IODP

doi:10.2204/iodp.sp.343.2012

Drilling, coring, and instrumentation plan

The general operations plan and time estimates are provided in Table T1 and Figure F3A and F3B. Although site survey data analysis, contingency planning, engineering, and working out the technical details for placing instruments in the boreholes are ongoing at the time of writing this Scientific Prospectus, the prioritization of the main activities and overall sequencing is not expected to change. However, given the compressed time frame for planning the project and the likelihood of encountering difficult drill site conditions, we expect that some details of the operational plan will be modified before and during the expedition. Purposefully including some redundancy in the instrumentation plan, contingency planning, and identification of alternate drill sites will greatly contribute to successfully meeting the scientific goals of the expedition.

The main operations to be completed during Expedition 343 consist of drilling two boreholes (Holes A and B) at a single location (e.g., proposed Site JFAST-3); the first hole is dedicated to collecting downhole geophysical data, and the second hole is dedicated to retrieving core samples from across the Tohoku earthquake slip surface. An observatory will be established to acquire temperature within each borehole from across the fault and fluid pressure data in one hole from inside and outside the fault zone. The sequencing of operations reflects the prioritization of the activities and engineering constraints.

The most important goal of the project is to gather repeated temperature measurements across the fault zone to determine the amount of frictional heat generated by the earthquake and constrain the dynamic frictional strength of the fault. Accordingly, each borehole will be instrumented with a multisensor temperature measurement string. Observations of a decaying temperature signal will be used to estimate the level of dynamic friction during the large rupture. To obtain good resolution of the thermal signal, calculations show that initial measurements need to be started within about 2 y following the earthquake, with measurements continued over subsequent years. In addition to the temperature observations, other time-sensitive measurements such as borehole stress provide insights on the rupture mechanics. Observatory data that are recorded in the borehole will be retrieved later using the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) remotely operated vehicle (ROV) Kaiko.

The sequence of operations to be completed during Expedition 343 consist of

  1. Drilling the pilot hole with logging while drilling (LWD)/measurement while drilling (MWD) to total depth (TD), currently planned for ~900 mbsf;

  2. Running casing and completion assembly (including long-term temperature monitoring with the autonomous string) in LWD/MWD hole to ~900 mbsf (casing to 100 m below fault, ~900 m);

  3. Drilling second hole with rotary coring barrel (RCB) drilling (with center bit) to ~500 mbsf and continuation with RCB coring to ~900 mbsf; and

  4. Running casing and completion assembly for the second hole (including long-term temperature and pressure monitoring with the telemetered string) to ~900 mbsf (casing to 100 m below fault, ~900 m).

Proposed Sites JFAST-3 and JFAST-4

The drilling target is the top of the oceanic basement below the plate interface at the toe of the frontal prism. At the proposed sites, the target is the seismic reflector that is interpreted as the top of the Pacific plate basaltic crust. Based on experience drilling other accretionary wedge settings (e.g., Barbados, Costa Rica, and Nankai) and the Taiwan Chelungpu Fault Drilling Project, we anticipate that identifying one or more plate boundary fault(s) that slipped during the Tohoku event will be possible on the basis of cores and logs. While drilling Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) holes through the toe regions of the frontal and splay thrust, the main faults were located slightly above the strong reflector imaged by multichannel seismic data (Expedition 316 Scientists, 2009). Logistic plans are not affected if the exact depth of the target fault is unknown until logging and coring is completed.

Preliminary analysis of seismic data indicates the primary site (JFAST-3) is located at a water depth of 6910 m along seismic Lines HD33B and HS41B, and the target fault lies at ~800 mbsf (Fig. F4). The alternate site (JFAST-4) is located at a water depth of 6830 m along seismic Lines HD33B and HS40B, and the target fault lies at ~880 mbsf.

Hole A

The first borehole will be rotary drilled with LWD and MWD tools to TD, currently planned for ~900 mbsf. LWD/MWD will acquire important geophysical data and provide information on the location of the fault zone (e.g., IODP Expedition 314 [Kinoshita et al., 2008]). LWD-derived resistivity is an effective way to identify the depth and width of a frontal megathrust fault zone, which will be used to refine strategies for coring in the second borehole. Resistivity images with 360 coverage of the borehole are also essential for identifying the geometry of faults and measuring in situ stress by stress-induced borehole compressive failures (borehole breakouts) and drilling-induced tensile fractures (DITFs). Other key LWD measurements could characterize important fault physical properties.

After completing LWD/MWD drilling operations, casing and a completion assembly (including a temporary temperature observatory) will be run in the LWD/MWD hole (8.5 inch diameter) to ~900 mbsf.

Hole B

The second borehole will be drilled using an RCB bit (10⅝ inch diameter) to a TD of ~900 mbsf. The second highest priority for the expedition is to retrieve core samples from across the slipped fault zone. However, the time required for running casing and placing observatory instruments in both boreholes will not allow enough time to core the entire length of the second borehole. Accordingly, we plan continuous RCB coring only over a 300–400 m interval spanning the fault zone. The remainder of the borehole (the top 500 m) will be drilled using RCB with a center bit. The exact depth range for coring will be determined on the basis of high-resolution seismic site survey data and LWD/MWD data from the first borehole. Successful RCB coring will allow for careful analysis of fault zone structures, as well as provide fault rock samples for experimentation designed to determine the mechanical properties that allowed such large slip to occur to the trench. Core studies will characterize important attributes of the fault including composition, mineralogy, grain size and fabric, as well as damage zone fracture density. Several different techniques will be employed to identify the main slip zone, which may be as thin as several millimeters and may occur within a damage zone as wide as several hundred meters. As was done during IODP Expedition 316 in the Nankai accretionary margin, X-ray computed tomography (CT) scans, visual core study, and microscopy will be used to identify the location of the recent rupture (Expedition 316 Scientists, 2009). Evidence in the core of coseismic slip may include structures indicative of fault gouge fluidization and injection of gouge into wall rock, grain size segregations and other fault rock textures, and petrographic, geochemical or geophysical signatures of transient heating. Detailed multisensor core logger (MSCL) profiles will provide important complementary information about physical properties such as density and magnetic susceptibility across the slip zone. Core-based, 3-D anelastic strain recovery measurements will be used to augment the determination of in situ stress conditions from geophysical data. Biostratigraphic age analysis may also play a critical role in detecting a stratigraphic age gap expected across the main slip zone.

If there is excellent core recovery, including capturing an identifiable rupture zone from the Tohoku earthquake, then various onshore core analyses and experiments will be performed, particularly to determine petrophysical and friction properties. Even if core recovery does not include an identifiable rupture zone, experiments can still be performed on in situ sediments that will characterize useful frictional parameters of the sediments within the fault zone.