[S-CG64] 海洋底地球科学

[SCG64-06] バーティカルケーブルサイスミック探査手法を用いた中部沖縄トラフ伊是名海穴Hakureiサイト海底熱水鉱床の海底下構造

*多良 賢二¹、芦 寿一郎¹、淺川 栄一²、村上 文俊²、塚原 均²、溝畑 茂治²、Jamali Hondori Ehsan²、徳山 英一³ (1.東京大学大学院、2.株式会社地球科学総合研究所、3.高知大学海洋コア総合研究センター)

キーワード：海底熱水鉱床, バーティカルケーブルサイスミック, 中部沖縄トラフ, 海底下構造

Seafloor hydrothermal deposits are formed in the deep sea around hydrothermal venting sites, where abundant metals precipitate. The seafloor locations of hydrothermal deposits are revealed by the exploration of geophysical, geochemical and submersible investigations. Hydrothermal deposits are known to form chimneys or mounds. Scale of hydrothermal mound is thought to be a hundred meters square by recent surveys. The sub-seafloor distribution of hydrothermal deposits are identified partially by drilling core data, however, a continuous structure has not been fully understood yet. To improve the quality of the sub-seafloor imaging of hydrothermal deposits, high-resolution geophysical data acquisition is required. We conducted two Vertical Cable Seismic (VCS) surveys at Hakurei site, Izena Hole in the Mid-Okinawa Trough to reveal sub-seafloor structure of hydrothermal deposits. In September 2011, we carried out 28 survey lines around the hydrothermal mound in the Hakurei site using an air-gun source during KY11-E04 cruise. In August 2013, we carried out 28 survey lines around the southern part of KY11E-04 survey area in the Hakurei site using a sparker source during KY13-E02 cruise. VCS surveys successfully imaged the shallow sub-seafloor structures of the hydrothermal field. We identified a mound shape reflector at the sub-seafloor. In addition, we estimated the seismic velocity at the sub-seafloor by the common reflection point (CRP) gathers. Some of the reflectors of the CRP gather show high velocity zone at the sub-seafloor. Previous drilling survey reported that hydrothermal deposits are buried by volcanic sediment in the Hakurei site. These results suggested that high velocity zone between reflectors at sub-seafloor is sediment-buried hydrothermal deposit.