The IODP Nankai Trough Seismogenic Zone Experiment

The Integrated Ocean Drilling Program’s (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) attempts to drill into, sample, and instrument the updip portion of the Nankai subduction megathrust asperity. Access to the interior of active faults in which in situ processes can be monitored and fresh fault zone materials can be sampled is of fundamental importance to the understanding of earthquake mechanics. Great subduction earthquakes (with their accompanying tsunamis) represent one of the greatest natural hazards on the planet. Accordingly, drilling into and instrumenting an active interplate seismogenic zone was identified as a very high priority in the IODP Initial Science Plan, 2001. Through a decade-long series of international workshops, a consensus was made that the Nankai Trough subduction zone was an ideal place to attempt drilling the seismogenic plate interface. The first stage of NanTroSEIZE drilling operations is now scheduled for the late summer of 2007. It involves parallel deployment of the new U.S. Scientific Ocean Drilling Vessel (SODV) and the riser vessel Chikyu.


Introduction
The Integrated Ocean Drilling Program's (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) attempts to drill into, sample, and instrument the updip s to drill into, sample, and instrument the updip to drill into, sample, and instrument the updip updip portion of the Nankai subduction megathrust asperity.
Nankai subduction megathrust asperity.asperity. .Access to the interior of active faults in which in situ processes can be monitored and fresh fault zone materials can be sampled is of fundamental importance to the understanding of earthquake mechanics.Great subduction earthquakes Great subduction earthquakes subduction earthquakes (with their accompanying tsunamis) represent one of the represent one of the greatest natural hazards on the planet.Accordingly, drilling into and instrumenting an active interplate seismogenic zone was identified as a very high priority in the IODP Initial identified as a very high priority in the IODP Initial Science Plan, 2001.Through a decade-long series of international workshops, a consensus was made that the Nankai , a consensus was made that the Nankai a consensus was made that the Nankai was made that the Nankai that the Nankai Trough subduction zone was an ideal place to attempt drilling the seismogenic plate interface.The first stage of NanTroSEIZE drilling operations is now scheduled for the late summer of 2007.It involves parallel deployment of the new U.S. Scientific Ocean Drilling Vessel (SODV) and the riser vessel Chikyu.
The fundamental goal of the NanTroSEIZE science plan (Tobin and Kinoshita, 2006;Tobin and Kinoshita, 2006;Kinoshita et al., 2006) is the creation of a ) is the creation of a is the creation of a is the creation of a distributed observatory spanning the updip limit of seismogenic and tsunamigenic behaviors at a location where great s at a location where great at a location where great subduction earthquakes occur, allowing us to observe the hydrogeologic behavior of subduction megathrusts and the aseismic-to-seismic transition of the megathrust -seismic transition of the megathrust 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. 1 and 2).At this location, the plate . 1 and 2).At this location, the plate 1 and 2).At this location, the plate interface and active megasplay faults implicated in causing tsunami are accessible to drilling in the region of coseismic rupture in the 1944 Tonankai magnitude 8.0 earthquake.The most ambitious objective is to access and instrument the Nankai plate interface within the seismogenic zone.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 that may accommodate a major portion of coseismic and tsunamigenic slip, and (c) the main plate interface at depths of ~4-6 km.
In addition to NanTroSEIZE, there is a burgeoning interest n addition to NanTroSEIZE, there is a burgeoning interest , there is a burgeoning interest there is a burgeoning interest there is a burgeoning interest here is a burgeoning interest in active fault drilling, represented by the San Andreas Fault Fault Fault Observatory at Depth (SAFOD), Taiwan Chelungpu Fault Drilling Project (TCDP), Corinth Rift Laboratory (CRL), (TCDP), Corinth Rift Laboratory (CRL), (TCDP), Corinth Rift Laboratory (CRL), Nojima Fault Drilling Project, and other active projects on projects on projects on land and at sea (Reches and Ito, 2007).This is taking place in (Reches and Ito, 2007).This is taking place in .This is taking place in the context of rapidly growing research efforts on the context of rapidly growing research efforts on the context of rapidly growing research efforts on the mechanics and dynamics of faulting processes that integrate processes that integrate processes that integrate rock mechanics, seismology, geodesy, frictional physics, and and and fluid-fault interactions.Despite recent advances, there is at present no unified theory of fault slip to account for earth-theory of fault slip to account for earth-theory of fault slip to account for earthquake nucleation and propagation, nor to explain the mecha-the mecha-the mechanisms of strain across the spectrum of observed deformation rates ranging from seconds to years.Consequently, the ranging from seconds to years.Consequently, the ranging from seconds to years.Consequently, the question of whether precursor signals exist for major earth-exist for major earth-exist for major earthquakes, even in theory, remains under discussion.Progress on these topics is severely limited by a lack of information on  studies suggest that only here did past coseismic ruptures clearly extend shallow enough shallow enough shallow enough for drilling (Baba and Commins, 2005;Ichinose et al., 2003), and, , consequently, an updip zone of an updip zone of updip zone of updip zone of large slip (asperity) has been identified and targeted.Coseismic slip during events like slip during events like 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 is permissible given is permissible given the available data.Therefore, the refore, the the the megasplay fault 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 behavior-have long been 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 normal stress magnitude, pore-fluid normal stress magnitude, pore-fluid pressure, and strain rate may affect the transition from from from aseismic to seismic slip (Saffer and Marone, 2003).NanTroSEIZE will sample fault rocks over a range of pressure fault rocks over a range of pressure fault rocks over a range of pressure and temperature (P-T) conditions across the aseismicseismogenic transition; the composition of faults and fluids and associated pore pressure and state of stress and will pressure and state of stress and will pressure and state of stress and will address partitioning of strain spatially between the décol-the décol-the décollement and splay faults.NanTroSEIZE will also install faults.NanTroSEIZE will also install .NanTroSEIZE will also install borehole observatories to provide to provide to provide in situ monitoring of these critical parameters (by monitoring of seismicity, strain, tilt, by monitoring of seismicity, strain, tilt, seismicity, strain, tilt, pressure, and temperature) over time, and test whether and temperature) over time, and test whether and temperature) over time, and test whether interseismic variations or detectable precursory phenomena precursory phenomena precursory phenomena exist prior to great subduction earthquakes.
The overarching hypotheses to be addressed are as overarching hypotheses to be addressed are as overarching hypotheses to be addressed are as follows: Systematic, progressive material and state changes control the onset of seismogenic behavior on subduction behavior on subduction 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 coseismic frictional slip in a coseismic frictional slip in a concentrated zone.Physical properties, chemistry, and state of the fault zone change systematically with time throughout the with time throughout the with time throughout the earthquake cycle.The mega splay (out-of-sequence thrust; OOST) thrust fault system slips in discrete events, which may include events, which may include events, which may include tsunamigenic slip during great earthquakes.

