Nobuo Shuto (Japan)

Ocean bottom topography very much affects tsunami generation and propagation. Most of tsunamis are generated by sub-marine earthquake that is a result of plate tectonic movement. Sea trough where a plate is subducting underneath another plate is the nest of submarine earthquake, and therefore that of tsunami. Large-scale sea bottom topography, such as a horst-and-graven structure, determines a tsunamigenic- or a tsunami-earthquake. Sea ridges and continental shelves are effective wave-guides for a far-field tsunami. Coastal topography, through shoaling, concentration and resonance effects, governs the amplification of tsunamis. Ria coast is a typical tsunami amplifier. Selective resonance occurs according to the bay characteristics. A long bay shows resonance to a far-field tsunami and a short bay to a near-field tsunami. A tsunami a few meters high in the deep ocean increases its height to tens meters at the bottom of a V-shaped bay. Some of coastal topography is, in return, a result of tsunami action. Habu harbour in Izu-Ohshima Island was made by an ancient tsunami. Another remarkable example is the Hamana freshwater Lake in Shizuoka Prefecture that was made a seawater lake. A large sandy hill 8 meters high in Izu Peninsula was built by the 1854 Ansei-Tokai tsunami.