Study on Reconstruction and Prediction of Active Fault Activity in the Nankai Trough using Pinpoint Sediment Sampling

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Detailed distributions of active faults have been revealed by recent high-resolution survey of crustal structure and seafloor topography. In contrast, study on sediment sample around faults is not investigated well in deepsea although samplings in shallow water have been conducted. ORI, Univ. Tokyo developed a remotely operated vehicle dedicated to pinpoint sampling in deepsea in 2002. Primary objective of our study is to reveal timing and deformation style of active faults in the Nankai area by pinpoint sampling around faults. Secondary objective is to obtain seep activity data as basic information for future long-term monitoring along active faults. Main results are shown in below.1. Ryuyo Canyon : It is an abandoned canyon without sediment source in shallow water. Core samples at two sites indicate turbidite layers deposited every three to six hundred years.2. Kumano Mud Volcano : Mud volcanoes composed of erupted materials from subbottom are developed along the active fault. Samples were taken from the area characterized by debris flow from the summit. Three debris mud layers were recognized suggesting eruptions in every ten to two thousands years.3. Splay fault : Talus deposits are existed at the small terrace where a large splay fault cuts the seafloor. Core samples from the talus are interpreted to be seismic trigged deposits and suggest every one thousand year's recurrence.We were also able to obtain flow data at cold seep sites for fifteen month suggesting fade-out of activity Moreover, video images at sea water column were successfully taken just after the 2004 southeast off Kii peninsula earthquake. Cloudiness between the seafloor and 200 meters above it suggests occurrences of landslides near the epicenter