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

近年来的高分辨率地壳结构和海底地形测量揭示了活动断裂的详细分布。与此相反，虽然在浅水中进行了取样，但在深海中对断层周围的沉积物样品的研究还不够深入。2002年，东京大学的ORI开发了一个专门用于深海精确取样的遥控潜水器。本研究的主要目的是通过断层周围的定点采样，揭示南开地区活动断层的形成时间和变形方式。第二个目标是获得渗漏活动数据，作为今后长期监测活动断层沿着的基础信息。主要结果如下：1. Ryuyo峡谷：这是一个废弃的峡谷，在浅水中没有沉积物来源。两个地点的岩心样品显示浊积岩层每隔三到六百年沉积一次。熊野泥火山：泥火山由海底喷发物组成，沿着活动断层发育。样品取自山顶以泥石流为特征的地区。三个碎屑泥层被确认为每十到两千年喷发一次。展布断层：岩屑沉积物存在于一个大型展布断层切割海底的小阶地。岩屑堆的岩芯样品被解释为地震沉积物，并建议每一千年一次的复发。我们还能够获得15个月的冷渗漏点的流量数据，表明活动的淡出。此外，在2004年纪伊半岛东南地震后不久，成功拍摄了海水柱的视频图像。海底和200米以上的云层表明震中附近发生了山体滑坡

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