if the Continental Shelf. They appear to have been underlain also by- 

 he fundamental free oscillation of the shelf (T =^ 5 hours). The 

 lechanism of generation envisioned is that the initial forced dipole 

 sunami immediately divided into two gravity wave systems along its 

 approximate) 65O kilometers front, one of which propagated seaward in 

 , southeasterly direction as an initially positive wave, and the other 

 .orthwestward as an initially negative wave. The pseudoperiod of the 

 'orced dipole-wave was apparently so close to the second mode free 

 iscillation of the shelf that pseudoresonance from repeated reflections 

 iff the coast and the Continental Shelf oscillation rapidly developed. 



The tsunami, propagating off the Continental Shelf, spread across 

 he entire Pacific Ocean. It probably expanded across the deep ocean 

 n fairly pure form as a modulated system of waves approximating 1.8 

 .ours in period. This system can be discovered in all the tide gage 

 races from recording stations around the Pacific Ocean, and the pattern 

 if beats is reasonably consistent (Figiires US to 66). At many places 

 hese long waves are unaccompanied by waves of higher frequency, thus 

 .eading to the above inference. At other places the influence of con- 

 inental and insular shelves or of island barriers on the long period 

 'aves is evident through the development of local oscillations as a 

 ■esult of some transference of the tsunami energy to higher frequencies, 

 'he exact mechanism for this phenomenon is not yet properly understood, 

 lut both observational and experimental evidence support the fact that 

 n the presence of discontinuities of depth, waves have a tendency to 

 .evelop their odd harmonic frequencies of which the third and fifth 

 larmonics usually carry most strength next to the fundamental, but in 

 rhich still higher frequencies may well arise if favored by local 

 ■esonances. 



A specific example of shelf and local resonance effect appears to 

 )e evident at Hilo Bay, Hawaii, whose fundamental free oscillation period 

 .s about the same as the fifth harmonic of the tsunami (T^ = 21.5 minutes). 

 although unproved at this time, it is believed that the coupled Hilo Bay- 

 .nsular shelf system has a fundamental free oscillation approaching 33 

 linutes in period, appropriate for stimulation by the third harmonic of 

 ;he tsunami waves (T3 - 36 minutes). Wave energy spectra of the tide 

 ;age record for Hilo at the time of the tsunami tend to show prominences 

 it or near these periods, along with peaks which are higher mode fre- 

 quencies of these bay or quasi-basin oscillations (Figure 67). The 

 jpectra fail to register the residual energy of the fundamental tsunami 

 it the wave period (T ^ 1.8 hours), (see Figure 59), because of an 

 inadequate resolution of the analysis at low frequencies. 



At San Francisco, California, the reaction of the bay to the tsunami 

 ^as clearly one of near-resonant response to both the fundamental tsunami 

 period and its third harmonic. The development of the latter as an ex- 

 citation may have been brought about by the entrance constriction to San 

 ^'rancisco Bay or by the Continental Shelf. 



The case of Crescent City, California, has been considered. It has 

 Deen largely a mystery why Crescent City was inundated by such large waves. 



351 



