Section V. EFFECTS OF THE MAIN TSUNAMI AND OF LOCAL 

 SEISMIC SEA WAVES IN PRINCE WILLIAM SOUND 



1. The Gross Picture of Behavior of Prince William Sound 



The tectonic movement which occurred during the earthquake has 

 ;en defined in the Prince William Sound area with greater accuracy than 

 lywhere else. The extent of vertical uplift or subsidence and the magni- 

 ide of horizontal displacement are illustrated in Figures 9, 10, and l6. 

 :gure 10 shows that the major portion of the Sound was uplifted in the 

 )rm of a massive tilting of the seabed toward the northwest. 



Figure 157 shows the main "bathymetry of the region. The coastline 

 ; labyrinthine in complexity and resembles a jigsaw puzzle, but in the 

 'OSS the Sound can be approximated as a triangular basin ABC (Figure 157) 

 Lth average depth of about 800 feet. This basin was effectively tilted 

 r the earthquake about a hinge line roughly parallel to the base line BC 

 1 Figure 157 and about 2/3 the perpendicular distance of the base BC from 

 le vertex A. 



In spite of the roughness of the approximation, it is of interest 

 ) examine the oscillating characteristics of this triangular basin, re- 

 irding it as having an open mouth along the line BC. This base line is 

 ■ course, virtually a closed boundary, but in the presence of very long 

 ;riod excitation such as the astronomical tides or the tsunami generated 

 1 the Continental Shelf, it may be approximated as an open basin. 



The fundamental period T]_ is obtainable from Lamb (1932 Ed.) (cf. 

 Lso Wilson, I966), namely 



Ti = 2,6i6L/ /id (^5) 



aere L is the perpendicular length of the embaymert from A to BC (Figure 

 57) and d is the mean depth. Adopting L = i+ x 105 feet and d = 8OO feet, 

 = find 



T-|_ ^ 110 minutes (^6) 



D the nth mode of oscillation (n = 1, 2, 3, ..) the periods T^ may be 

 hown to be 



T^ - 110; 1+8; 30; 22; . . . . minutes (1+7) 



he dominant period of the shelf-generated tsunami has already been shown 

 o be about the same as Equation (U6). This would imply that Prince 

 illiam Sound would have been a responsive sounding box for the external 

 timulation penetrating the straits around Montague Island, as well as 

 or its own upheaval, which according to Figure 3^ , would initially have 

 ome of the same character as the externally generated tsunami. 



The tortuous coastline would no doubt have a rather profound effect 

 n modifying and attenuating the main tsunami oscillations, but the 



247 



