le main earthquake occurred as a differential thrust dislocation of 



bout 20 meters along the second of these fault planes. However, the 



Dssibility that some dislocation occurred also along the near -vertical 



ault plane cannot be discounted. The overall length of the fault (or 



aults) appears to be about 800 kilometers and accords well with an 



npirical statistical result for world data relating fault length to 



arthquake magnitude, Equation (2) (p. 28). 



The above inferences on earthquake mechanism and ground deformation 

 ver the length of the fault, are supported by evidence from tsunami 

 scordings. By following the tsunami wave fronts backward in distance 

 rom tide gage stations throughout the Pacific, over the time intervals 

 etween the occurrence, of the earthquake and the arrival of the first 

 ave at the stations, it is found that the generation front of the 

 sunami tends to lie along the Aleutian Trench axis off Prince William 

 ound and Kodiak Island, terminating opposite the Trinity Islands, at 

 he southwest extremity of Kodiak Island (Figure 27). The evidence 

 apports the view that the crest trough (dipole) type of earth deforma- 

 ion was largely sustained at the magnitudes measured in the region of 

 ontague Island, along the entire length of the fault. 



The areal extent of the tsunami generation region conforms well to 

 n empirical statistical result based on Japanese data (Equation (3), 

 . 39 ) relating equivalent diameters to earthquake magnitude M. 



Some speculation is made regarding the fact that the Alaskan earth- 

 uake occurred on Good Friday, near sunset and shortly after the vernal 

 quinox. At this time the earth, moon, and sun were in opposition at 

 yzygy and ocean tides were at maximum range (springs). A sampling of 

 ix other great earthquakes, whose magnitudes exceeded 8.2, shows that 

 11 occurred when the relative sun-earth-moon positions were at or near 

 yzygy, either in opposition or conjunction. From these data it is 

 nf erred that earth-tides, which are maximum under these conditions, as 

 Iso oceanic tides, are perhaps important triggering loads for releasing 

 ent-up seismic strain in the earth's crust. 



The actual time of occurrence of the Alaskan earthquake conforms 

 o the time at which earth-tide would have produced maximum tangential 

 ompressive stress in the earth's crust in an almost due north-south 

 irection at the epicenter, in keeping with the observed directionality 

 if the inferred thrust faulting on the low angle fault plane. The fact 

 ,hat the oceanic tides were at their lowest in Prince William Sound and 

 Llong the Gulf of Alaska coastline, and at their highest at the head of 

 look Inlet, could have provided an important additional triggering load 

 md moment for unleashing the earthquake. 



Attention is drawn to a peculiarity of a water level record for 

 'ensacola, Florida, which, besides showing impulsive type decaying 

 oscillations at a time that seismic Love and Rayleigh waves could be 

 ^xpected to reach Pensacola from the hypocenter of the earthquake, also 

 shows similar water oscillations occurring prior to the earthquake. 



349 



