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doi:10.2204/iodp.sp.nantroseize1.2006

BACKGROUND

NanTroSEIZE overall goals

The IODP Nankai Trough Seismogenic Zone Experiment will, for the first time ever, attempt to drill into, sample, and instrument the seismogenic portion of a plate boundary fault or megathrust within a subduction zone where great earthquakes (M ~ 8.0) have repeatedly occurred in the past (Tobin and Kinoshita, 2006). Access to the interior of active faults where in situ processes can be monitored and fresh fault zone materials can be sampled is of fundamental importance to the understanding of great earthquake mechanics. As the December 2004 Sumatra earthquake and Indian Ocean tsunami so tragically demonstrated, great subduction earthquakes represent one of the greatest natural hazards on the planet. Accordingly, drilling into and instrumenting an active interplate seismogenic zone is a very high priority in the IODP Initial Science Plan (2001). Through a decade-long series of national and international workshops, a consensus emerged that the Nankai Trough is an ideal place to attempt drilling and monitoring of the seismogenic plate interface. The first stage of NanTroSEIZE drilling operations is now scheduled to start in September 2007. Stage 1 involves parallel deployment of both the new U.S. SODV and the riser-capable drilling vessel Chikyu.

The fundamental goal of the NanTroSEIZE Complex Drilling Project (CDP) (www.iodp.org/NanTroSEIZE) is the creation of a distributed observatory spanning the updip limit of seismogenic and tsunamigenic behavior at a location where great subduction earthquakes occur, allowing us to observe the hydrogeologic behavior of subduction megathrusts and the aseismic–seismic transition of the megathrust system. This will involve drilling of key elements of the active plate boundary system at several locations off the Kii Peninsula of Japan, from the shallow onset of the plate interface to depths where earthquakes occur (Figs. F1, F2, F3). At this location, the plate interface and active megasplay faults implicated in causing tsunami are accessible to drilling within the region of coseismic rupture in the 1944 Tonankai (magnitude 8.1) great earthquake. The most ambitious objective is to access and instrument the Nankai plate interface within the seismogenic zone at proposed Sites NT2-03 and NT3-01, at depths of ~3.5 and ~6 km below the seafloor, respectively. The science plan entails sampling and long-term instrumentation of (a) the inputs to the subduction conveyor belt, (b) faults that splay from the plate interface to the surface and that may accommodate a major portion of coseismic and tsunamigenic slip, and (c) the main plate interface at a depth of up to 6 km.

There is a burgeoning interest in active fault drilling, represented by the San Andreas Fault Observatory at Depth (SAFOD), Taiwan Chelungpu Fault Drilling Project (TCDP), Corinth Rift Laboratory (CRL), Nojima Fault Drilling Project, and other active projects on land and at sea in addition to NanTroSEIZE. This is taking place in the context of rapidly growing research efforts on the mechanics and dynamics of faulting processes that integrate rock mechanics, seismology, geodesy, frictional physics, and fluid-fault interactions. Despite recent advances, there is at present no unified theory of fault slip to account for earthquake nucleation and propagation, nor to explain the mechanisms of strain across the spectrum of observed deformation rates ranging from seconds to years. Consequently, the question of whether precursor signals exist for major earthquakes, even in theory, remains under discussion. Progress on these topics is severely limited by a lack of information on ambient conditions and mechanical properties of active faults at depth. Extant rheological models for how faults behave depend on specific physical properties at the fault interface and in the surrounding rock volume. Coefficients of friction, permeability, pore-fluid pressure, state of stress, and elastic stiffness are examples of such parameters that can best (or only) be measured through drilling and through geophysical sensing of the surrounding volume.

