leeward waves. The same phenomenon i§ found in vaves reflected from 

 promontories or headlands, and has "been otgerved and demonstrated in 

 model experiments. Theoretical understanding of this phenomenon, however, 

 appears to be meager. The only studies known to us are those of Biesel 

 (1966), but the subject is far from being fully explored. Munk and his 

 collaborators (cf. Munk, et al 1959; Snodgrass, et al, 1962; Munk, I962) 

 have shown from analyses of long period wave spectra that the Continental 

 Shelf plays a great part in trapping long-wave energy over the shelf and 

 in promoting local oscillations. All of the subjective analyses in Fig- 

 ures k3 to 66 indicate that seismic waves, incident on a coastline, tend 

 to induce instantaneous response in a wide variety of frequencies. 



a. San Francisco Bay . A good example is at San Francisco, 

 California where a regular oscillation of average period (38.5 minutes) 

 developed instantaneously with the tsunami, and appeared to be modulated 

 by the same modulating influences governing the primary waves (Figure 

 50d). The period 38.5 minutes is close to being the third harmonic of 

 the l.T3-hour period of the main tsunami. However, one of the free 

 periods of oscillation for San Francisco Bay is in the range 3^-^l 

 minutes and the bay's fundamental free period is I.90 hours (cf. Wilson, 

 1966; Thornton, 19U6; Honda, Terada and Isitani, I908). Wot only then 

 were the tsunami waves near-resonant for San Francisco Bay, but their 

 third harmonic, developing no doubt from the entrance constriction, as 



a process of energy transfer at an obstruction, also was enabled to 

 resonate on its own. Thus, large effects developed in San Francisco Bay 

 despite its very protected location and narrow entrance. 



b. Hilo Bay, Hawaii . Explanations of this kind are not so 

 readily made for the local oscillations at other places, because knowledge 

 of the inherent oscillating characteristics is largely lacking. Hilo, 

 Hawaii, is one exception. Hilo Bay, with reference to a base line drawn 

 across the mouth, has the approximate shape of an acute-angled triangle 

 (Figure T2a). Schematically it may be idealized to the shape of the 

 isosceles triangle shown in Figure T2c . The bed of the bay can be con- 

 sidered a uniformly inclined plane (Figure 72b), although some departure 

 from a linear depth profile takes place near the mouth. In general the 

 bay is a good case for the application of the geometrical analogy shown 



in Figures 72 c and d. 



The natural periods T^ of oscillation for a triangular bay with a 

 uniformly sloping bed, from hydrodynamic theory (Lamb, 1932; Wilson, 

 1966) are: 



T 



/id-^/L = 3.306; 1.786; 1.237; 0.936; . . . (29) 



where n(=l,2,3...) is an integer defining the mode number, L is the length 

 of the bay and d-|_ its depth at the mouth. For an axial length L = 30,U00 

 feet and depth d-^ = 200 feet , the first four modal periods are 



T =20.9; 11.3; 7.8; 5. 9 minutes 



107 



