ascribed to the lesser amount of tsunami energy focused toward Hilo from 

 the Gulf of Alaska. The exception to this generalization might be a large 

 earthquake causing little or no vertical ground motion, as seems to have 

 occurred at San Francisco in I906. 



c. Crescent City . Returning to Crescent City, our subjective 

 analysis of Figure U9 has suggested large amplitude (30 to 35-minute 

 period) oscillations occurring on top of the 13. U feet high tsunami waves 

 with a l.T7-hour period (Table III). We note at once that the 30 to 35- 

 minute oscillations would accord with the third harmonic of the incident 

 tsunami waves. That this frequency could have gained such large response 

 suggests that some topographical feature of the region must provide 

 resonant conditions. 



Crescent City occupies a position on a concave coastline southeast 

 of Point St. George, from which a submerged reef extends seaward in a 

 continuation of the coastal area (Figure T3a). Moreover, at the two 

 extremities of the arc, off Point St. George and off Rocky or Patrick's 

 Point (at the southern end), the Continental Shelf width narrows appre- 

 ciably. The coast and shelf markedly conform to a semi-elliptic basin, 

 open-mouthed along its major axis at the edge of the shelf (Figure 73a). 

 The dimensions of this basin are such that the ratio of the half-lengths 

 of the major and minor axes is close to U/3; the half-length L of the 

 minor axis is 17-25 nautical miles. The depth profile along the minor 

 axis is approximated very closely by a parabola, and the trend of the 

 depth contours indicates that the entire shelf in the area is pseudo- 

 ellipsoidal to a maximum depth d-i_ of 300 feet (Figures 73 a and b). 



The approximation of the shelf and coast to a geometrical form that 

 can be described mathematically makes possible some conclusions regarding 

 oscillating characteristics. For this we adapt the work of Goldsbrough 

 (1930) to the situation of an open-mouth basin (cf. Wilson, I966). For 

 this particular shape of elliptic basin, extrapolation from previous 

 calculations (Wilson, I966), suggests that the Continental Shelf off 

 Crescent City has the following natural periods of oscillation, Tm: 



T /gdi 



-2L^ = k.kkk; 3.528; 2.930; 2.3^0; 1.785; . . . (32) 



The subscript m is an arbitrary integer (m = 1, 2, 3 ...) to describe the 

 mode order in terms of period value or frequency. All these modes re- 

 quire that a node of the free oscillation shall lie along the major axis 

 at the mouth of the basin (or the edge of the shelf). The fundamental 

 mode represented by m = 1 (Ti = 79.1 minutes) would be a simple uninodal 

 oscillation about the major axis. The second mode (T2 = 62.8 minutes) 

 would represent a binodal oscillation with nodes along both axes of the 

 semi-ellipse. Another binodal oscillation (T3 = 52.1 minutes) would 

 involve a hyperbola as the second node, symmetrical with respect to the 

 minor axis, and intersecting the coast probably near Point St. George and 

 Patrick's Point. The fourth mode (Ti = kl.6 minutes) effectively would 

 be trinodal for the bay, with two nodal hyperbolas, each symmetrical 



110 



