TSUNAMI PREDICTIONS FOR THE COAST OF ALASKA 

 KODIAK ISLAND TO KETCHIKAN 



PART I: INTRODUCTION 

 Background 



1. Of all water waves that occur in nature, one of the most destructive 

 is the tsunami. The term "tsunami," originating from the Japanese words "tsu" 

 (harbor) and "nami" (wave), is used to describe sea waves of seismic origin. 

 When they occur along the seabed, tectonic earthquakes (i.e., earthquakes that 

 cause a deformation of the earth's crust) appear to be the principal seismic 

 mechanism responsible for the generation of tsunamis. Coastal and submarine 

 landslides and volcanic eruptions also have triggered tsunamis. 



2. Tsunamis are principally generated by undersea earthquakes of magni- 

 tude greater than 6.5 on the Richter scale with focal depths less than 



30 miles.* They are very long-period waves (5 min to several hours) of low 

 height (a few feet or less) when traversing water of oceanic depth. Conse- 

 quently, they are not discernible in the deep ocean and go unnoticed by ships. 

 Tsunamis travel at the shallow-water wave celerity equal to the square root of 

 acceleration from gravity times water depth even in the deepest oceans because 

 of their very long wavelengths. This speed of propagation can be in excess of 

 500 mph in the deep ocean. 



3. When tsunamis approach a coastal region where the water depth de- 

 creases rapidly, wave refraction, shoaling, and bay or harbor resonance may 

 result in significantly increased wave heights. The great periods and wave- 

 lengths of tsunamis preclude their dissipating energy as a breaking surf; in- 

 stead, they are apt to appear as rapidly rising water levels and only occa- 

 sionally as bores. 



4. Over 500 tsunamis have been reported within recorded history, and 

 virtually all of them have occurred in the Pacific Basin. Most tsunamis are 

 associated with earthquakes, and most seismic activity beneath the oceans 



is concentrated in the narrow fault zones adjacent to the great oceanic trench 



* A table of factors for converting non-SI units of measurement to SI 

 (metric) units is presented on page 3. 