Subduction zones like the Nankai Trough (Fig. F1A), on which great earthquakes (M = 8.0) occur, are especially favorable for study because the entire width (dip extent) of the seismogenic zone ruptures in each great event, so the future rupture area is perhaps more predictable than for smaller earthquakes. The Nankai Trough region is among the best-studied subduction zones in the world. It has a 1300 year historical record of recurring and typically tsunamigenic great earthquakes, including the 1944 Tonankai (M = 8.1) and 1946 Nankaido (M = 8.3) earthquakes (Ando, 1975; Hori et al., 2004). The rupture area and zone of tsunami generation for the 1944 event are now reasonably well understood (Fig. F1B, F1C) (Ichinose et al., 2003; Baba and Cummins, 2005). Land-based geodetic studies suggest that the plate boundary thrust here is strongly locked (Miyazaki and Heki, 2001). Similarly, the relatively low level of microseismicity near the updip limits of the 1940s earthquakes (Obana et al., 2004) implies significant interseismic strain accumulation on the megathrust; however, recent observations of very low frequency (VLF) earthquake event swarms apparently taking place within the accretionary prism in the drilling area (Obara and Ito, 2005) demonstrate that interseismic strain is not confined to slow elastic strain accumulation.

The Kumano Basin region, off Kii Peninsula (Fig. F1A), was chosen based on three criteria: (1) the updip end of the seismogenic zone must be definable based on slip in past great earthquakes, (2) seismic imaging must present clear drilling targets, and (3) deep targets must be within the operational limits of riser drilling by Chikyu (i.e., maximum of 2500 m water depth and 7000 m subseafloor penetration). In the Kumano Basin, the seismogenic zone lies ~6000 m beneath the seafloor (Nakanishi et al., 2002). Slip inversion studies suggest that only here did past coseismic rupture clearly extend shallow enough for drilling (Ichinose et al., 2003; Baba and Cummins, 2005), and an updip zone of large slip (asperity) has been identified and targeted (Fig. F1B). Coseismic slip during events like the 1944 Tonankai earthquake likely occurred on the megasplay fault rather than on the décollement beneath it, though slip on either plane is permissible given the available data. The megasplay fault therefore is a primary drilling target equal in importance to the basal décollement zone.

Overall project scientific objectives

Conditions for stable versus unstable sliding—which define seismic versus aseismic behavior—have long been the subject of research and debate, as has the frictional strength of likely fault zone material. Fault zone composition, consolidation state, normal stress magnitude, pore-fluid pressure, and strain rate may affect the transition from aseismic to seismic slip (e.g., Saffer and Marone, 2003). NanTroSEIZE will sample fault rocks over a range of pressure and temperature (P-T) conditions across the aseismic–seismogenic transition; the composition of faults and fluids and associated pore pressure and state of stress and will address partitioning of strain spatially between the décollement and splay faults. NanTroSEIZE will also install borehole observatories to provide in situ monitoring of these critical parameters (seismicity, strain, tilt, pressure, and temperature) over time and test whether interseismic variations or detectable precursory phenomena exist prior to great subduction earthquakes. The overarching hypotheses to be addressed are as follows (please refer to the CDP document at www.iodp.org/NanTroSEIZE for detailed discussion of these hypotheses):

• Systematic, progressive material and state changes control the onset of seismogenic behavior on subduction thrusts.
  
• Subduction zone megathrusts are weak faults.
  
• Within the seismogenic zone, relative plate motion is primarily accommodated by coseismic frictional slip in a concentrated zone.
  
• Physical properties, chemistry, and state of the fault zone change systematically with time throughout the earthquake cycle.
  
• The megasplay (out-of-sequence thrust; OOST) thrust fault system slips in discrete events which may include tsunamigenic slip during great earthquakes.
  

Overall project drilling targets

We plan to drill at eight sites (Figs. F2, F3) to achieve the scientific objectives of the NanTroSEIZE project. Two sites target the incoming plate section, one will sample the frontal thrust of the accretionary wedge, three sites target the megasplay fault system at different depths, one site will sample the megasplay uplift history recorded in the forearc basin sediments, and one ultra-deep site targets the plate interface in the seismogenic zone.

