NanTroSEIZE Stage 2: NanTroSEIZE riser/riserless observatory, IODP Expedition 319 Scientific Prospectus

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) program is a coordinated, multiexpedition drilling project designed to investigate fault mechanics and seismogenesis along subduction megathrusts through direct sampling, in situ measurements, and long-term monitoring in conjunction with allied laboratory and numerical modeling studies. The fundamental scientific objectives of the NanTroSEIZE drilling project include characterizing the nature of fault slip and strain accumulation, fault and wall rock composition, fault architecture, and state variables throughout the active plate boundary system. The primary goals for Integrated Ocean Drilling Program (IODP) Expedition 319 are riser drilling, analyses of cuttings and limited cores, logging, and casing at proposed Site NT2-11B (or alternate proposed Site NT2-11A) and riserless drilling, logging while drilling (LWD), and casing at proposed Site NT2-01J (or alternate proposed Site NT2-01K). Operations at both sites will include placing a wellhead and drilling and casing to the target depth in preparation for installation of monitoring equipment planned for future expeditions. This Scientific Prospectus outlines the scientific rationale, objectives, and operational plans for drilling these two sites. Proposed Site NT2-11B is a riser drilling site located in the Kumano forearc basin in the upper plate above the seismogenic and presumed locked portion of the plate boundary thrust system. During this expedition, proposed Site NT2-11B will be drilled to 1600 meters below seafloor (mbsf) to (1) determine the composition, physical properties, and stratigraphy of the basin sediments, (2) conduct downhole measurements to determine stress magnitude and orientation and pore pressure magnitudes, (3) install casing in preparation for a riser observatory, and (4) acquire data from a two-ship vertical seismic profile experiment to characterize the rock volume surrounding and below the borehole, including the subduction thrust at a depth of ~10 km. The observatory (planned for installation in 2010 or 2011) will monitor seismicity, volumetric strain, tilt, pore pressure, and temperature, with the goals of characterizing strain accumulation and release, microseismicity, hydrologic transients associated with strain events, and ambient pore pressure and temperature. Proposed Site NT2-01J is a riserless drilling site located 3.5 km along strike of previously drilled and cored IODP Site C0004 and will penetrate a major splay fault ("megasplay") at ~410 mbsf. The objectives at this site are to (1) collect measurement while drilling (MWD) and LWD data that will complement LWD and core data collected during IODP Expeditions 314 and 316 at Site C0004, (2) install casing in preparation for installation of an observatory (planned for 2010 or 2011) that will cross the fault zone and extend into the footwall, and (3) install a temporary instrument package that will monitor pore pressure and temperature within a screened interval spanning the shallow megasplay fault zone. The temporary instrument package will provide continuous monitoring in the time between drilling of the site and final observatory installation (planned for 2010 or 2011). The observatory planned for 2010 or 2011 at this site will monitor seismicity, volumetric strain, tilt, pore pressure, and temperature. Together with the observatory at proposed Site NT2-11B, this will form the beginning of a distributed observatory network that spans the region above the aseismic–seismic transition on the plate boundary at depth. Contingency plans for Expedition 319 include: (1) drilling, casing, and installing a temporary instrument package at IODP Site C0002 (former proposed Site NT3-01) in the outer more seaward portion of the Kumano Basin similar to the package planned for proposed Site NT2-01J and (2) coring at proposed Site NT1-01A (located seaward of the Nankai Trough on the incoming Philippine Sea plate).