate the megasplay system is active and that it may accommodate an appreciable component of plate boundary motion. However, the partitioning of strain between the 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 are not understood. One of the main objectives of the NanTroSEIZE project 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.

IODP Expeditions 314, 315, and 316 were carried out as a unified drilling program collectively known as NanTroSEIZE Stage 1 (Tobin et al., 2009a). Eight sites were selected for riserless drilling to target the frontal thrust region, the midslope megasplay fault region, and the Kumano forearc basin (Figs. F1, F2). IODP Site C0002 was the preparatory pilot site 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 et al., 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 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 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 (Expedition 319 Scientists, 2010; Underwood et al., 2010; Expedition 332 Scientists, 2011a; Expedition 333 Scientists, 2012a).

NanTroSEIZE Stage 3 started with IODP Expedition 326, during which the first casing string was installed in Hole C0002F to 860 meters below seafloor (mbsf) (Expedition 326 Scientists, 2011). IODP Expedition 338, the ninth NanTroSEIZE drilling expedition, was planned to deepen Hole C0002F to investigate the properties, structure, and state of stress within the hanging wall above the locked plate boundary at Site C0002. 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 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). To access, sample, and monitor these deeper zones, Hole C0002F will be further deepened later in 2013–2015, with the ultimate goal of penetrating the megasplay fault and for the future installation of a long-term observatory (Fig. F3).

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