Sampling of the sediments, fluids, and crustal rocks seaward of the deformation front will characterize the subducting materials before deformation. It has been hypothesized that sediment type (especially clay mineral content), fluid content, and basement relief on the incoming plate govern the mechanical state of the plate interface at depth and influence the formation of fault-zone asperities. Two sites (NT1-01 and NT1-07) are planned to sample the entire sedimentary section and as much as 100 m of the basement, respectively, on and off of a preexisting basement high that controlled deposition of thick turbidites in the lower part of the stratigraphic section. Long-term monitoring of pore pressure, seismicity, and other observations in these boreholes will define the hydrological and stress conditions and microseismicity at the point where sediments enter the subduction zone.

Three drill sites targeting the megasplay fault zone (NT2-01, NT2-02, and NT2-03) and one site targeting the frontal thrust (NT1-03) are designed to document the evolution of fault rock properties and the state of stress, fluid pressure, and strain at different P-T conditions. These sites will access faults from ~500 to 3500 m depth below the seafloor. Sealed borehole observatories at some of these sites will monitor pore-fluid pressure, crustal deformation, seismicity, and other properties to document the physical state of the fault zone and its wallrock environment. Proposed Site NT2-03 will cross the seismically reflective megasplay at a depth of 3000–3500 m, in a location where slip probably propagated in 1944.

After initial instrumentation at proposed Site NT2-03, our attention will turn to the 5500–6000 m deep Site NT3-01. Drilling there will pass through both the megasplay fault system and the basal detachment, bottoming in the oceanic crustal rocks of the subducting plate. Drilling of these deep objectives requires novel borehole engineering. Project scientists envision a multipath approach to allow collection of both logs and cores from deep target zones, as well as implementation of a comprehensive monitoring system (Fig. F4), and we are working with the borehole engineers to determine how best to implement this ambitious plan.

In addition to the primary fault zone targets, proposed Site NT3-01 will pass through ~1000 m of the Kumano forearc basin section, including an apparent gas hydrate reflector, several thousand meters of the active accretionary wedge, and a zone of potential underplated rocks below the splay fault. Proposed Sites NT3-01 and NT2-04 together will document the history and growth of the Kumano forearc basin, which has formed as a response to slip on the megasplay fault system, as well as processes of accretionary wedge growth. The basinal history will shed light on the evolution of this long-lived, mid-wedge fault that may be a primary feature of many subduction zone forearcs that produce great earthquakes (e.g., Wells et al., 2003).

Drilling will yield both geophysical logs and physical samples of the rocks and sediments (cores and cuttings), as well as formation fluids. Logging and borehole imaging will determine in situ physical properties and help define stress state (e.g., through borehole breakout and tensile fracture studies). Sampling the inputs and splay faults at several depths, and the plate interface at great depth, will provide key data on the evolution of fault zone composition, fabric development, and lithification state as a function of pressure, temperature, and cumulative slip. Finally, long-term monitoring through downhole instrumentation will yield in situ time-series data sets after the drilling disturbance signals have subsided, possibly including the preseismic near term for a future great earthquake. Ideally, thermal signals, fluid pressure, geochemical tracers, tilt and volumetric strain, microseismicity, and time-varying seismic structure will all be monitored at several locations.

The IODP portion of the NanTroSEIZE project will span a number of years, many individual expeditions, multiple drilling platforms, and several significant stages to achieve all of the proposed scientific objectives, with onboard and shore-based scientific teams matched to the goals of each individual drilling expedition. In response to these challenges, the IODP Operations Task Force formed a project management team (PMT) consisting of two co-chief project scientists (the authors), several additional lead scientists, representatives of the implementing organizations (Center for Deep Earth Exploration [CDEX], and the U.S. Implementing Organization [USIO]), and IODP management (IODP-MI) to help craft strategies to ensure the overall science objectives of the NanTroSEIZE project are maximized. We anticipate that the composition of the PMT will continue to change over time to address the evolving planning and implementation requirements as the multistage NanTroSEIZE project progresses.

