We return then to a consideration of the circiomstances of the Alaskan 

 earthquake of March 27, 196^+. In Figure 30 the lunisolar relationship is 

 shown in greater detail at the time(5:36 AST), close to sunset, when the 

 earthquake occurred. Because of the earth's position, just teyond the 

 vernal equinox, the earth's axis was almost coplanar with the great circle 

 normal to the line joining the sun and the moon. Consequently at 5:36 

 p.m. the hypocenter c.:' the earthquake moved into a position on this plane 

 where the crust became subject to the maximum vertical inward squeeze of 

 the earth tide that can result at any time from both the sun and moon, 

 acting in lonison. Within the crust, this squeeze would have produced a 

 ring-type tangential compressive stress in an almost due north-south di- 

 rection, which presumably was a sufficient increment of stress in the 

 right direction to trigger the earthquake and cause a massive release 

 of rock strain. 



It is worthy of note that besides the earth tide effect, there would 

 have occurred an additional triggering load from the oceanic tides. Figure 

 31 shows the tidal situation at the time of the earthquake. The tides in 

 Prince William Sound are in rhythmic opposition to the tides at the head 

 of Cook Inlet. At the moment of the earthquake, the tide in the Inlet 

 was at its highest (about +l8 feet above MSL) and at its lowest in Prince 

 William Sound (about -7-5 feet below MSL). 



The weight of water impounded above MSL in Cook Inlet was about 

 2.7 X lOlO short tons. On the opposite side of Kenai Peninsula, approxi- 

 mately 9.3 X lOlO tons of water had been drained (with reference to MSL) 

 from the roughly triangular area of Prince William Sound, out to the 

 boundaries of no vertical earthquake ground motion (Figure 31). The 

 centers of mass of these water bodies occur in the locations shown at a 

 moment-arm distance of about 131 miles. This moment arm is seen to be 

 approximately normal to the hinge axis of no vertical ground movement. 

 The tilting moment of the entire water mass, which thus favored vertical 

 ground motion in the manner in which it occurred, amounted to about 

 k X lOlo foot-tons. It seems almost certain that this moment must have 

 been an additional factor in triggering the rupture that caused the 

 earthquake. It is of general interest that the centroid of negative 

 water load lies on the axis of highest uplift through Montague Island. 



There remains one other remarkable feature of the Alaskan earthquake 

 which warrants attention. The earthquake caused many remote effects in 

 the form of seismic seiches and disturbances in lakes, ponds, canals, and 

 waterways over a large part of the United States (cf. Vorhis, 19^7; McGarr 

 and Vorhis, 1967; Spaeth and Berkman, 1967). The maximum concentration 

 of these effects occurred along the U. S. Gulf of Mexico coastline. 

 Examples of the effects registered on the tide gages in the old Brazos 

 River channel at Freeport , Texas, and at Pensacola, Florida, are repro- 

 duced in Figure 32. What is remarkable in Figure 32 is that in addition 

 to the obvious distiirbances triggered at Freeport and Pensacola about 25 

 minutes after the earthquake, there occurred also on the tide record at 

 Pensacola, similar type vibrations of the water surface at regular inter- 

 vals preaeding by several hours the occurrence of the Alaskan earthquake 

 of March 27, 196U. 



43 



