Written Paper

24. 1969年8月北海道東方沖地震による津波  [1970]

Hatori, Tokutaro

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A tsunami was generated off Shikotan Island, in eastern Hokkaido, at 21 h 28m (GMT) on August 11, 1969. According to JMA, the epicenter of the main shock was at 42°42'N, 147°37'E, with a depth of 30km and the earthquake magnitude 7.8. Tsunami height observed at the coast of eastern Hakkaido was the largest among tsunamis generated recently in the vicinity of the South Kuril Islands. At Hanasaki, the maximum height of 1.5 m was recorded. Waveheights in northeastern and southwestern Japan are in the ranges of 40~60 cm and 20~30 cm, respectively. The initial motion of the tsunami was in an upward direction at most stations, but at Hanasaki, the initial disturbance began with a conspicuous downward motion, suggesting the subsidence of the sea bottom in a part of the tsunami source. The source area estimated by means of an inverse refraction diagram lies on the continental slope, extending about 170km parallel to the Kuril Arc. Judging from the attenuation of the initial wave height with distance, the total wave energy seems to be equal to that of the 1958 Iturup tsunami. The predominant period of tsunami incident to the shelf is inferred to be about 30min by comparing the spectrum of tsunami with that of the records at the ordinary time near Miyagi-Enoshima.|1969年8月12日6時28分(JST),北海道色丹島沖の地震に伴ない津波が発生した.気象庁の地震月報によると,震央は42°42'N,147°37'E,深さ30km,地震のマグニチュードはM=7.8である.この津波は,本震が起った約30分以後から各地の検潮所で観測された.津波の第1波は,本州とオホーツク海に面した北海道沿岸では押し波で始まつた.しかし,波源に近い花咲では明瞭な引き波が記録され,波源域の1部分