著者

羽鳥 徳太郎

出版者

公益社団法人 日本地震学会

雑誌

地震. 2輯 (ISSN:00371114)

巻号頁・発行日

vol.51, no.2, pp.203-210, 1998-10-15

参考文献数

19

被引用文献数

1

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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 (<i>M</i> 9) in the Cascadia subduction zone (SATAKE <i>et al.</i>, 1996). In the present paper, tsunami magnitudes on the Imamura-Iida scale, <i>m</i>, 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, <i>M</i>_s, 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 <i>m</i>=3 is the largest. The magnitude values in the California region are mostly <i>m</i>=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 (<i>M</i>_s=8.3) is <i>m</i>=-4. On the contrary, that of the 1927 Lompoc tsunami caused by a high-angle thrust earthquake (<i>M</i>_s=7.0) is <i>m</i>=1, and this tsunami was observed in Hawaii and Japan. According to the epicenter distribution of the earthquakes (<i>M</i>_s&ge;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.