NanTroCORK: Development of hydrologic long-term CORK observatories in the seismogenic Nankai Trough subduction zone, Japan
Zusammenfassung der Projektergebnisse
NanTroSEIZE (Nankai Trough Seismogenic Zone Experiment) is currently the flagship project in lODP and to date had three expeditions in 2007/2008 (Stage 1), two expeditions in 2009 (Stage 2), and will have two more expeditions in 2010/2011 (Stage 3). The results of the former 5 cruises are largely published online at the lODP website. After completion of Stage 1, the NanTroSEIZE project management team (PMT) decided that a change in observatory strategy was needed to accommodate for a) delays in the drilling plan, and b) the strong Kuroshio current which at present makes the deployment of complex CORK systems very unlikely. The German component of the borehole observatory network planned for NanTroSEIZE was initially aiming at the toe of the accretionary prism as well as a location outboard of the trench for direct comparison. Since both sites were announced lower priority by the PMT in 2008, we changed our strategy and focused on the splay fault system within the prism (all in accordance with DFG). At two sites, pore pressure (as a proxy for strain) and temperature are lo be monitored in sediments of the accretionary wedge to illuminate the activity of the up-dip end of the splay fault over time. The specific objectives of monitoring are to document (i) formation pore pressure, both to determine ambient values and to record hydrologic transients and temporal pattems of strain, (ii) formation pressure response to known tidal loading to constrain formation compressibility and hydraulic diffusivity, and (iii) formation temperature as a function of active fluid flow, the latter of which has been demonstrated to be elevated in Nankai further to the west. Collocated in situ long-term monitoring of seismicity, pore pressure, and strain are needed to test hypotheses for strain localization, locking along faults, and plate boundary weakness that invoke suprahydrostatic fluid pressures. The other sites of monitoring are to be instrumented by American and Japanese colleagues in a joint, intensely coordinated effort and with commingled funds. The results of the NanTroCORK project can be wrapped up as follows: 1) We have successfully developed a new "mini-CORK", which in essence is an instrumented, retrievable bridge plug of little more than 1 m length with pore pressure and temperature sensors. 2) We have jointly built this instrument with colleagues at PGC Canada, where substantial knowhow transfer occurred (this having been a goal of the proposal too). 3) During lODP expediton 319, the first of the two instruments was deployed successfully in hole C0010 into an interval where casing was perforated so that the in situ properties of the splay fault can be monitored. This has been the only successful long-term installation in NanTroSEIZE to date! 4) We have then further developed an add-on component to the "mini-CORK" which also samples fluids percolating through the splay faull (osmo-sampler unit) and does in situ microbial cultivation experiments (so called FLOC syslem). This instrument will be replacing the first generation "mini-CORK" in fall 2010 during the upcoming expedition 332. 5) Finally, we contributed to the joint German-US-Japanese non-Riser CORK system, which has been built and is currendy tested at JAMSTEC and onboard Chikyu. It is aimed for a permanent installation in Hole C0002, equally in the up-dip portion of the splay fault system.
