Seismographic array observation in and around Iceland by use of ocean bottom seismographs and land seismometers.

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Mid-Atlantic Ridge is the place where plate is being born. Iceland is situated immediately on the Ridge. There was no detailed study on crustal and upper mantle structure of any ridge. Also there was no detailed seismicity study around ridge. This study has revealed a very detailed seismicity in and around Iceland which made use of sensitive ocean bottom Seismograph (OBS) array.Also, crustal structure off north of Iceland was obtained in this study.Twenty OBS were transported from Japan to Iceland twice in 1990 and 1991. The OBS were deployed by an Icelandic Coast Guard Ship, for both years. In 1990, the OBS array was along Reykjanes Ridge, whose length is 150 km, and width is 40 km. The dense array observation was operated for one month continuously.The result shows that earthquake hypocenters are distributed only along the very crest area of the ridge, and no earthquake was observed outside of the ridge crest. We found some patches of concentrations of earthquakes along the ridge axis, which show a good correspondence to the past submarine volcanic activities. The depth distributions of observed earthquakes were nearly vertical, and the maximum depth was 12 km. Two microearthquake swarms were caught during the observation period, which shows a very concentrated epicenters with vertical chimney-like distribution of hypocenters.In 1991 similar OBS expeditions were made to the north off Iceland, where tectonics is more complicated than usual mid-oceanic ridges. 21 OBS as well as 15 land-based seismographs were deployed for the observation. Explosions were made for controlled source study of crust and upper mantle. The expedition was quite successful, and obtained data are now being processed.As a results of this study in 1990 and 1991, microseismicity of mid-oceanic ridge in Icelandic region has been clearly revealed, which is for the first time in any ridge. Also, a detailed crustal structure of the ridge has been obtained.