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Overview of the NanTroSEIZE complex drilling project

Subduction zones account for the majority of global seismic moment release, and slip along subduction megathrusts generates both damaging earthquakes and tsunamis (e.g., Lay et al., 2005). Understanding the processes that govern the distribution, mechanics, and style of slip along subduction and other plate boundary fault systems is essential to earthquake and tsunami hazard assessment. 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), and Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) drilling (Tobin and Kinoshita, 2006a, 2006b).

NanTroSEIZE is a multiexpedition, multistage IODP drilling project focused on understanding the mechanics of 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 (Fig. F1), culminating in drilling, sampling, and instrumenting the plate boundary fault system near the updip limit of inferred coseismic slip, at 5~7 km below seafloor (Tobin and Kinoshita, 2006b) (Fig. F2) and installation of a distributed network of integrated borehole observatories to monitor strain and seismological, thermal, and hydrological processes. The main scientific objectives of drilling and monitoring are to understand and test

  • Hypothesized mechanisms controlling the transition from predominantly aseismic creep at shallow depths and near the trench to seismic slip and interseismic locking at greater depths along the megathrust fault system;

  • Frictional and hydrologic processes governing the mode of fault slip, 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.

In the NanTroSEIZE study area, high-resolution seismic reflection profiles across the outer rise clearly document a major out-of-sequence thrust fault system (megasplay fault, after Park et al., 2002) that branches from the décollement close to the updip limit of inferred coseismic rupture in the 1944 Tonankai M 8.2 earthquake (Fig. F2). Several lines of evidence indicate that the megasplay system is active, may accommodate a significant fraction of plate boundary motion, and may slip coseismically; these are discussed in more detail in “Geological setting” below (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. Thus, the megasplay and the region near its updip terminus comprise one of the primary drilling and monitoring targets for NanTroSEIZE.

In late 2007 through early 2008, NanTroSEIZE Stage 1 was conducted as a unified program of drilling, comprising IODP Expeditions 314, 315, and 316 (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009). Drilling during Stage 1 included riserless drilling along a transect of eight sites that targeted the frontal thrust region near the trench, the megasplay fault region, and the Kumano forearc basin region (Fig. F2). One of these sites, IODP Site C0002, also served as a preparatory pilot hole for planned future deep riser drilling. The other drill sites primarily targeted fault zones and accretionary prism sediments 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 a comprehensive logging-while-drilling (LWD) program (Kinoshita et al., 2008). Expedition 315 focused on coring and downhole temperature measurements at one site in the megasplay region (IODP Site C0001) and one in the forearc basin (Site C0002) (Ashi et al., 2008). Expedition 316 cored and conducted downhole temperature measurements across the frontal thrust and megasplay fault near their updip termini (Kimura et al., 2008).

NanTroSEIZE Stage 2 included two expeditions in 2009 (IODP Expeditions 319 and 322), with the aims of building on the results of Stage 1 and preparing for observatory installations. Expedition 319 investigated the properties, structure, and state of stress within the hanging wall above the locked plate boundary at IODP Site C0009 and across the shallow megasplay fault at IODP Site C0010. In addition, the first temporary long-term borehole observatory, a SmartPlug, was successfully installed at Site C0010 in an interval of screened casing across the megasplay fault (Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010). Expedition 322 sampled and characterized the properties of sediments entering the subduction zone on the subducting Philippine Sea plate (Saito, Underwood, Kubo, and the Expedition 322 Scientists, 2010).

IODP Expedition 332 continued NanTroSEIZE Stage 2 operations, using data generated during previous expeditions in Stages 1 and 2 as well as infrastructure provided by the cased Hole C0010A to continue riserless observatory operations along the NanTroSEIZE transect.