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doi:10.2204/iodp.pr.319.2009

Abstract

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) 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 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. As part of the NanTroSEIZE program, operations during Integrated Ocean Drilling Program (IODP) Expedition 319 included riser drilling, analyses of cuttings and core samples, downhole measurements and logging, and casing at Site C0009 in the Kumano forearc basin as well as riserless drilling, logging while drilling (LWD), casing, and observatory operations at Site C0010 across a major splay fault (termed the "megasplay") that bounds the seaward edge of the forearc basin near its updip terminus. In addition, we drilled at contingency Site C0011 to collect logging-while-drilling data in advance of planned coring operations scheduled as part of IODP Expedition 322.

Site C0009 marked the first riser drilling in IODP history. This allowed several scientific operations unprecedented in IODP, including carefully controlled measurements of in situ pore pressure, permeability and minimum principal stress magnitude, real-time mud gas analysis, and laboratory analyses of cuttings throughout the entire riser-drilled depth range (~700–1600 meters below seafloor [mbsf]). We conducted a leak-off test at one depth interval and successfully deployed the wireline Modular Formation Dynamics Tester 12 times to directly measure in situ stress magnitude, formation pore pressure, and permeability. During all phases of riser drilling, we collected mud gas for geochemical analyses and cuttings samples were collected throughout the entire riser-drilled depth range. Integration of data from cuttings, wireline logging, and cores (from a limited depth interval) allowed definition of a single integrated set of lithologic units and comparison with previously drilled IODP Site C0002 to determine the evolutionary history of the forearc basin. After casing the borehole, we conducted a long-offset (up to 30 km) two-ship active seismic experiment, recording shots within the borehole to image the megasplay and master décollement beneath the borehole, and to evaluate seismic velocity and anisotropy of the forearc basin and accretionary prism sediments around the borehole.

At riserless Site C0010, operations included drilling with measurement while drilling (MWD)/LWD across the megasplay fault to 555 mbsf, casing the borehole with screens at the depth of the fault, conducting an observatory dummy run to test future strainmeter and seismometer deployment procedures, and installation of a temporary pore pressure and temperature monitoring system in advance of planned future permanent observatory emplacement. The observatory system (termed a "smart plug") marks the first observatory installation of the NanTroSEIZE program. MWD/LWD data at this site were used to define unit boundaries and the fault zone target interval for placement of the casing screens. Through comparison with previously drilled Site C0004 these data also provide insights into along-strike differences in the architecture of the megasplay fault and hanging wall.

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