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doi:10.2204/iodp.sp.322.2009

Introduction

Overview of NanTroSEIZE complex drilling project

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 nature and distribution of slip along these plate boundary fault systems is a crucial part of evaluating earthquake and tsunami hazards. More generally, characterizing fault behavior through direct sampling, near-field geophysical observations, and measurement of in situ conditions at the depths of coseismic slip is a fundamental goal of modern earth science. To this end, several recent and ongoing drilling programs have targeted portions of active plate boundary faults that either slipped coseismically during large earthquakes or nucleate clusters of smaller events. These efforts include the San Andreas Fault Observatory at Depth (Hickman et al., 2004), the Taiwan-Chelungpu Drilling Project (Ma et al., 2006; Hirono et al., 2006), and IODP NanTroSEIZE drilling (Tobin and Kinoshita, 2006a, 2006b).

NanTroSEIZE is a multiexpedition, multistage project focused on understanding the mechanics of seismogenesis and rupture propagation along plate boundary faults. The IODP science plan outlines a coordinated effort to sample and instrument the plate boundary system at several locations offshore the Kii Peninsula (Figs. F1, F2). The main objectives are to improve understanding of

• The aseismic–seismic transition of the megathrust fault system,

• The mechanics of earthquake and tsunami generation, and

• The hydrologic behavior of the plate boundary and subduction margin (Tobin and Kinoshita, 2006a, 2006b).

As NanTroSEIZE progresses, scientists 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 IODP 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 (i.e., they slip under relatively low stress).

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 laterally extensive "megasplay" fault system slips in discrete events that may include tsunamigenic slip during great earthquakes. The fault remains locked during the interseismic period and accumulates strain.

6. To test these hypotheses, we need to document initial conditions within subducting sediment and basalt, beginning at "reference sites" seaward of the deformation front.
   

Sediment-dominated subduction zones such as Nankai margin are characterized by repeated occurrence 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 zone is thought to correlate with a topographic break along the outer rise of the forearc (e.g., Byrne et al., 1988; Wang and Hu, 2006). At Nankai, high-resolution seismic reflection profiles clearly document an out-of-sequence thrust or megasplay fault system that branches from the plate boundary (décollement) within the coseismic rupture zone of the 1944 Tonankai M 8.2 earthquake (Park et al., 2002) (Fig. F2). As stated above, two of the first-order goals of this project are to document the role of the megasplay fault in accommodating plate motion and to characterize its mechanical and hydrologic behavior. Ultimately, we plan to intersect the plate interface itself at seismogenic depths.

The Japanese Center for Deep Earth Exploration (CDEX) conducted three coordinated riserless expeditions during 2007–2008 as Stage 1 of NanTroSEIZE, drilling a series of sites across the continental margin offshore the Kii Peninsula. The transect is located within the inferred coseismic slip region of the 1944 Tonankai M 8.2 earthquake (Figs. F1, F2) (Tobin and Kinoshita, 2006a, 2006b). The first expedition (IODP Expedition 314) successfully obtained a comprehensive suite of geophysical logs and other downhole measurements at sites along the transect using state-of-the-art logging-while-drilling (LWD) technology (Kinoshita et al., 2008). Unfortunately, the expedition ended before LWD data could be obtained from any of the "subduction input" sites. This was followed by a coring expedition (IODP Expedition 315) to collect materials from and to characterize in situ conditions within the accretionary wedge and Kumano forearc basin at IODP Sites C0001 and C0002 (Ashi et al., 2008). The third expedition (IODP Expedition 316) collected core samples from shallow fault zones, including the frontal thrust near the trench (IODP Sites C0006 and C0007) and the older accretionary prism and megasplay fault at 400 meters below seafloor (mbsf) (IODP Sites C0004 and C0008) (Kimura et al., 2008).

NanTroSEIZE proceeds to Stage 2 in 2009. IODP Expedition 319 will drill two holes for future long-term observatories, which in conjunction with a planned dense ocean floor network system will monitor earthquakes and tsunamis (Kinoshita et al., in press). The first-ever riser hole in scientific ocean drilling history will be drilled and cased to 1600 mbsf at a site just above the locked zone of the plate interface. A riserless cased hole for another long-term observatory is also included in the expedition plan.

Some of the tasks remaining from NanTroSEIZE Stage 1 will be implemented following the riser expedition. Expedition 322 will characterize the sedimentology, physical properties, physical and chemical hydrogeology, and in situ conditions of the incoming sediment and uppermost igneous crust at proposed Site NT1-07A. A companion site (proposed Site NT1-01A) is included in the contingency plan.

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