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doi:10.2204/iodp.proc.314315316.131.2009

Introduction

Overview of NanTroSEIZE

Subduction zones account for 90% of global seismic moment release, generating damaging earthquakes and tsunamis with potentially disastrous effects on heavily populated coastal areas (e.g., Lay et al., 2005). Understanding the processes that govern the strength, nature, and distribution of slip along these plate boundary fault systems is a crucial step toward evaluating earthquake and tsunami hazards. More generally, characterizing fault behavior at all plate boundary types through direct sampling, near-field geophysical observations, and measurement of in situ conditions at depths of coseismic slip is a fundamental and societally relevant goal of modern earth science.

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 (Hickman et al., 2004), the Taiwan-Chelungpu Drilling Project (Ma, 2005), and the Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) drilling (Tobin and Kinoshita, 2006a, 2006b).

NanTroSEIZE is a multiexpedition, multistage IODP effort focused on understanding the mechanics of seismogenesis and rupture propagation along subduction plate boundary faults. The drilling program represents a coordinated effort to sample and instrument the plate boundary system at several locations offshore the Kii Peninsula (Figs. F1, F2), with the main objectives of understanding (Tobin and Kinoshita, 2006a, 2006b) the following:

• The aseismic–seismic transition of the megathrust fault system,
• Processes of earthquake and tsunami generation, and
• The hydrologic behavior of the plate boundary.

The drilling program will evaluate a set of core hypotheses through a combination of riser and riserless drilling, long-term observatories, and associated geophysical, laboratory, and numerical modeling efforts. The following hypotheses are paraphrased from the original proposals and outlined in Tobin and Kinoshita (2006a, 2006b):

1. Systematic, progressive material and state changes control the onset of seismogenic behavior on subduction thrusts.
2. Subduction megathrusts are weak faults.
3. Plate motion is accommodated primarily by coseismic frictional slip in a concentrated zone (i.e., the fault is locked during the interseismic period).
4. Physical properties of the plate boundary system change with time during the earthquake cycle.
5. A significant, laterally extensive fault system (the “megasplay” fault; Park et al., 2002) slips in discrete events that may include tsunamigenic slip during great earthquakes. During the interseismic period, the megasplay remains locked and accumulates strain.

Sediment-dominated subduction zones such as the East Aleutian, Cascadia, and Nankai margins are characterized by repeated occurrences of great earthquakes of >M 8.0 (Ruff and Kanamori, 1983). Although the causative mechanisms are not well understood (e.g., Byrne et al., 1988; Moore and Saffer, 2001; Saffer and Marone, 2003), the updip limit of the seismogenic zones at these margins is thought to correlate with a topographic break along the outer rise (e.g., Byrne et al., 1988; Wang and Hu, 2006). Accretionary prisms in these subduction zones are separated into two parts by this topographic break (Wang and Hu, 2006; Kimura et al., 2007a), and the inner and outer wedges are located above the seismogenic plate boundary and aseismic décollement, respectively.

At the Nankai Trough, high-resolution images of the outer rise from seismic reflection profiles clearly document a large out-of-sequence thrust (OOST) fault system (the megasplay fault after Park et al., 2002) that branches from the plate boundary décollement within the coseismic rupture zone of the 1944 Tonankai M 8.2 earthquake (Fig. F2). Several lines of evidence indicate that the megasplay system is active and may accommodate an appreciable component of plate boundary motion. 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. As stated in the fifth hypothesis above, one of the first-order goals in characterizing the seismogenic zone along the Nankai Trough—and one which bears on understanding subduction zone megathrust behavior globally—is to document the role of the megasplay fault in accommodating plate motion and to characterize its mechanical and hydrologic behavior.

NanTroSEIZE Stage 1 consisted of three coordinated riserless drilling expeditions to drill several sites across the continental slope and rise offshore the Kii Peninsula, within the inferred coseismic slip region of the 1944 Tonankai M 8.2 earthquake (Figs. F1B, F2) (Tobin and Kinoshita, 2006a, 2006b). The first of these was a logging-while-drilling (LWD) expedition that served as a geophysical baseline (IODP Expedition 314: LWD Transect; D/V Chikyu) to define physical properties, lithology, and structural information in advance of coring operations. This was followed by a coring expedition (IODP Expedition 315: Megasplay Riser Pilot; Chikyu) that sampled the materials and characterized in situ conditions within the accretionary prism to a depth of 458 meters below seafloor (mbsf) at Site C0001 (proposed Site NT2-03) and 1057 mbsf at Site C0002. This expedition (IODP Expedition 316; Shallow Megasplay and Frontal Thrusts; Chikyu) was designed to characterize two major thrust fault systems at relatively shallow depths where they are accessible to riserless drilling (Table T1).

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