42. Ocean-bottom Seismographic Observation at the offside of Japan Trench near the Erimo Seamount-Seismic Activity of the Oceanic Lithosphere and Velocity Structure around the Geophysical "Ocean-Continent Boundary"
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The ocean bottom seismographic .observation was performed at the offside of the Japan trench, λ=145°18.'3, ψ=40°37.'0, near the Erimo-Seamount in the time from May 7 to 9, 1969 by the Research Vessel M/S "Hakuho-Maru" of the Ocean Research Institute, University of Tokyo. 1. The deep ocean-bottom seismographic observation, about 5.6km deep, was succesf ully performed. The observation is of the anchored buoy system. The instruments and the rope-buoy system for mooring were newly developed for this operation. The operations of deployment and recovery were very smooth and efficient. 2. As many as 52 earthquakes were registered by the ocean-bottom seismograph during the two days recording period. This shows that the seismic activity in this region is very high. 3. The presence of the small earthquakes and micro-earthquake activity was discovered on both sides of the deepest part of the Japan trench. The frequency of earthquakes with amplitudes larger than 100μkine is six earthquakes per day. 4. The presence of such seismic activity evidences that the trench tectonics in this area is active at present. 5. The presence of small earthquakes and micro-earthquakes in the oceanic lithosphere evidences that the oceanic lithosphere is competent for generating earthquakes. 6. The value of Omori's coefficient K is determined at the offside of the Japan trench and is about K=10.9～9.4km/sec, which is much larger than the common value in the continental lithosphere. 7. The value {(Vp/Vs)-1} for shallow earthquakes in the oceanic lithosphere is obtained as about 0.76. 8. Large travel time anomalies are observed at the ocean-bottom seismo-graphic station. 9. From these travel time anomalies the velocity structure around the ocean-continent boundary is obtained. The velocities Vp and Vs in the high velocity zone along the deep seismic surface are 8.4km/see and 4.65km/sec respectively. 10. The amount of the velocity-anomaly in the high velocity zone is about 10% both for P-wave and S-wave. 11. The value of Omori's coefficient K and the velocity distributions agree very well with the Utsu-Oliver Model for the geophysical ocean-continent boundary structure around the oceanic trench.|えりも海山の南東,日本海溝外側の海洋性地殻の上,東径145°18'.3,北緯40°37'.0,水深5610mの地点において海底地震観測を行った.この観測は東京大学海洋研究所研究船"白鳳丸"によって(KH-6-2),1969年5月7日～9日に行われた.
- 1970-12-05
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