Seafloor geodetic measurements and modeling of the subduction processes along the southernmost Ryukyu trench

Research Project

Project/Area Number	17K05630
Research Category	Grant-in-Aid for Scientific Research (C)
Allocation Type	Multi-year Fund
Section	一般
Research Field	Solid earth and planetary physics
Research Institution	Shizuoka University
Principal Investigator	Ando Masataka 静岡大学, 防災総合センター, 客員教授 (80027292)
Co-Investigator(Kenkyū-buntansha)	中村衛琉球大学, 理学部, 教授 (60295293) 生田領野静岡大学, 理学部, 准教授 (60377984)
Project Period (FY)	2017-04-01 – 2020-03-31
Project Status	Completed (Fiscal Year 2019)
Budget Amount *help	¥4,420,000 (Direct Cost: ¥3,400,000、Indirect Cost: ¥1,020,000) Fiscal Year 2019: ¥1,430,000 (Direct Cost: ¥1,100,000、Indirect Cost: ¥330,000) Fiscal Year 2018: ¥1,430,000 (Direct Cost: ¥1,100,000、Indirect Cost: ¥330,000) Fiscal Year 2017: ¥1,560,000 (Direct Cost: ¥1,200,000、Indirect Cost: ¥360,000)
Keywords	背弧拡大 / 海底地殻変動 / GNSS / 音響 / プレート間の固着 / 前弧のせり出し / GNSS 音響 / スラブサクション / 海底地殻変動観測 / 音響測距 / 1771年八重山地震 / プレート間固着率 / 琉球海溝 / 沈み込み帯 / 背弧海盆拡大 / 巨大地震 / 津波 / KGPS/音響測距
Outline of Final Research Achievements	We have conducted a seafloor geodesy measurement at the southern end of Ryukyu trench. The site is located on the fore-arc basin south off Hateruma Island. It is known that Ishigaki, Miyako and the surrounding islands along the southern Ryukyu Trench suffered a significant damage from a large Tsunami in 1771.　Nakamura [2009] proposed that this Tsunami was generated by a large tsunami earthquake of Mw8.0 that occurred along the southern Ryukyu Trench. We deployed a seafloor benchmark at the fore-arc basin to observe the inter-plate coupling between the subducted Philippine sea slab and the fore-arc wedge. Six campaigns were conducted within five years from 2014 to 2019. As a result, the benchmark velocity is estimated to be 5.2±1.2 cm/yr southward with reference to the Eurasian plate, which is comparable with the southward movement of Hateruma Island. This suggests that there is no or very weak inter-plate coupling at the plate boundary just beneath our seafloor benchmark.
Academic Significance and Societal Importance of the Research Achievements	琉球海溝南端部における、巨大プレート境界地震の発生源の有無について、初めて海域下の実際の地殻変動を高精度で計測し、固着の有無を検討した。結果、巨大プレート境界地震を発生するような強い固着が認められなかったことは新たな発見である。さらに、本研究では着脱式、低コストのGNSS-音響測位装置を開発し、十分な精度で計測ができることを示した。現在台湾のグループが同じ方式で観測システムを作成しているが、装置の低コスト化と観測の簡易化に道を開いたことも大きな意義である。