Joint sea-land seismological observation at peculiar mid-oceanic ridges.

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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 … More smic structures.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