PART II: METHODS FOR ELEVATION PREDICTION 



8. FEMA requires the 100- and 500-year combined tsunami and tide ele- 

 vations for locations along the coast of Alaska bordering the Gulf of Alaska. 

 Because of the isolation of the area, there are no historical data of tsunami 

 occurrence at most of these locations. At the few locations where tsunami 

 occurrence has been documented, the unreliability of the data and the brevity 

 of the record made it impossible to perform the required frequency analyses. 

 Therefore, it was necessary to synthesize a record of tsunami activity 

 throughout the study area and to assign a probability of occurrence to each 

 tsunami in the synthetic record. 



9. The method for determining the 100- and 500-year combined tsunami 

 and tide elevations is summarized in this paragraph and discussed in detail 

 in the following sections. First, a record of tectonic deformations of the 

 seabed was synthesized. A numerical model was then used to simulate propa- 

 gation of the tsunami caused by each of the synthetic seabed deformations. 

 The numerical model produced tsunami elevation time-histories at numerical 

 gage locations throughout the study area. Model results were used to estab- 

 lish the intensity of each tsunami, and probability of occurrence was assigned 

 to each tsunami according to its intensity. Finally, a numerical procedure 

 was used to combine the effects of astronomical tides and tsunamis to deter- 

 mine the 100- and 500-year combined tsunami and tide elevations at the numer- 

 ical gage locations. 



Synthetic Record of Tectonic Deformations 

 of the Seabed 



10. To synthesize a record of tectonic deformations of the seabed, three 

 characteristics of each deformation must be defined: the shape of the rupture 

 zone (defined here as the area of the ground that is deformed by an earth- 

 quake), the distribution of uplift over the rupture zone, and the location of 

 the rupture zone. 



Rupture zone locations 



11. Tsunamis of distant origin are not considered a threat to the study 

 area. Furthermore, large tsunamis have not historically originated in the 

 eastern Gulf of Alaska because this region borders an area of strike-slip 

 faulting along the border of the North American and Pacific tectonic plates. 



