It is speculated that these prequake water oscillations may have been 

 evidence of a forced vibration of the earth in its second (football) 

 spheroidal mode, induced by the lunisolar earth tides. Seismic seiches 

 induced by the earthquake in lakes, canals, and ponds reached their 

 greatest amplitudes in the Gulf of Mexico region, suggesting that the 

 epicenter of the earthquake formed the antinode of a binodal (football) 

 mode of free oscillations of the earth which followed the earthquake. 

 The epicenter would also have been on the antinode of the forced binodal 

 (football) mode of earth vibration caused by the lunisolar earth tides 

 at the time of the earthquake. 



3. Generation, Propagation and Dispersion of the Main Tsunami Waves 



The emergent picture of the earth deformation along the length 

 of the fault is that of a skew thrust and dipole heave which would have 

 had the general effect upon the sea of a gigantic wave paddle. 



Attempts are made to envision the magnitude and nature of the 

 ground movement in profile along typical cross sections transverse to 

 the Continental Shelf between Prince William Sound and Kodiak Island. 

 From these the heights of the initial tsunami wave are inferred to lie 

 between 10 and 20 meters with greatest heights occurring southwest of 

 Montague Island and perhaps at the extremity of the fault length, south 

 of the Trinity Islands and Kodiak Island, where a high center of strain 

 release has been detected. These heights are generally consonant with 

 the runup heights of waves observed along the coasts of Kodiak Island 

 and the Kenai Peninsula (allowing for some amplification). This range 

 of wave heights accords well with statistical trends of relationship 

 between tsunami wave height near the source and earthquake magnitude as 

 established from Japanese data (see Figure ^l). An approximate predic- 

 tion formula relating initial tsunami height H to earthquake magnitude 

 M is thus available in Equation (6) (p. 62)- 



All tide gages operating in the Alaskan coastal region (at Kodiak 

 and Seward) were rendered inoperative by the earthquake and sea waves. 

 Definitive knowledge of the types of waves encountered in the earthquake 

 region would be almost entirely lacking were it not for the log of wave 

 heights and times kept by Lt . C. R. Barney of the U. S. Fleet Weather 

 Central at the Naval Station, Womens Bay, Kodiak, and for the observations 

 of C. R. Bilderback of wave runup heights at Yakataga. These data, 

 plotted against recorded time, make it possible to infer equivalent 

 marigraras. 



Subjective analysis of these marigrams (Figure 38) leads to the con- 

 clusion that the dominant wave system to strike the coastline of Kodiak 

 and Alaska and penetrate Cook Inlet was a modulated beat of very long 

 period waves (T ~ 1.8 to 2.5 hours) whose first wave was a negative 

 trough created by the subsidence of land northwest of the hinge line of 

 zero vertical earth movement. These waves, however, were overridden by 

 large amplitude modulated waves of somewhat shorter period (T = 80 to 

 110 minutes) representing apparently the second mode free oscillations 



350 



