LANDSLIDE TSUNAMI GENERATION ON THE PACIFIC COAST OF NORTH-AMERICA AND EFFECT OF TIDES

The Pacific coasts of Alaska and British Columbia are characterized by a high risk of catastrophic tsunamis generated by landslides. Many la ndslides and tsunamis have been triggered not by earthquakes but as a result of hydrometeorological factors. In the present study, the role of tides is considered as a triggering mechanism for coastal slope fai lures and resultant tsunamis. It is shown that low tides (especially s pring tides) dramatically increase the probability of landslide failur e in unstable sediment accumulations. Analyses of landslide tsunamis i n Kitimat Arm (27 April, 1975) and in Skagway Harbour (3 November, 199 4) demonstrate that landslide occurrence was related to lowest low wat er of spring tides. The Bingham plastic-viscid model was used to expla in the phenomenon and to determine the stability of the landslide bodi es. A ''factor of safety'', describing the degree of slope stability o n a steep coast, is introduced, This parameter diminished by a factor of 1.2-2 at the time of full drainage of a landslide body (i.e. at low tide).