IODP Expedition 319, NanTroSEIZE Stage 2: First IODP Riser Drilling Operations and Observatory Installation Towards Understanding Subduction Zone Seismogenesis

Abstract

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a major drilling project designed to investigate fault mechanics and the seismogenic behavior of subduction zone plate boundaries. Expedition 319 is the first riser drilling operation within scientific ocean drilling. Operations included riser drilling at Site C0009 in the forearc basin above the plate boundary fault, non-riser drilling at Site C0010 across the shallow part of the megasplay fault system-which may slip during plate boundary earthquakes-and initial drilling at Site C0011 (incoming oceanic plate) for Expedition 322. At Site C0009, new methods were tested, including analysis of drill mud cuttings and gas, and in situ measurements of stress, pore pressure, and permeability. These results, in conjunction with earlier drilling, will provide a) the history of forearc basin development (including links to growth of the megasplay fault system and modern prism), b) the first in situ hydrological measurements of the plate boundary hanging wall, and c) integration of in situ stress measurements (orientation and magnitude) across the forearc and with depth. A vertical seismic profile (VSP) experiment provides improved constraints on the deeper structure of the subduction zone. At Site C0010, logging-while-drilling measurements indicate significant changes in fault zone and hanging wall properties over short (<5 km) along-strike distances, suggesting different burial and/or uplift history. The first borehole observatory instruments were installed at Site C0010 to monitor pressure and temperature within the megasplay fault zone, and methods of deployment of more complex observatory instruments were tested for future operations.

Distribution of stress state in the Nankai subduction zone, southwest Japan and a comparison with Japan Trench

“…The occurrence of an extensional regime at higher structural levels where slope sediments are dominant (e.g., Site C0001) and the identification of a relatively weak footwall (Olcott and Saffer, 2012;McNeill et al, 2010) are consistent with this interpretation. We therefore propose that the hanging wall of the megasplay is relatively strong and supports a compressional stress state that is consistent with plate convergence.…”

“…Shipboard analysis of the borehole breakouts showed a consistent pattern above the megasplay with the maximum horizontal stress trending NW-SE. McNeill et al (2010) noted that the megasplay corresponds to a seismic reflector…”

Operational Review of the First Wireline In Situ Stress Test in Scientific Ocean Drilling

“…Deployment of the MDT tool deeper within the forearc and in the vicinity of major fault zones in future riser holes will constitute a major breakthrough in understanding subduction zone fault mechanics, and it is a critical part of the NanTroSEIZE program Saffer et al, 2009;McNeill et al, 2010). Operation of this tool was complex both in the planning and execution phase, but valuable experience has been gained by both CDEX and NanTroSEIZE scientists.…”

“…The fracture is then extended for 1-5 minutes before the interval is isolated and the pump stopped (shut-in), the pressure decline then observed, and then followed by a series of injection/fall-off cycles (two to ing (30 km) to improve velocity information of the deep plate boundary and mega-splay faults-the ultimate targets of the project Saffer et al, 2009;McNeill et al, 2010;Expedition 319 Scientists, 2010). In a vertical borehole, the orientation of breakouts is a well-established indicator of the orientation of the horizontal maximum principal stress in the present-day stress field (Zoback et al, 2003).…”

“…This is the case for stress orientation but not for stress magnitude. Thus, for further understanding the state of stress, the hydraulic fracturing (HF) method was used to measure stress magnitude in Expedition 319 (Ito et al, 2009;Saffer et al, 2009;McNeill et al, 2010;Expedition 319 Scientists, 2010).…”