RESEARCH OF DETAILED STRUCTURE BENEATH ICELANDIC PLATEAU BY USE OF OCEAN BOTTOM SEISMOGRAPHS, WITH A COOPERATION BETWEEN JAPAN AND NORWAY

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By using ocean bottom seismograph (OBS) which were developed in our institutes, We made researches of detailed crustal structures of Sognefjord, which is the longest fjord in Norway with a length of 200 km) in 1987, and a very detailed three dimensional structure beneath northeast Atlantic Ocean bottom between Norwegian coast and Mohns Ridge in 1988. Also, we made a detailed study of natural earthquake activities off Norway in 1989.In the controlled source experiments, 20 - 25 OBS were deployed on ocean bottom by research ships of Noway and West Germany. A powerful airgun array was used for the controlled source.In the 1987 experiment, we obtained a detailed structure beneath Sognefjord where we find a good correspondence with a surface geological structure and traced a deeper extension of the boundary.In the 1988 experiment, two successive experiments were carried out, each of which made use of 20 - 25 OBS, Both were the highest density experiment which were made to date. The region off Lofoten is covered by a flow lava which is impossible to penetrate by multichannel reflection study so that only OBS could study the crustal structure beneath the layer. The result has helped to study the opening of the Atlantic Ocean. In the 1989 experiment, detailed seismic activity with very small earthquakes has been studied for the first time in a region between Norwegian coast and Mohns Ridge. The activity had not been able to be observed from the observations on land. The research has helped the study of the intraplate earthquake in the region which had not been well known.