Relationship between the upper mantle structure of the Okinawa Trough, large earthquake in the back-arc basin, and expansion of the back-arc basin

2022 Fiscal Year Final Research Report

• Project/Area Number: 18H01306
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Review Section: Basic Section 17040:Solid earth sciences-related
• Research Institution: Tokyo University of Marine Science and Technology
• Principal Investigator: nakahigashi kazuo 東京海洋大学, 学術研究院, 教授 (90709346)
• Co-Investigator(Kenkyū-buntansha): 山田 知朗 東京大学, 地震研究所, 助教 (40323654) ; 八木原 寛 鹿児島大学, 総合科学域総合研究学系, 准教授 (60295235) ; 山下 裕亮 京都大学, 防災研究所, 助教 (80725052)
• Project Period (FY): 2018-04-01 – 2022-03-31
• Keywords: 沖縄トラフ

Outline of Final Research Achievements

Five seafloor seismometers were deployed around the Danjyo Basin, located in the northern part of the Okinawa Trough. As a result of hypocenter relocation using the obtained seafloor seismic data and remote island observation data, it was found that the distribution of hypocenters around the Danjyo Basin tends to be linearly aligned and in the form of several clusters. The strike of these clusters is oriented in various directions, indicating that the fault zone forms a complex fault geometry. We also found that the distribution of earthquakes was continuous from the upper crust to the lower crust after the M6.2 earthquake that occurred during the observation period. This seismic activity in the lower crust may be related to the presence of fluid supplied by the uppermost mantle.

Free Research Field

海洋底科学

Academic Significance and Societal Importance of the Research Achievements

沖縄トラフは現在背弧海盆拡大の最も初期の段階にあると考えられており、その北部延長線上には2016年熊本地震(マグニチュード7.3)が発生した「別府-島原地溝帯」に接続していると考えられている。北部沖縄トラフでは、2015年にM7.1の地震が発生した。この地震の余震分布をみると熊本地震と同じくライン状に震源が分布し、複数の断層帯が複雑に分布していることが分った。熊本地震と同様に日本では珍しい伸長場で発生した大地震の発生様式を考察することは、今後、別府-島原地溝帯で発生の可能性がある地震を考察する上でも重要である。