1792年島原眉山崩壊に伴つた津波の数値実験

抄録

On May 21, 1792, a gigantic collapse of Mt. Mayuyama in Shimabara Peninsula, Kyushu, occurred. Following this event, a severe tsunami of about 10 meters in height was generated by the landslide and attacked the coast of Ariake-kai, killing more than 14, 500 persons. Many historical documents tell us the phenomena of this tsunami in fair details, so that we attempted to reconstruct a numerical model of the tsunami consistent with the historical data. In the numerical computation, a finite difference method with a leap-frog system is adapted, and two kinds of source input are tried; one is the prescrived water mass transport normal to shore line and the other the vertical displacement of sea bottom. When the transport of 18, 000m³/min (current speed-20m/sec) per unit length of shore on the center line of landslide area is assumed to be continued during 2 to 4 min, the computed waves agree fairly well with the real tsunami behaviors, the height of tsunami in various places along the coast and the order of the maximum crest in the sequence of a wave train. Therefore, it seems probable that the extraordinary flow of water normal to the shore occurred by some physical mechanisms of the mountain collapse.
The energy of this tsunami is estimated to be about 5×10¹⁹erg, and this is about 1/100-1/1000 of the available potential energy of the slided material due to the collapse of the mountain. It is significant that the tsunami energy is several times larger than that of the 1968 Hyuganada Earthquake (M=7.5). The wave spreaded over a wide area and gave distructive damages to the coast more than 120km on both side of Ariake-kai.