The nature of the modulated waves of the second wave system in 

 Figure 38d strongly suggests they represent a second mode oscillation 

 of the free oscillation of the Continental Shelf, forced initially by a 

 second harmonic of the main tsunami. The free oscillation of the Conti- 

 nental Shelf for the profile DD ' of Figure 37a may be calculated by using 

 the impedance principle adopted by Neumann (l9^8) for investigating the 

 eigenfrequencies of coupled basins. The profile of section DC is best 

 approximated by considering the shelf as comprising two coupled basins 

 with horizontal beds, one IT nautical miles long and U50 feet deep, and 

 the second 50 nautical miles long and 26O feet deep. The shallow basin 

 is considered to be open at both ends and the deep basin to be closed at 

 one end. The calculation yields a fundamental mode oscillation of period 

 T-^ = 300 minutes or 5 hours and a second mode of period T2 = 99-5 minutes. 

 These periods could reasonably be reduced somewhat because of the shorter 

 overall length of the shelf off Kodiak as compared with profile DD ' of 

 Figure 3Ta. 



The main tsunami in Figure 38c also reveals a much longer period 

 interference with wave crests 5 hours apart. It seems reasonable to 

 adduce that this represents the development of a fundamental shelf oscil- 

 lation, attaining maximum amplitude at the time the tsunami oscillation 

 attained its maximum amplitude in the beat. 



The approximate parallel of Figures 38c and 58d with the theoretical 

 examples of impulsive water upheaval calculated by Kranzer and Keller 

 (1959) - see Figure 39 - is notable and suggests that the mechanism 

 envisioned by Van Dorn (196U) and further exemplified here in Figures 

 3^, 36, and 37 is a likely representation of what occurred. 



It appears to have been fortunate for the Naval Station at Womens 

 Bay, Kodiak, that the phasing of the primary and secondary wave systems 

 of Figure 38 was toward producing ultralow rather than ultrahigh water. 

 That Kodiak City was slightly less fortunate in this respect will be 

 shown later. We merely note here that while Womens Bay reported a highest 

 runup level of I8.8 feet above MLLW (Figure 38a), Kodiak City experienced 

 a maximum runup to 21 feet above MLLW. 



At various places along the coast in the earthquake zone, accounts 

 have indicated runup heights of waves to 60 or 80 feet (58 feet west of 

 Narrow Cape, Kodiak; 80 feet Aialik Bay, Kenai Peninsula) (Berg, et al , 

 196h; Chance, I968). These accounts are not too well established, and 

 they may refer to swash marks left by slide-generated waves or later 

 seismic sea waves that occurred on the high tide. On Kodiak Island coast 

 between Cape Chiniak and Narrow Cape, however, it is fairly well docu- 

 mented (see Berg, et al, 196U; Plafker and MayD,1965; Chance, I968; 

 Plafker and Kachadoorian, 1966) that waves reached 30 to kO feet above 

 the low tide level soon after the earthquake (Figure kO) . If we antici- 

 pate a conclusion to be presented later that the runup R of such long- 

 period tsunamis on the coast invoked only a modest amplification factor 

 of about 1.5 on the wave height H, a nominal wave height near the coast 

 might be considered to be of the order of 26 feet (8 meters). At the 

 generation point we have already inferred wave heights of the order of 



59 