A significant volume of site survey data have been collected in the drilling area over many years, including multiple generations of two-dimensional (2-D) seismic reflection (e.g., Park et al., 2002), wide-angle refraction (Nakanishi et al., 2002), passive seismicity (e.g., Obana et al., 2004), heat flow (Kinoshita et al., 2003), side-scan sonar, and swath bathymetry, and submersible and remotely operated vehicle (ROV) dive studies (Ashi et al., 2002). In 2006, Japan and the United States conducted a joint, three-dimensional (3-D) seismic reflection survey over a ~11 km x 55 km area, acquired by PGS Geophysical, an industry service company. This 3-D data volume, the first deep-penetration, fully 3-D marine survey ever acquired for basic research purposes, will be used to refine selection of drill sites and targets in the complex megasplay fault region, to define the regional structure and seismic stratigraphy, to analyze physical properties of the subsurface through seismic attribute studies, to expand findings in the boreholes to wider areas, and to assess drilling safety.

Drilling activities are basically organized into four stages, each of which will include multiple individual expeditions.

NanTroSEIZE Stage 1 calls for drilling and sampling in riserless mode at six of the sites (see "Stage 1 expeditions" below):

• The incoming sediment of Shikoku Basin and underlying oceanic crust (two sites),
  
• The frontal thrust system at the toe of the accretionary wedge,
  
• The mid-wedge megasplay fault system, and
  
• Approximately 1000 m deep holes at the two sites planned for later deep penetrations of the seismogenic zone faults (two sites; one of which will have a subseafloor observatory).
  

"Stage 1 expeditions" contains further information about the specific breakdown of individual expeditions in Stage 1. In brief, comprehensive coring and logging of the boreholes is planned, including extensive use of logging-while-drilling (LWD) technology to obtain high-quality logs. One borehole observatory installation is planned for a pilot hole at proposed Site NT3-01 to monitor pore-fluid pressure, strain, temperature, and seismicity above the plate boundary. This observatory deployment (see Becker and Davis, 2005; Araki et al., 2004) will serve as a prototype and testbed for some of the technologies that might be used in future deeper borehole observatories. IODP currently plans to allocate approximately 8 months of ship time, divided between the new U.S. SODV and the Chikyu, for NanTroSEIZE Stage 1 drilling to take place from September 2007 through April 2008.

NanTroSEIZE Stage 2 will involve drilling the first of the two planned deep riser holes using the drillship Chikyu, targeting the megasplay fault zone at ~3000–3500 m below the seafloor at proposed Site NT2-03. Extensive coring, use of LWD, downhole experiments to measure pore pressure and seismic properties, and an initial, retrievable, long-term monitoring package are all planned for this site. Additional Stage 2 operations will include deepening and installing borehole observatory systems at several of the Stage 1 drill sites. Stage 2 is likely to begin in mid to late 2008 and extend into 2009.

NanTroSEIZE Stage 3 will focus on 5500–6000 m deep drilling into the seismogenic zone and across the plate interface into subducting crust at proposed Site NT3-01. This unprecedented deep ocean borehole will be accomplished through a program of riser-based drilling and a carefully planned casing program. The results of pilot-hole operations, 3-D seismic data, and real-time LWD and measurement-while-drilling (MWD) monitoring will guide the borehole design. Given the frontier nature of this drilling, it is uncertain how long it will take to complete the borehole, but it may exceed a full year of drilling. Once drilling is complete, an initial monitoring system will be deployed in the borehole, components of which remain to be designed. We intend for this monitoring system to remain in place for a period of 1–2 y, while the "final" long-term monitoring package is readied.

NanTroSEIZE Stage 4 will be concerned with installing the final long-term observatory systems into the two ultra-deep boreholes (Fig. F4). This monitoring installation will be planned as much as possible for robust, long-duration deployment, such that data pertinent to the behavior and evolution of the plate interface fault system during a significant portion of the seismic cycle can be recorded. In Japan, a system is proposed to deploy a seafloor fiber-optic network for seismic monitoring and other applications in the Kumano Basin region. One exciting possibility is that the NanTroSEIZE boreholes ultimately could be connected to this network in Stage 4, allowing real-time access to the data. We envision tentatively that Stage 4 installations will be completed sometime in 2012 or 2013; however, the schedule depends heavily on the previous stage success and readiness of sensors and infrastructure for borehole observatories.

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