Joint sea-land seismological observation at peculiar mid-oceanic ridges.

特殊洋中脊海陆联合地震观测.

• 批准号: 04044014
• 负责人: SHIMAMURA Hideki
• 金额: $ 8.83万
• 依托单位: Hokkaido University
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for international Scientific Research
• 财政年份: 1992
• 资助国家: 日本
• 起止时间: 1992 至 1993
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-04044014/
• 关键词: Mid-Atlantic Ridge; Spreading or Atlantic Ocean floor; Ocean Bottom Seismographic Observations; Ocean Bottom Seismograph; Triple Junction; Microseismicity; Hypocentral distribution; Plate tectonics

Mid-Atlantic Ridge is the place where plate is being born. Azores Islands are situated immediately on the triple junction where three plates, Eurasian plate, North American Plate and African Plate, are simultaneously born. Also, the Azores-Gibraltar Fracture Zone(AGFZ) extends from Azores Triple Junction (AZT) to the east until Gibraltar strait. The largest European earthquake in the history, Lisbon earthquake, 1755, took place on this fracture zone.However, both seismic activity and underground structure for AZT and AGFZ have not been well investigated since all the seismological observations had been so far made from thousands of km apart, i.e. from both sides of Atlantic Ocean, which consequently restricted the detailed studies.We believe that our Ocean Bottom Seismographs (OBSs) are powerful tool to solve these problems. Our OBS is small, reliable and highly sensitive. Deployments of tens of OBSs enable to illustrate a very detailed seismic activity as well as three dimensional sei … More smic structures.We brought about 30 OBSs to the Atlantic in 1992 and in 1993. In 1992 we made a dense array measurements of seismic activity in the AZT area, and in 1993 we made again the similarly extensive experiments in the AGFZ area. More than 1000 micro earthquakes were caught by our OBSs for both expeditions. Apart from the Azores area, we also made a dense OBS studies, in which we used 30 OBSs, to illustrate a very detailed crustal structure off Norway, where a remnant initiation of the Atlantic Ocean, which has been believed to initiate 55 million years ago, is kept beneath the flow basalt. The flow basalt is very impenetrable rock for the usual seismic reflection method used in the sea, however our OBS refraction survey can penetrate beneath the layer.In the 1992 experiment, we deployed 29 OBSs along Azores Islands, which covered 250 x 150km area. We obtained a valuable help from Lisbon University and Portuguese Coast Guard for logistics. University of Paris deployed seven radio-wave telemetry seismographs for this study on some islands.As a result, we found that microearthquakes occurred only along a narrow tectonic belt which connect Azores Islands, and no seismicity elsewhere. The earthquake epicenters are in a form of an echelon, which is clockwise to the tectonic belt. Our result suggests that microplates with rather small size behaves as books on the bookshelf, which is a discovery for the tectonics of the triple junction.In 1993 we deployed 25 OBSs in the vicinity of AGFZ. The experiment was two-fold : microearthquake survey and refraction study by the use of air-guns. Lots of earthquakes were caught by the OBS, and the refraction study was successful. Because the obtained data were vast, the analysis is still being done, however we are optimistic to obtain various findings for the tectonics of the region. Less

