The earthquake caused a vast deformation of the land in a vertical 

 sense about a hinge line in parallel with the structural system mentioned. 

 Northwest of the hinge, subsidence of the land occurred; southeast, the 

 land was uplifted. The hinge line (see Figure 33) flanks the southeast 

 coast of Kodiak Island, skirts the southeast coast of the Kenai Peninsula, 

 across which it also penetrates into the northwest corner of Prince 

 William Sound. 



The major part of Prince William Sound was thus uplifted and was 

 raised to its maximum extent of 33 feet along the axis of Montague Is- 

 land at its southern end. Southwest of Montague Island to a distance 

 of about 10 nautical miles intensive surveying has indicated an uplift 

 of the seabed in excess of 50 feet. Deductive reasoning suggests that 

 this crest of maximum uplift extends in line with Montague Island paral- 

 lel to the hinge line along the entire Continental Shelf, perhaps as far 

 as the south of Kodiak Island. The trough of the subsidence area is also 

 roughly parallel with the hinge line and has its maximum depression of 

 about 8 feet on the Kenai Peninsula (see Figure 8). 



In addition to the vertical earth movements, the entire south- 

 central area of Alaska, out to the shelf edge appears to have suffered 

 a differential horizontal displacement in directions varying from south- 

 east in the Prince William Sound area to south and southwest in the 

 Kodiak Island region. The zero line of this movement parallels the Knik 

 Arm of Cook Inlet and runs south-southwest across the Kenai Peninsula. 

 Extrapolation might suggest that it would coincide with the northeast- 

 southwest axis of Kodiak Island and the Barren Islands at the Cook Inlet 

 entrance. North of this zero line, horizontal land displacement was 

 apparently directed to the northwest (5 to 10 feet along the Kenai coast 

 of Cook Inlet). Southeast of the zero line, the amount of the horizontal 

 displacement appears to have reached a peak of over 80 feet on the Con- 

 tinental Shelf southwest of Montague Island (see Figure l6) approximately 

 along the ridge of highest vertical uplift. The approximate extent of 

 maximum resultant earth movement appears to be predictable in terms of 

 earthquake magnitude (see Figure IT). 



The shot-scatter of aftershock epicenters, following the earthquake, 

 covered an area about 800 kilometers long in the northeast-southwest 

 direction and 250 kilometers wide over the Continental Shelf and coast 

 of Alaska. Determinations of the fault plane mechanism of the earthquake 

 and aftershocks by various seismologists suggest the possibility that two 

 fault planes, mutually at right angles, may have been involved. One of 

 these planes is alomost vertical, and would, in effect, underlie the 

 hinge line of zero vertical earth movement without physically breaking 

 surface there. The second plane would underlie the hinge line at the 

 surface by about 50 kilometers, dipping northwest at a small angle, and 

 would probably intersect the seabed along the northwest wall or bottom of 

 the Aleutian Trench. 



Computations of ground deformation based on dislocation theory, in 

 comparison with the deformations observed, tend to favor the view that 



348 



