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doi:10.2204/iodp.pr.322.2009

Background

The entirety of the NanTroSEIZE science plan entails sampling, logging, downhole measurements, and long-term instrumentation of three geologic domains: (1) inputs to the subduction "conveyor belt," (2) faults that splay from the plate interface to the surface and accommodate a major portion of coseismic and tsunamigenic slip, and (3) the main plate interface at 6–7 km below seafloor. The project has progressed into Stage 2 by following a research strategy that integrates rock mechanics, seismology, geodesy, frictional physics, and fluid-fault interactions to shed light on the mechanics and dynamics of faulting processes. Despite recent advances, there is neither a unified theory of fault slip to account for earthquake nucleation and propagation nor is there a theory to explain the mechanisms of strain across the spectrum of observed deformation rates ranging from seconds to years. Consequently, the simple question of whether or not precursor signals exist for major earthquakes, even in theory, remains under scientific debate.

Progress on these topics in earthquake science is limited by a lack of information on ambient conditions and mechanical properties of active faults at depth. Extant rheological models for how faults behave require choices to be made for 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 be measured best (or only) through drilling and geophysical sensing of the surrounding rock volume. Conditions for stable versus unstable sliding (i.e., seismic versus aseismic behavior) have long been debated. The frictional strength of likely fault zone materials is another topic of great interest. Fault zone composition, consolidation state, normal stress magnitude, pore fluid pressure, and strain rate may all affect the transition from aseismic to seismic slip (e.g., Moore and Saffer, 2001; Saffer and Marone, 2003). To tease apart the contributions of each, the NanTroSEIZE project will: (1) sample fault rocks over a range of pressure-temperature (P-T) conditions across the aseismic–seismogenic transition; (2) document the composition of fault rocks and fluids, as well as associated pore pressure and state of stress; and (3) address spatial partitioning of strain 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 or not interseismic variations or detectable precursory phenomena exist prior to great subduction earthquakes.

The overarching hypotheses to be tested by the project are as follows:

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

In order to test the first two hypotheses, initial conditions along the subduction conveyor must be established in comprehensive detail and with clarity. We will achieve this goal by sampling the incoming sedimentary strata and the top of igneous basement prior to subduction. It is only through such sampling that we can pinpoint how various properties (e.g., clay composition, fluid production, and pore pressure) change in space and time. This relatively simple combination of observational data and laboratory measurements represents the fundamental contribution of Expedition 322 toward the overall success of NanTroSEIZE.

Geological setting

The purpose of Expedition 322 is to document the characteristics of incoming sedimentary strata and igneous basement prior to their arrival at the subduction front of the Nankai Trough. The Shikoku Basin, in which the subducting sediments accumulated, formed during the early and middle Miocene epochs by seafloor spreading along the backarc side of the Izu-Bonin volcanic chain (Okino et al., 1994; Kobayashi et al., 1995). The subducting Philippine Sea plate is currently moving toward the northwest beneath the Eurasian plate at a rate of ~4 cm/y (Seno et al., 1993), roughly orthogonal to the axis of the Nankai Trough. Deposits within the Shikoku Basin and the overlying Quaternary trench wedge are actively accreting at the deformation front, as demonstrated by IODP Expeditions 314, 315, 316 (Stage 1) in the Kumano transect area (Tobin et al., 2009).

As summarized by Underwood (2007), our knowledge of inputs to the Nankai subduction zone is rooted in discoveries from numerous boreholes that were drilled previously along the Muroto and Ashizuri transects (Deep Sea Drilling Project [DSDP] Legs 31 and 87 and Ocean Drilling Program [ODP] Legs 131, 190, and 196) (Karig, Ingle, et al., 1975; Kagami, Karig, Coulbourn, et al., 1986; Taira, Hill, Firth, et al., 1991; Moore, Taira, Klaus, et al., 2001; Mikada, Becker, Moore, Klaus, et al., 2002). Those studies demonstrated, among other things, that the plate boundary fault (décollement) propagates through Miocene strata of the lower Shikoku Basin facies, at least near the toe of the accretionary prism (Taira et al., 1992; Moore et al., 2001). Along the Kumano transect (Fig. F2), seismic reflection data show that the décollement is hosted by lower Shikoku Basin strata to a distance of at least 25–35 km landward of the trench (Fig. F3). Farther landward, the plate boundary fault steps downsection to a position at or near the interface between sedimentary rock and igneous basement (Park et al., 2002). Thus, if the goal is to track physical/chemical changes down the plate interface from shallow depths toward seismogenic depths, then the relevant targets for sampling prior to subduction lie within the lower Shikoku Basin.

