PART III: EXPLANATION OF RESULTS 



34. The 100- and 500-year combined tsunami and tide elevations were pre- 

 dicted at 1,249 sites in the study area. The latitude and longitude of each 

 site and the 100- and 500-year elevations (in feet) are listed in Table 1. The 

 elevations in this report are referenced to the local mean sea level (msl) 

 datum. The locations of the sites are shown in Plates 1-77. 



35. At some locations an apparently contradictory result is found: the 

 predicted 100-year combined tsunami and tide elevation is less than the maxi- 

 mum tide. At Anchorage, for example, the predicted 100-year elevation is 

 15.7 ft, and the maximum computed tide occurring in 1964 is 16.7 ft. This 

 result is correct, however, since the predicted combined tsunami and tide ele- 

 vations are determined given the occurrence of a tsunami. The combined ele- 

 vation occurrence probabilities are dominated by the low probability of 

 tsunami occurrence. The arrival of a tsunami at a time during the year when 

 it will result in a greater water surface elevation than the maximum for the 

 year is an event with a return period greater than 100 years. This kind of 

 result indicates that severe tsunami damage is not likely at locations, such 

 as Anchorage, where tsunami amplitudes are small compared to the tide range. 



Shoreline Elevation Versus Runup Elevation 



36. The tsunami elevations presented in Table 1 are elevations at the 

 shoreline. They were determined by a finite difference solution to the linear 

 shallow-water equations. Only tsunamis resulting from the tectonic-scale per- 

 manent vertical deformation of the seafloor were considered. Local phenomena 

 such as seafloor slumping or small-scale local features of the faulting were 

 not considered. The steepness of the tsunami associated with the tectonic- 

 scale faulting is so small that it precludes the possibility of the tsunami 

 breaking as it moves onshore. Hence, except as discussed in the remainder of 

 this section, elevations presented in this report also can be considered runup 

 elevations. 



37. Three situations in which the runup elevation is not equal to the 

 shoreline elevation are: 



a. Where the tsunami intrudes into a river and creates a bore. 



b. Where dunes prevent flooding except through inlets. 



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