Report on DELP 1988 Cruises in the Okinawa Trough : Part 2. Seismic Reflection Studies in the Southwestern Part of the Okinawa Trough

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概要

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• Multi- and single-channel seismic reflection profiles along four track lines in the southwestern part of the Okinawa Trough were obtained during the 1988 DELP cruise. The trough is a back-arc depression located between the Ryuku Arc and the shallow shelf of the East China Sea. The acoustic sequence in the trough can be divided into three units depending mainly on the varying degrees of deformation and reflection character. Widespread deformation by normal faulting is especially developed in the lowest unit which displays horst and graben structures. The deformation would have originated at about the Plio-Pleistocene boundary and formed the prototype of the trough. Reflectors observed within the upper unit show the tilting and subsidence of the trough floor, and essentially no deformation by faulting except for the Yaeyama Graben area located in the axial part of the trough. Just along the Yaeyama Graben, rifting and subsidence with normal faulting are in progress. Some volcanic intrusions observed in the Yaeyama Graben indiccate that the bimodal and/or island-arc type volcanisms have occurred. These tectonic events suggest that the origin of the trough is the continental rifting and that the trough is in a state of the rifting stage. Also, the events suggest that the back-arc spreading might have recently initiated only in the Yaeyama Graben.1988年7月,沖縄トラフ南西部を中心にDELP研究航海が実施された.本航海では,トラフ南西部を南北に横断する4測線において反射法地震探査が行われた.その結果,トラフに分布する音響層は,正断層による変形の程度および堆積構造から,前期中新世から現在までに堆積したと考えられる3層準に区分される.広域かつ規模の大きい変形が認められるのは,トラフ底において島尻層群に相当すると考えられる音響基盤である.上位の堆積層中に発達する反射面は,トラフの中軸部に向って傾動しており,トラフ底全体が沈降を続けてきたことが推定される.トラフの中軸部には,現在も活動的であると思われる正断層によって形成された明瞭な海底地溝が認められる.この海底地溝に沿った断層に伴うトラフ底の沈降は,特に顕著なものであり,往復走時で2.5秒を越える堆積物が認められる.地震探査記録に認られたこれらの構造から,沖縄トラフ南西部の形成は大陸地殻のriftingによるものであることが示唆される.また,その形成は,おもに第四紀以降に行われたことが推定される.

• 東京大学地震研究所,Earthquake Research Institute, University of Tokyo,Department of Earth Sciences, Kobe University,Ocean Research Institute, University of Tokyo,Deparment of Earth Sciences, Chiba University,Observation Center for Predictions of Earthquakes and Voの論文
• 1991-06-28

著者

• 古川 雅英

  放射線医学総合研究所人間環境研究部

• Tokuyama Hidetoshi

  Faculty Of Pharmaceutical Sciences The University Of Tokyo

• TOKUYAMA HIDEKAZU

  Ocean Research Institute, University of Tokyo

• Nishizawa A

  Maritime Safety Agency Tokyo Jpn

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• Report on DELP 1988 Cruises in the Okinawa Trough : Part 2. Seismic Reflection Studies in the Southwestern Part of the Okinawa Trough
• Report on DELP 1987 Cruises in the Ogasawara Area : Part III : Seismic Refraction Experiment Using Airgun-OBSH for Fine Crustal Structure across the Ogasawara Trough along East-West Track Line
• Report on DELP 1987 Cruises in the Ogasawara : Part II : Seismic Reflection Studies in the Ogasawara Trough
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• Report on DELP 1987 Cruises in the Ogasawara Area : Part IV : Explosion Seismic Refraction Studies
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• Report on DELP 1985 Cruises in the Japan Sea : Part III : Seismic Reflection Studies in the Yamato Basin and the Yamato Rise Area
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