43. Ocean-bottom Seismographic Observation off Sanriku-Aftershock Activity of the 1968 Tokachi-Oki Earthquake and Its Relation to the Ocean-Continent Boundary Fault
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In order to investigate the aftershock activity of'the 1968 Tokachi-Oki earthquake at the period of one year after the main shock, an oceanbottom seismographic observation was performed off the coast of Sanriku along the Japan trench in May～June of 1968. The main results obtained are as follows.(1)The aftershock activity one year after the main shock was still high and the number of earthquakes which are observed at the ocean-bottom seismographic station in the aftershock area with amplitudes larger than 100μkine was twenty-two earthquakes per day on the average. This activity is about three times higher than the foreshock activity.(2)Omori's coefficient K off the coast of Sanriku is about 8.6 km/sec for the earthquakes with S-P time smaller than 20 seconds.(3)The epicenter determination was performed by the two ocean-bottom seismographic stations and one land station. The length of the three stations network is about 250km.(4)The aftershocks are distributed in two groups: one is off the coast of Urakawa and the other is at the steep slope along the Japan trench.(5)The distribution of epicenters along the steep slope of the Japan trench corresponds to the boundary fault of the geophysical ocean and continent.(6)The boundary fault of the geophysical ocean and continent is tectonically active at present. Its activity seems to be one of the main causes of the genesis of the large earthquake activity in the province off the coast of Sanriku.|1968年十勝沖地震本震発生後1年経過した時点における余震活動を調べるために,1969年5月～7月にわたって約40日間,三陸沖において海底地震観測を行った.主な結果は次の通りである.
- 1970-12-05
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