著者

羽鳥 徳太郎

出版者

東京大学地震研究所

雑誌

東京大学地震研究所彙報 (ISSN:00408972)

巻号頁・発行日

vol.62, no.3, pp.p297-309, 1988-01

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寛政5年1月7日(1793年2月17日)宮城沖に発生した地震の震度分布およびそれに伴う津波の高さを,新史料をもとに調べ,近年の宮城沖地震との比較から地震と津波の規模および波源域を考察した.各地の史料を調べた結果,震度5の範囲は岩手県中部から福島県北部に至る内陸部に分布し,震度4の範囲は東北地方から関東地方に広くまたがることが示された.1933年三陸地震・1978年宮城県沖地震などの震度分布との比較から,寛政地震のマグニチュードはM=7.8と推定された.一方,この地震に伴う津波の高さは,岩手県中部~牡鹿半島沿岸で3~5m,福島県沿岸では2~3mと推定された.筆者の方法(羽鳥,1986)によれば,津波マグュチュード(今村・飯田スケール)はm=2.5と見つもられ(1968年十勝沖津波と同じ規模),従来推定されていた値よりもやや大きい.震度および津波の高さの分布から,波源域は1897年8月の宮城沖津波の波源域を含むかたちで海溝付近にあり,長さ200km,幅80km程度の大きさであったと考えられる.The Miyagi-Oki tsunamigenic earthquake of Feb. 17, 1793 (Jan. 7, Kansei 5) hit the Tohoku district, and its aftershock activity continued for a long time. According to the old documents, in the Sendai region 12 persons were killed and 1060 houses were destroyed by the earthquake. Along the Pacific coast from Aomori to Fukushima 22 or more persons were drowned and about 500 houses and many ships were damaged by the tsunami. In this paper the distributions of the seismic intensity and tsunami behavior are investigated with the addition of newly collected data. The results are summarized as follows: 1) A seismic intensity of 5 (JMA scale) occurred inland for 230 km from Morioka to Fukushima, and an intensity of 4 was widely distributed in the Tohoku to Kanto districts. The earthquake magnitude is inferred as M=7.8 via a comparison with the seismic intensity pattern of other Miyagi-Oki earthquakes. The epicenter is estimated to have been 38.5°N, 143.5°E near the trench. 2) Considering the ground level of the damaged regions, the tsunami heights were inferred to have been three meters (above M.S.L.) along the Iwate to Miyagi coasts and locally to have reached 4~5 meters. The tsunami heights along the Fukushima coast were 2~3 meters. The head of the large bay with seiche periods of 30min or more was con- spicuously damaged, suggesting that the longperiod waves were predominant. By use of the author's method based on the classification of tsunami heights along the coast, the tsunami magnitude (Imamura-Iida scale) is determined as being m=2.5. 3) The estimated source area is lapped on the tsunami source of Aug. 5, 1897, lies near the trench. According to the statistical relation of tsunami magnitude, the source area may be 80×200 km2. It is pointed out by many seismologists that a seismic gap exists near the trench far east of Miyagi Prefecture. For future tsunamis generated in this region, the behaviors of the 1793 tsunami suggest useful information.