It is well known that the faulting of most tsunamigenic earthquakes is of the 

 dip-slip type and that very few large tsunamis have been generated by strike- 

 slip faulting. Therefore, the eastern Gulf of Alaska was not considered as a 

 tsunamigenic region. 



12. In the western Gulf of Alaska, the Aleutian trench-arc system repre- 

 sents a subduction zone where the Pacific plate sinks beneath the North Ameri- 

 can plate. Faulting along the trench is believed to be predominantly of the 

 dip-slip type. That the trench has historically been a region of high seismic 

 and tsunamigenic activity is well documented. In particular, in a catalogue 

 of Alaskan tsunamis, Cox, Pararas-Carayannis, and Calebaugh (1976) list at 

 least 10 large tsunamis that have been generated along the Aleutian trench 

 since 1870. As discussed in the remainder of this subsection, the eastern end 

 of the trench was considered to be the only tsunamigenic region which could 

 produce tsunamis capable of causing great enough runup in the study area to 

 affect the 100- and 500-year combined tsunami and tide elevations. 



13. Rupture zones of tsunamigenic earthquakes along deep sea trenches at 

 Pacific margins are generally elliptically shaped with the major axis of the 

 ellipse parallel to the trench. The majority of the energy of a large tsunami 

 will propagate from the source in a direction normal to the major axis of the 

 ellipse. If H * is the wave height emitted in the direction parallel to the 

 major axis of length a by a tsunami with an elliptically shaped rupture zone 

 and H b is the wave height emitted in the direction parallel to the minor 

 generation axis of length b , then experimental research of tsunami gener- 

 ation has shown that H fa /H a « a/b (Hatori 1963). For a large tsunami H b can 

 be as much as five or six times greater than H a . This fact and consider- 

 ation of the alignment of the Aleutian trench relative to the study area (Fig- 

 ure 2) indicated that only tsunamis originating at the eastern end of the 

 trench would cause significant runup in the study area. Furthermore, geophys- 

 ical evidence (discussed in the following paragraph) permitted all rupture 

 zones in the synthetic record to be placed coincident with that of the 1964 

 great Alaskan earthquake. 



14. Davies, et al. (1981) state that the Alaska-Aleutian portion of the 

 North American plate consists of tectonic blocks which are delimited along the 



* For convenience, symbols and abbreviations are listed in the Notation 

 (Appendix A) . 



