High-resolution seismic profiling in the trench axis area of the Japan Trench and its preliminary correlation with the results of IODP Expedition 343

IODP Expedition 343 (Japan Trench Fast Drilling Project: JFAST) was conducted in April-May 2012 using D/V Chikyu. For the JFAST project, a high-resolution multi-channel seismic (MCS) survey was carried out to clarify the structure near the drill site. We present the characteristics of the seismic data from the site survey and briefly introduce the preliminary correlation between seismic and drilling results. The high-resolution seismic data were obtained during the KY11-E05 cruise in Oct-Nov 2011. The sounding source was a 320 cubic inch cluster gun array, and the receiver was a 1.2-km, 192-channel streamer cable. The seismic sections clearly show sedimentary strata overlaying the igneous crust in the outer trench slope. The strata and igneous crust are offset by normal faults producing a horst-graben structure. The strata are also deformed in the vicinity of the trench axis by compaction. Comparison with the results from DSDP Site 436 suggested that the Pacific plate has been subducting with a chert-rich layer preserved above the igneous crust layer, which was confirmed by the drilling results. The hanging wall in the lowermost landward trench slope has acoustically chaotic character, and no continuous strong reflectors which could suggest the possible 'splay' faults are not observed. Sonic, and density logging nor vertical seismic profiling (VSP) were made, therefore a synthetic seismogram was calculated using basically only resistivity log data. The time-depth relation was calibrated by cross-correlating the synthetic seismogram with the seismic section for the interval around the strong reflector corresponding to the top of the chert-rich layer. The synthetic seismogram also shows some strong reflections in the shallower part, but they are not well correlated with the seismic profiles. We will continue detailed investigations in the future through integration of core, log and seismic data.