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Geological setting and previous NanTroSEIZE drilling

The Nankai accretionary complex off the coast of southwest Japan is formed by subduction of the Philippine Sea plate beneath the Eurasian plate along the Nankai Trough (Kinoshita et al., 2007). The southwest–northeast striking accretionary wedge mainly consists of off-scraped and underplated materials from the trench-fill turbidites and the incoming Shikoku Basin hemipelagic sediments (Moore et al., 2009). Much of what is known about the Nankai Trough subduction zone results from earlier scientific drilling in the Deep Sea Drilling Project and Ocean Drilling Program as well as an extensive 3-D seismic reflection survey (Moore et al., 2007, 2009). Drilling and geophysical surveys along two transects in the western (off Ashizuri Peninsula) and central part (Muroto Peninsula) of the study area shed light on the regional geology of both the incoming and accreted sediment (e.g., Taira, Hill, Firth, et al., 1991; Moore et al., 2001; Ike et al., 2008). A third transect offshore of the Kii Peninsula, now termed the NanTroSEIZE transect, was the target of subsequent 3-D seismic reflection work (Moore et al., 2007, 2009; Bangs et al., 2009; Strasser et al., 2009). Apart from the master décollement, a prominent splay fault system was imaged that is invoked as a candidate for coseismic slip (e.g., Moore et al., 2007).

From 2007 until the present, a series of expeditions took place under the umbrella of the IODP complex drilling project NanTroSEIZE (for an outline of the objectives, see Tobin and Kinoshita, 2006). One of the key scientific goals is to shed light on the nature of the different fault zone materials within the Nankai Trough accretionary complex and how their physical properties vary as a function of depth and distance from the deformation front (i.e., with increasing pressure and temperature [P-T]). Several hypotheses are to be tested with respect to the downdip transition from aseismic deformation at shallow depth to stick-slip behavior and earthquake rupture at depths greater than ~5–6 km (e.g., Tobin and Kinoshita, 2006). This approach requires a transect of holes spanning from the undeformed sediment of the incoming Philippine Sea plate across the frontal to the central portion of the accretionary wedge (Fig. F1A). During three stages of drilling, drill sites have sampled the downgoing plate (IODP Sites C0011 and C0012), the frontal wedge (IODP Sites C0006 and C0007), sediments draping the midslope (IODP Site C0008), the shallow splay fault (IODP Sites C0001, C0003, C0004, and C0010) and the Kumano forearc basin (IODP Sites C0002 and C0009) (Tobin et al., 2009; Screaton et al., 2009; Ashi et al., 2009; Saito, Underwood, Kubo, and the Expedition 322 Scientists, 2010; Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010). A second key objective of the NanTroSEIZE project is to develop and install a distributed network of borehole observatories spanning the upper transition from aseismic to seismic slip in order to provide long-term and continuous records of subsurface fluid pressure, temperature, strain, tilt, and seismicity (e.g., Tobin et al., 2009).

The borehole observatory systems installed to date have focused on the shallow portion of the megasplay fault and the seaward edge of the Kumano Basin (Expedition 319 Scientists, 2010). Here, we describe the temporary instrument installations at Site C0010 (SmartPlug and GeniusPlug); a companion manuscript by E. Araki et al. (unpubl. data) describes the permanent observatory system and its installation at Site C0002. The megasplay fault was penetrated at two sites along the NanTroSEIZE transect: (1) Stage 1 drilling at Site C0004 (Tobin et al., 2009), which collected logging-while-drilling (LWD) data during IODP Expedition 314 and continuously cored the section during IODP Expedition 316, and (2) Stage 2 drilling, casing, and SmartPlug installation at Site C0010, located ~3.5 km along strike of Site C0004 to the southwest, during Expedition 319 (Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010). At both sites, a similar sequence was drilled (Fig. F1B).

Combined analysis of 3-D seismic and age constraints from drilling indicate that the tectonostratigraphic system in this area evolved ~2.2 m.y. ago in a frontal prism toe position when an emerging trench-slope basin was formed in concert with in-sequence forward imbrication of accreted strata (Strasser et al., 2009). Splay fault movement initiated ~1.95 m.y. ago as an out-of-sequence thrust (OOST) in the lower part of the prism. Since ~1.55 Ma, this initial OOST was uplifted and became reactivated, favoring ongoing “megasplay” slip along it (Strasser et al., 2009). Along the NanTroSEIZE drilling transect, it appears that displacement along the shallow segment of the megasplay fault ceased at ~1.24 Ma, suggesting that it only experienced a relatively short period of high activity between ~1.55 and 1.24 Ma (Strasser et al., 2009). Seismic data clearly show that the megasplay truncates very young sediments near the seabed west of the study area (Moore et al., 2007), suggesting that segments of the fault system are currently or recently active. However, variations in fault activity and architecture along strike indicate that in some areas, the megasplay fault system includes several branches and that fault activity may be distributed along these branches as well as expressed by distributed deformation of the overlying slope sediments (e.g., Moore et al., 2009; Kimura et al., 2011). Thus, splay fault activity and deformation in the surrounding geologic bodies are interrelated. The wealth of information in the area adjacent to Sites C0004 and C0010 indicates that monitoring strain as a function of modern splay fault activity represents a promising target for NanTroSEIZE observatories.

The geology at Site C0010 includes a ~200 m thick sequence of slope apron deposits composed of silty mudstone with some thin sand and ash layers overlying a ~210 m thick wedge of fractured mudstone comprising a thrust sheet in the hanging wall of the megasplay fault (e.g., Moore et al., 2009; Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010). The fault juxtaposes the thrust wedge above with overridden slope apron sediments below, which consist of silty mudstone with numerous sand beds and some ash (Kimura et al., 2008; Moore et al., 2009). The character of the megasplay fault zone in seismic reflection images differs markedly between Sites C0004 and C0010 (e.g., Flemings et al., 2009). At Site C0004, there are two distinct reflectors at the base of the thrust wedge; both coring and LWD data document the presence of two main fault zones separated by a ~50 m thick “fault-bounded package” (e.g., Kimura et al., 2008). In contrast, at Site C0010, the megasplay is imaged as a single sharp reflector in the seismic data, suggesting that it would be thinner and perhaps have a simpler architecture than the fault at Site C0004. During Expedition 319, a bottom-hole assembly was made up of a 12¼ inch bit with an 8¼ inch LWD geoVISION tool measuring natural gamma ray and resistivity and the measurement while drilling (MWD) PowerPulse measuring direction and inclination, torque, and weight on bit. After MWD and LWD drilling to 402 mbsf, 20 inch casing was run into the hole. Two joints of screened casing (22 m length) were placed at 387–409 mbsf to span the fault zone at 407 mbsf as interpreted from LWD and seismic reflection data (Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010). This configuration is outlined schematically in Figure F1B.