東海道沖におこった歴史津波の数値実験
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概要
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地震断層モデルを波源とする津波数値実験が,東海道沖においても有用であることは,1944年東南海津波の例によって認められた.ここでは数値実験によって,1854年安政東海津波,1707年宝永津波,1605年慶長津波,1498年明応津波の波源断層モデルのパラメータを求めた.いずれも東海道に問題を限って議論される.この中,安政,宝永津波については,かなり信頼度の高いモデルが確定された.しかし慶長,明応津波については,史料の乏しいことなどもあって,信頼度が低いことは否めない.これらの津波の特徴を比較すると,地域によっては明応津波が最も高く,災害予測の面から見落すことのできない津波であると考えられる.また局地モデルによる清水港の陸上遡上計算の結果,防波堤,埋立地などの港湾工事が,津波の高さを軽減するのにかなり効果を持っていることがわかった.また津波危険度の局地性についても,計算上の結果が得られた.Several tsunamis accompanying large earthquakes occurred in the coastal vicinity of the Tokaido district, Central Japan. All these tsunamis differed greatly in behavior. The numerical experiments are curried out for these tsunamis to prepare for tsunami hazards and to clarify some of the characteristics of earthquakes in this region. Linear shallow water equations are solved by a space staggered finite difference scheme which is telescoped at selected coastal regions. Many historical documents describing the 1854 Ansei-Tokai tsunami still remain and some investigators have used these documents to estimated the inundation heights at about 50 sites. The numerical experiment of the tsunami based on the published parameters of this earthquake explains satisfactorily the distribution of the real tsunami heights along the coast. Documents describing the 1605 Keicho and the 1498 Meio tsunamis are so scarce that estimates of the real inundation heights may be somewhat inaccurate. The source models for these tsunamis are also determined from among several hypothetical models. They are located south of the generating area of the 1854 tsunami. Descriptions of the inundations caused by the 1498 and the 1605 tsunamis in the district's typical harbors of Shimizu and Shimoda are not found in historical documents. The tsunami heights in these two harbors estimated by simulations of these tsunamis are considerably higher than those of recent tsunamis. Local model experiments including the effects of inundated water on land and quadratic bottom friction are made for the Shimizu harbor. It is clearly seen that the difference of the inundation pattern was caused by changes in the coastal geometry such as harbor construction. Attempts are made to zone tsunami hazards by estimating the hydraulic pressure caused by inundated water.
- 1981-09-30
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