Regional-scale analyses of seismic reflection data from the Shikoku Basin reveal a large amount of complexity and variability in terms of acoustic character and stratigraphic thickness (Ike et al., 2008a, 2008b). Previous drilling also demonstrated that seafloor relief (created during construction of the underlying igneous basement) strongly influenced the basin's early depositional history (Moore et al., 2001; Underwood, 2007). As an example of such influence, a prominent basement high marks the axis of a fossil (middle Miocene) backarc spreading center; younger seamounts of the Kinan chain are superimposed on the fabric of the extinct ridge (Kobayashi et al., 1995). Evidently, elevation of the seafloor along the seamount chain inhibited transport and deposition of sand by gravity flows. As a consequence, Miocene–Pliocene sediments above the ridge consist almost entirely of hemipelagic mudstone, whereas coeval Miocene strata on the flanks of the Kinan basement high consist largely of sand-rich turbidites (Moore et al., 2001). There are also important differences along strike in clay mineral assemblages and the progress of clay–mineral diagenesis (Underwood, 2007; Saffer et al., 2008). To learn more about how basement might have exerted control over stratigraphic architecture elsewhere in the Shikoku Basin, Expedition 322 was designed to drill two reference sites: one at the crest of a bathymetric high (Kashinosaki Knoll) and the other along the northwest flank of the knoll (Fig. F4).

Site survey data

In addition to data from previous drilling transects, site survey data near the two proposed sites include multiple generations of two-dimensional (2-D) seismic reflection lines (e.g., Park et al., 2002), heat flow (Yamano et al., 2003), side-scan sonar, swath bathymetry, visual observations from submersible and remotely operated vehicle dives (Ashi et al., 2002), and a three-dimensional (3-D) seismic reflection survey (Moore et al., 2007). A smaller 3-D survey was completed in 2006 by the Japan Agency for Marine-Earth Science and Technology–Institute for Frontier Research on Earth Evolution (JAMSTEC-IFREE) (Park et al., 2008). Prestack depth migration of the mini-3-D data led to refinements of velocity models and revised estimates of sediment thickness and total drilling depths. The project also greatly benefited from the acquisition of logging-while-drilling (LWD) data from Hole C0011A (proposed Site NT1-07A) during the final days of Expedition 319 (Saffer et al., 2009). Among other things, the LWD data allowed for shipboard adjustments to seismic velocity models and more accurate time-depth conversions.

Our expectations regarding the subsurface facies relations across the flank of the seamount were heavily influenced by coring results from ODP Site 1177, where the boundary between upper Shikoku Basin facies and lower Shikoku Basin facies is a dividing line between abundant layers of volcanic ash above and monotonous mudstone below (Moore et al., 2001). At Site 1177, the lower Shikoku Basin also contains four discrete packets of sandy turbidites between ~450 and 750 m core depth below seafloor (CSF). Near Site C0011, we see one distinctive interval of discontinuous high-amplitude seismic reflectors from ~350 to 500 m seismic depth below seafloor (SSF) (Fig. F4), and that interval may coincide with a facies that contains abundant sand beds. However, it is interesting to note that this packet of reflectors can be traced up and over the crest of the Kashinosaki Knoll. Deeper in the seismic section the inferred Miocene turbidites of the lower Shikoku Basin thicken and display outstanding acoustic continuity. That seismic interval begins at ~700 m SSF and clearly pinches down against the basement high toward the southeast (Fig. F4). These two acoustic intervals, with their inferred contents of sandy sediment, should have a significant influence on 3-D changes in hydrogeological, geotechnical, and frictional properties.

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