Section III. GENERATION, PROPAGATION, AND DISPERSION 

 OF THE MAIN TSUNAMI 



1. The Inferred Mechanism of Tsunami Generation 



In Section II, paragraph 6, we considered some of the inferences 

 that could be drawn from tsunami evidence regarding the character of the 

 seabed disturbance resulting from the earthquake. Along with seismic 

 evidence and the measured horizontal and vertical displacements of land 

 in the meizoseismal area, the emergent picture is that of a skew thrust 

 in a south and southwesterly direction (see Figure l6), of the earth's 

 crust over a slightly dipping fault plane that probably intersects the 

 earth's surface deep in the Aleutian Trench. It is possible that the 

 skewness of the thrust was partly the result of a strike and dip slip 

 along a nearly vertical fault that has no intersection with the earth's 

 surface. The vertical ground deformations have been shown to be compatible 

 with both forms of faulting, although the evidence strongly favors thrust 

 faulting on a low angle plane as being the primary mechanism of the 

 earthquake . 



The dipole character of the vertical ground movement led Van Dorn 

 (196U, 1965) to conclude that the corresponding initial form of sea- 

 surface disturbance must also have been of this form. This conclusion 

 may be substantiated also on theoretical grounds (Wilson, et al, 1962; 

 Kajiura, 1963; Van Dorn, I965) which show that the initial surface effect 

 from a sudden bottom disturbance tends to be a smoothed representation 

 of the bed displacement. In this picture the new element, whose effect 

 has no background of mathematical exploration, is the horizontal dis- 

 placement of the seabed, occurring simultaneously with the vertical 

 uplift. Hinchinbrook Island, Montague Island, and the entire emerging 

 ridge to the southwest moved forward through kO or more feet and thus 

 functioned as a gigantic wave-generating paddle (Figure 16). Further, 

 if the extrapolations of Figure 16 are correct to any degree, the thrust 

 from this movement was increasingly skew toward the southwest, implying 

 that near the end of the fault, off Kodiak and the Trinity Islands, its 

 direction was almost entirely southwest. 



2. The Heights of Tsunamis Near Their Source 



Tide gage records of the seismic sea waves, outside the immediate 

 earthquake area on the Pacific Ocean side, show an initial rise of water 

 indicating a positive sea wave resulting from the upward (and forward) 

 motion of the seabed (Coast and Geodetic Siirvey, I96U ; Brown, I96U; 

 Spaeth and Berkman, I965). The initial water movement within the Giilf 

 of Alaska and along its coast is not so well known because no tide gages 

 were operating between Yakutat and Kodiak or in Prince William Sound. 



In the region of uplift, immediate water withdrawals were reported 

 at Boswell Bay (Hinchinbrook Island), Cape St. Elias , and Middleton 

 Island (Van Dorn, 196^+), and also at Cape Yakataga (Berg, et al, I96U ; 

 Brown, I96U; Chance, I968); - see Figure 1 for locations. 



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