1991 年 44 巻 2 号 p. 85-91
Accompanied with the Mariana Trench earthquake (epicenter: 15.226°N, 147.529°E, d=32km, Ms=7.5, USGS) of April 6, 1990, a tsunami was observed at many tidal stations in Japan. The travel times at these stations were about 3 hours, and the maximum wave at each station was observed at the time about 2 hours later than the initial wave. The average double-amplitude was about 20cm, but the amplitudes at Muroto and Tosa-Shimizu near the tip of peninsula reached 40cm with the wave period of 8 minutes. Judging from the diagram of the attenuation of wave-height with distance, the tsunami magnitude on the Imamura-Iida scale was determined to be m=2. This value is one grade (5 times for wave energy) high for an earthquake having a magnitude of Ms=7.5. Though the energy concentrated in Japan by the effect of submarine topography, the tsunami might be generated by a high-angle thrust fault.