2000 Fiscal Year Final Research Report Summary
Study on the Mantle-Upwelling Structure of Non Island-Arc Type West off Kyushu
| Project/Area Number |
10304034
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| Research Category |
Grant-in-Aid for Scientific Research (A).
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
固体地球物理学
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| Research Institution | KYUSHU UNIVERSITY |
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Principal Investigator |
SUZUKI Sadaomi KYUSHU UNIVERSITY Faculty of Science, Prof., 大学院・理学研究院, 教授 (00001850)
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| Co-Investigator(Kenkyū-buntansha) |
TAKENAKA Hiroshi KYUSHU UNIVERSITY Faculty of Science, Ass.Prof., 大学院・理学研究院, 助教授 (30253397)
SHIMIZU Hiroshi KYUSHU UNIVERSITY Faculty of Science, Prof., 大学院・理学研究院, 教授 (50178985)
NAKADA Masao KYUSHU UNIVERSITY Faculty of Science, Prof., 大学院・理学研究院, 教授 (50207817)
SHINOHARA Masanao Univ.Tokyo, Earthq.Res.Inst., Ass.Prof., 地震研究所, 助教授 (90242172)
KAME Nobuki KYUSHU UNIVERSITY Faculty of Science, Res.Ass., 大学院・理学研究院, 助手 (90304724)
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| Project Period (FY) |
1998 – 2000
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| Keywords | West off Kyushu / Back-arc of Kyushu / Crustal Strucuture / Mantle-Upwelling / Seismic Tomography / Okinawa Trough / Seismic Travel Time |
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| Research Abstract |
The Okinawa trough which is considered to be in an early stage of backarc spreading locates in the backarc region of Ryukyu-Kyushu arc. Understanding mechanism of backarc spreading and backarc volcanism of Kyushu, we conducted an explosion seismology in the northeastern Okinawa trough west off Kyushu by using ocean bottom seismographs (OBSs) in fall of 1999. Twenty OBSs were deployed at intervals of 15 km in the NNE-SSW seismic profile of 300 km in length along the trough center. We used airguns and 76 explosives as controlled sources. During shooting of the airguns, reflection waves were recorded by 24-ch hydrophone streamer towed from the shooting ship. An obtained seismic reflection profile shows large lateral heterogeneity in the upper crust in spite of smooth seafloor topography of the surveyed area. On the distance-time sections of OBSs, apparent velocities of first arrivals vary due to the heterogeneity. The sedimentary section consists of two layers. The upper sedimentary layer
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has a thickness of 200-500m with P-wave velocity (Vp) of 1.7 - 1.9 km/s. A vertical velocity gradient of the upper layer is small. The lower sedimentary layer has Vp of 2.0 - 3.5 km/s with relatively large vertical velocity gradient. The thickness of the lower layer has large variety from 800 m to 3500m. The upper crust has large lateral heterogeneity and is also divided into two layers. Vp at the top of the uper crust vary in 3.0 - 4.9 km/s. The lower layer of the upper crust has Vp of 5.6 - 6.0 km/s. The thickness of the upper crust changes from 3 km to 9 km. The boundary between the upper and lower crust is about 10 km in depth below the sea surface. The lower crust has Vp of 6.5 - 6.7 km/s. Compared with the upper crust, the lower crust is relatively homogeneous. A thickness of the lower crust is about 15 km. The crust has a thickness of about 25 km in total and the Pn velocity is estimated to be 7.7 -7.8 km/s. The distribution of Vp and the thickness of the crust indicated that the crust of the northeastern Okinawa trough is similar to that of island arc than to that of ocean. It is suggested from the obtained seismic structure that the northeastern Okinawa trough is in the incipient stage of the back-arc rifting. Less
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