大西洋中脊是板块形成的地方。亚速尔群岛位于欧亚板块、北美板块和非洲板块三个板块同时诞生的三重连接点上。此外，亚速尔-直布罗陀断裂带从亚速尔三联点向东延伸，直至直布罗陀海峡。1755年欧洲最大的地震里斯本地震就发生在这一断裂带上，但由于迄今为止所有的地震观测都是在相距数千公里的大西洋两岸进行的，我们相信我们的海底地震仪（OBS）是解决这些问题的有力工具。我们的OBS体积小、可靠、灵敏度高。几十个地震观测站的部署，能够说明非常详细的地震活动以及三维地震活动。 ...更多信息 我们在1992年和1993年把大约30个OBS带到了大西洋。1992年，我们对AZT地区的地震活动进行了密集的测量，1993年，我们再次在AGFZ地区进行了类似的广泛实验。两次考察中，我们的OBS都捕捉到了超过1000次的微地震。除了亚速尔群岛地区之外，我们还进行了密集的OBS研究，其中我们使用了30个OBS，以说明挪威附近非常详细的地壳结构，其中大西洋的残余起源被认为是在5500万年前开始的，被保存在流动玄武岩下面。流动玄武岩是非常难以穿透的岩石，对于通常在海洋中使用的地震反射方法来说，但是我们的OBS折射测量可以穿透该层之下。在1992年的实验中，我们沿着亚速尔群岛沿着部署了29个OBS，覆盖了250 x 150 km的区域。我们得到了里斯本大学和葡萄牙海岸警卫队的后勤帮助。巴黎大学在一些岛屿上部署了7台无线电波遥测地震仪进行研究，结果发现，只有沿着连接亚速尔群岛的狭窄构造带发生了微震，其他地方没有地震活动。地震震中沿构造带顺时针方向呈雁列状。1993年我们在AGFZ附近布设了25个大洋钻探系统，发现了一些小的微板块，这些微板块就像书架上的书一样。实验有两个方面：微地震测量和使用气枪进行折射研究。OBS捕捉到了大量地震，折射研究也取得了成功。由于获得的数据量很大，分析仍在进行中，但我们对获得该地区构造的各种发现持乐观态度。少

项目成果

T.Watanabe,H.Shiobara,T.Kanazawa,R.Stefansson,H.Shimamura: "Microseismicity study on Reykjanes Ridge Crest area revealed by ocean bottom seismograph array" Geophysical Journal. (to be submitted).

T.Watanabe，H.Shiobara，T.Kanazawa，R.Stefansson，H.Shimamura：“海底地震仪阵列揭示的雷克雅内斯山脊顶部地区的微震活动研究”地球物理杂志。

B.Bryndis,H.Shiobara,G.Gudmundsson,H.Shimamura: "Crustal structure study north of Iceland revealed by ocean bottom seismographs and land-based seismograph array" Physics of the Planetary and Earth Interiors. (to be submitted).

B.Bryndis、H.Shiobara、G.Gudmundsson、H.Shimamura：“海底地震仪和陆基地震仪阵列揭示的冰岛北部地壳结构研究”行星和地球内部物理学。

L.A.Mendes Victor, H.Shimamura, A.Hirn, J.M.Miranda, J.Z.Simoes, J.F.Luis, I.Abreu, H.Shiobara, H.Nemoto, S.Kodaira, M.Mochizuki, J.C.Lepine: "Seismotectonics of the Azores Triple Junction 3 : IPG, University of Paris, Jussieu Paris Cedex 05, France" Tect

L.A.Mendes Victor、H.Shimamura、A.Hirn、J.M.Miranda、J.Z.Simoes、J.F.Luis、I.Abreu、H.Shiobara、H.Nemoto、S.Kodaira、M.Mochizuki、J.C.Lepine：《亚速尔群岛地震构造》

H.Shimamura,R.Stefansson,T.Watanabe,G.Gudmundsson,P.Einarsson,S.Kodaira,H.Shiobara: "An extremely narrow zone of seismicity along the crest of the Reykjanes Ridge revealed by a dense OBS survey" Geophysical Journal. (to be submitted).

H.Shimamura、R.Stefansson、T.Watanabe、G.Gudmundsson、P.Einarsson、S.Kodaira、H.Shiobara：“通过密集的 OBS 勘测揭示了雷克雅内斯山脊顶部的一个极其狭窄的地震活动区”地球物理

R.Stefansson, P.Einarsson, and H.Shimamura: "Seismic activity beneath mid-oceanic ridge crest area revealed by ocean bottom seismographic array" Geophysical Journal. (to be submitted).

R.Stefansson、P.Einarsson 和 H.Shimamura：“海底地震阵列揭示的洋中脊顶部区域下方的地震活动”《地球物理杂志》。