1998 年 51 巻 2 号 p. 203-210
The generating frequency of the North-West American tsunamis is relatively lower than that of the South American region, but there are historical records of a large tsunami accompaning with the January 1700 earthquake (M 9) in the Cascadia subduction zone (SATAKE et al., 1996). In the present paper, tsunami magnitudes on the Imamura-Iida scale, m, are investigated by using the diagram of wave-height attenuation with distance. The regional characteristics of tsunami magnitudes are discussed in relation to earthquake magnitudes, Ms, during the period from 1899 to 1997. The tsunami magnitudes in the South-East Alaska to Canada region are nearly normal compared to earthquakes with similar size in the other Pacific regions, and the 1899 Yakutat tsunami being m=3 is the largest. The magnitude values in the California region are mostly m=0 or less (amplitude: 50-100cm), but those of a few tsunamis vary by the faulting mechanism. For example, the magnitude value of the 1906 San Francisco tsunami accompaning with a strike-slip earthquake (Ms=8.3) is m=-4. On the contrary, that of the 1927 Lompoc tsunami caused by a high-angle thrust earthquake (Ms=7.0) is m=1, and this tsunami was observed in Hawaii and Japan. According to the epicenter distribution of the earthquakes (Ms≥6.5) since 1812, a seismic gap exists at the segment of 700km off the Washington to Oregon states. It should be considered a region of relatively high tsunami risk.