Ocean Bottom Seismographic Survey of the Earthquake Swarm which occur beneath Mid Atlantic Ridge.

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Mid-Atlantic Ridge is the place where plate (lithosphere) is being born.. Because usual land-based seismographs are too far to study Ridges, we made use of 30-50 sensitive portable OBSs (Ocean Bottom Seismographs) to make detailed microearthquake observations and refraction studies to study detailed crustal/upper mantle structures. Also we studied Atlantic passive margins, which is a remnant of spreading of the Atlantic Ocean.We brought about 30 OBSs to the Atlantic in 1994,1995 and 1996. In 1994 we made two experiments in the Atlantic : one on the Reykjanes Ridge and the other in the Bay of Biscay. In the former, we succeeded to obtain a detailed hypocentral distribution of microearthquakes, and the structure beneath the Ridge. In the latter, we succeeded to develop a powerful controlled source survey to study crustal/upper mantle structure, with a combination of the single-bubble pulse airgun and the sensitive OBS.In 1995 we made an extensive OBS-airgun refraction survey in a region … More from Kolbeinsey Ridge, which is a part of Mid-Atlantic Ridge north of Iceland, to Jan Mayn Ridge, which is thought a remnant continent before the Atlantic was born. We have succeeded to obtain a continuous detailed structure for the whole transect from Kolbeinsey Ridge to Jan Mayn Ridge, which is quite important to study the history of opening of the Atlantic Ocean.In 1996 we also made two experiments in the Atlantic : one on the Voring Basin off Norway and the other in the transition from Reykjanes Ridge to Iceland. The former was to illustrate a very detailed crustal structure off Norway, where a remnant initiation of the opening of the Atlantic 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 layr. The latter was to study micro-earthquakes beneath the transition from Reykjanes Ridge to Iceland which is essential to study how Iceland and the Ridge was formed. We also made controlled source study in this area to elucidate detailed crustal structure of the transition. We used 30 OBS plus 47 temporary land seismographs for this study. Less