Geological and Geophysical Background
Subduction zones like the Nankai Trough, on which great earthquakes (M>8.0)occur, are especially favorable for >8.0) occur, are especially favorable for 8.0) occur, are especially favorable for study because the entire width (dip extent) of the seismo-the seismo-the seismogenic zone ruptures in each great event.Thus, the future rupture area is perhaps more predictable than for smaller more predictable than for smaller more predictable than for smaller earthquakes.The Nankai Trough region is among the best-among the best-among the beststudied subduction zones in the world.It has a 1300-year -year year historical record of recurring and typically tsunamigenic record of recurring and typically tsunamigenic record of recurring and typically tsunamigenic great earthquakes, including the 1944 Tonankai (M8.1) and Tonankai (M8.1) and Tonankai (M8.1) and 1946 Nankaido (M8.3) earthquakes (Ando, 1975;Hori et al., al., al., 2004).The rupture area and zone of tsunami generation for the 1944 event are now reasonably well understood (Baba now reasonably well understood (Baba now reasonably well understood (Baba and Cummins, 2005;Ichinose et al., 2003).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 earthquakes in the 1940s (Obana et al., 2004) implies earthquakes in the 1940s (Obana et al., 2004) implies 1940s (Obana et al., 2004) implies significant interseismic strain accumulation on the megathrust; however, recent observations of very low observations of very low observations of very low frequency (VLF) earthquake event swarms apparently taking place within the accretionary prism in the drilling place within the accretionary prism in the drilling place within the accretionary prism in the drilling area (Obara and Ito, 2005) demonstrate that interseismic that interseismic that interseismic strain is not confined to slow elastic strain accumulation, but , but also is released as slow events.
released as slow events.as slow events.
In the Kumano Basin, the seismogenic zone lies ~6000 m Basin, the seismogenic zone lies ~6000 m Basin, the seismogenic zone lies ~6000 m beneath the seafloor (Nakanishi et al., 2002).Slip inversion Slip inversion Slip inversion

Figure 1 .
Figure 1.Bathymetric map of Kumano Basin region, with Kikuchi et al. (2003) inversion of the coseismic rupture area of the 1944 Tonankai earthquake (gray contours ), 3-D multichannel seismic survey area shown (bold black outline), and location of planned drill sites identified.Deep-penetration Multi Channel Seismic (MCS) Line 5 (thick black line) is shown in Fig. 2 with drill sites projected onto it.
Figure 1 mechanical properties of active faults mechanical properties of active faults mechanical properties of active faults at depth.Extant rheological models for how faults behave faults behave faults behave depend on specific physical properties at the fault interface and in the surrounding rock volume.Coefficients of friction, surrounding rock volume.Coefficients of friction, surrounding rock volume.Coefficients of friction, permeability, pore-fluid pressure, state of stress, and elastic state of stress, and elastic 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.volume.volume.

Figure 2 .
Figure 2. Prestack depth-migrated seismic line 5 of Park et al. (2002), with locations of planned drill sites.Riserless sites planned for Stage 1 drilling are shown in solid color, and riser-drilling sites are shown in outline.Faults labeled with red arrows are inferred possible co-seismic slip zones during 1944 event on basal decollement and mega-splay fault.Vertical exaggeration = 2×.