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

大西洋中部山脊是板块诞生的地方。冰岛就坐落在海脊上。没有对任何海脊的地壳和上地幔结构进行详细的研究。此外，还没有对山脊周围的地震活动进行详细的研究。这项研究利用灵敏的海底地震仪(OBS)阵列揭示了冰岛及其周边地区非常详细的地震活动。此外，这项研究还获得了冰岛北部的地壳结构。1990年和1991年，20架OBS从日本两次运往冰岛。两年来，OBS都是由冰岛海岸警卫队的一艘船部署的。1990年，OBS阵列位于长150公里、宽40公里的雷克雅内斯山脊上。密集台阵连续观测1个月，结果表明，地震震源仅沿脊区分布，脊外未发现地震。我们在海脊轴线上发现了一些地震集中的斑块，这与过去的海底火山活动有很好的对应关系。观测到的地震深度分布接近垂直，最大深度为12千米。观测期间捕获了两个微震群，震中非常集中，震中垂直分布呈烟囱状。1991年，类似的OBS探险也曾在冰岛以北进行过，那里的构造比通常的大洋中脊更复杂。部署了21台OBS地震仪和15台陆基地震仪进行观测。为了研究地壳和上地幔的可控源，进行了爆炸。这次考察相当成功，目前正在处理所获得的数据。作为1990年和1991年这项研究的结果，冰岛地区大洋中脊的微震活动得到了清晰的揭示，这在任何海脊中都是第一次。此外，还获得了海脊的详细地壳结构。

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.