The Japan Trench Fast Drilling Project (IODP Exp. 343&343T JFAST): Making Scientific Drilling History in the Japan Trench

The international scientific community began planning the Japan Trench Fast Drilling Project (JFAST) soon after the 11 March Tohoku Earthquake occurred. Predicted rapid decay of any thermal anomaly resulting from shear heating, which may allow the frictional strength of the main slip zone to be calculated, dictated that temperature measurements needed to be made within 18 months of the initial earthquake. Based on the drilling and observatory request from the science team, the Center for Deep Earth Exploration (CDEX) began scoping activities for this project, and rapidly became aware of some of the daunting technical challenges involved in drilling in approximately 7 km of water. The deepest water depth drilling in scientific ocean drilling history was in the Marianas Trench in 1978 in water depth of 7,049.5 m with 15.5 m penetration. The original plan of JFAST required logging-while-drilling (LWD) and sample collection from 1,000 m below the seafloor in 7,000 m water depth in the Japan Trench. Beyond this, temperature observatories needed to be deployed into the borehole. A scientific drilling proposal was submitted to the Integrated Ocean Drilling Program (IODP) by 1 August 2011 and our preparation for the operation began in parallel. To reach the plate boundary target, and to install an observatory, we had to develop several new tools (e.g., a casing running tool). The strength and performance of the drill string was also a major technical and engineering issue. Taking the limitations of the operational time window into account, our original strategy was, in about 7,000 m of water depth near the axis of the Japan Trench, to 1) drill a 8-1/2" hole with LWD and install an observatory 900 m below the seafloor, 2) drill a 10-5/8" hole with coring assembly and collect core samples from the deeper part of the hole then install another observatory 900 m below the seafloor. IODP Expedition 343 (JFAST) started on 1 April 2012 (less than 13 months after the earthquake). Several mechanical and weather issues prohibited completion of the above planned operations but we had reached the following operational targets by the end of this expedition of 24 May 2012: 1. Penetrated 850.5 m below seafloor and obtained geophysical data by LWD which allowed the plate boundary interface to be located. 2. Collected core samples from 648 m to 844.5 m below the seafloor, including samples of the plate boundary fault zone. Both operations were completed in water depth of 6889.5 m. However, due to a lack of operational time, the installation of temperature observatories was not performed. Consequently, IODP and CDEX/JAMSTEC decided to return to the site, in a follow-up expedition (IODP Expedition 343T), to install a temperature observatory into a borehole started by the original part of the expedition. IODP Expedition 343T began on 5 July 2012 and successfully installed a temperature observatory into an 854.8 m borehole in 6,897.5 m of water, operations completed on 19 July 2012.