Grant-in-Aid for international Scientific Research
|Allocation Type||Single-year Grants|
|Research Institution||Hokkaido University|
SHIMAMURA Hideki Hokkaido University, Professor (Faculty of Science), 理学部, 教授 (10011636)
セレボール マークバード ベルゲン大学, 地球物理, 教授
ステファンソン ラグナー アイスランド気象庁, 部長
MIRANDA Miguel Associate Professor, Lisbon University, Portugal, 助教授
SIMOES Jose Associate Professor, Lisbon University, Portugal, 助教授
メルザ バシレ 英国国際地震センター, 主任研究員
レピン ジャン クロード パリ大学, 理学部, 助教授
ヒルン アルフレッド パリ大学, 理学部, 助教授
メンデス ビクター ルイ リスボン大学, 理学部, 教授
KANAZAWA Toshihiko Assistant Professor, Tokyo University (Faculty of Science), 理学部, 助教授 (30114698)
KODAIRA Shuuichi Research Associate, Hokkaido University (Faculty of Science), 理学部, 助手 (80250421)
SHIOBARA Hajime Research Associate, Hokkaido University (Faculty of Science), 理学部, 助手 (60211950)
SELLEVOLL Markvard Professor, Bergen University, Norway
MARZA Vasile Professor, International Seismological Centre, U.K.
LEPINE Jean-claude Professor, University of Paris, France
HIRN Alfred Professor, University of Paris, France
STEFANSSON Ragnar Chief Geophysical Investigator, Icelandic Meteorological Office, Iceland
MENDES-VICTOR Luis Professor, Lisbon University, Portugal
MARKVARD Sel ノルウエー, ベルゲン大学・地球物理学研究所, 教授
ROLF Mjelde ノルウエー, ベルゲン大学・地球物理学研究所, 助教授
JOSE Simoes ポルトガル, リスボン大学・理学部, 教授
ALVARO Peret アルゼンチン国立極地研究所, 主任研究員
VASILE Marza 英国, 国際地震センター, 主任研究員
JEAN Claude フランス, パリ大学・地球物理学部, 助教授
ALFRED Hirn フランス, パリ大学・地球物理学部, 助教授
LUIS Mendes ポルトガル, リスボン大学・理学部, 教授
|Project Period (FY)
1992 – 1993
Completed(Fiscal Year 1993)
|Budget Amount *help
¥13,800,000 (Direct Cost : ¥13,800,000)
Fiscal Year 1993 : ¥7,100,000 (Direct Cost : ¥7,100,000)
Fiscal Year 1992 : ¥6,700,000 (Direct Cost : ¥6,700,000)
|Keywords||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
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