1983年日本海中部地震津波の波源数値モデル

1983年日本海中部地震津波の波源数値モデルによる数値実験を行い,波源の最適断層モデルを求めた.断層モデルは南北二つの部分から成り,南側断層は長さ40km,幅30km,傾斜角40°(東下がり),縦ずれ変位7.6m,北側断層は長さ60km,幅30km,傾斜角25°(東下がり),縦ずれ変位3.0m,左横ずれ変位0.54mの逆断層で,日本海沿岸10個所の検潮所の津波記録と相似な計算津波波形が得られた,この断層モデルによると,地震のモーメントマグニチュードは7.8,津波のエネルギーは4×10^<20>エルグと計算される.A very shallow earthquake with a magnitude of 7.7(JMA) occurred at 40.4°N and 139.1°E in the Japan Sea on May 26, 1983. The tsunami accompanying this earthquake hit the entire coast along the Japan Sea and inflicted serious damage including the loss of 100 human lives especially in Aomori and Akita prefectures and the southwestern part of Hokkaido. To examine the seismic fault model as the source of this tsunami, a numerical experiment was carried out on the basis of the shallow water equations. The most suitable fault model to the amplitudes of tsunami records at 10 reference tide-gage stations along the coast of the Japan Sea is composed of the southern and the northern parts. The southern fault has a length of 40km and a width of 30km dipping 40 degrees with the reverse dip slip of 7.6meters. The northern part has an area of 60×30km2 dipping 25 degrees with the reverse dip slip component of 3.0meters and left lateral component of 0.54 meters. It was also assured in this simulation that the period of the tsunami progressing toward the nearest coast to the source area was very short; the time interval DI the first and second up-crossing of the water level was 7.5 minutes. For this reason, some tide records at the stations near the tsunami source area suffered very large amplitude reductions due to the frequency response characteristics of tide-wells. Thus, these records were used only as secondary data. The local mean runup heights at coastal segments of about 15km each could be explained in the present simulation; the coastal height is about 2.5 times the wave height computed at a depth of 200 meters. However, extraordinary high runup heights at some restricted areas might be due to the formation of undular bore. The moment-magnitude of this earthquake inferred from this simulation is 7.8 and the tsunami energy is 4×10^<20> ergs