Recent installation of a well-head bridge plug at the A(advanced)CORK site in the Nankai Trough now permits continued monitoring of pressures with superior resolution in geological formations of a subduction zone (upper basement of incoming plate, plate boundary fault itself, and overlying sediments). This important time series of pressure measurements will be made from two ACORKs deployed during IODP leg 196: 1) a reference hole (1173B) through the sediment column in the oceanic plate and 2) at a deep site (hole 8081) at the frontal thrust of an accretionary prism. From these measurements relationships between subseafloor fluid pressures, fluid flow, and tectonic strain events will be made.
Broader Impacts: Relationships between subseafloor pressures and earth quake cycles have societal relevance. The research wiIl support graduate research, strengthen international collaborations between RSMAS-JAMSTEC-GSC, and make good use of existing ODP observatories such as ACORKs
The major goals of this project were to support the continuing participation of the principal investigator and graduate students in recurring Japanese submersible operations at, and analysis of data from, two long-term "Advanced CORK" (ACORK) subseafloor observatories at the Nankai Trough subduction zone offshore Shikoku Island. This is a long-term internationally collaborative program with scientists in both Canada and Japan. The ACORKs monitor formation pressures from several subseafloor zones down to 900 m below seafloor, with the aim of elucidating linkages among tectonic, earthquake, and fluid flow processes along the subduction zone boundary fault. The two ACORKs were originally installed during Ocean Drilling Program operations in 2001, with earlier support from NSF to the PI for the original instrumentation. One is located about 10 km seaward of the "deformation front" or seafloor location of the plate boundary, penetrates about 700 m below seafloor and serves as a reference site. The other is located about 2 km landward of the deformation front and penetrates about 900 m, nearly to the plate boundary fault. Since their installation they have continued to function as long-term pressure monitoring installations, and JAMSTEC (Japan Agency for Marine Science and Technology) has continued to support submersible revisits for data downloads every year or two. This project has helped to support installation of new equipment to extend the monitoring effort to more than double the original ten-year lifetime – well beyond the lifetime of this project. This is an important contribution to scientific infrastructure, and JAMSTEC will likely continue to support submersible dives for periodic data downloads in the future. The subseafloor pressures recorded in the ACORKs have shown signals that can clearly be associated with (1) local earthquakes at the Nankai Trough subduction zone, (2) distant earthquakes such as the destructive March 2011 Tohoku earthquake about 900 km to the northeast, and (3) a new class of non-destructive "slow-slip" or very-low frequency earthquake (VLFE) activity that has been detected at many subduction margins in the last decade and may account for a significant amount of the relative motion between plates at a subduction zone. The ACORK pressures therefore provide important constraints on geophysical modeling of earthquake processes (e.g., "elastic dislocation" models) and how the plates deform both in response to earthquakes and during intervening inter-seismic periods. A ten-year review of results has recently been published in the scientific literature with support from this project. In combination with seismic observations and subseafloor borehole observatory results in other subduction settings, the results are contributing toward understanding how plate tectonic, earthquake, and fluid flow processes might be related in subduction zones. This has important implications for understanding earthquake cycles in subduction settings and for mitigating or minimizing the impact of earthquake hazards near subduction zones.
