\ 



The reason for the active seismicity of the circum-Pacific belt is 

 still not clearly tmderstood but the area may be considered a zone of 

 structural weakness resulting from some tremendous upheaval of the earth 

 in the remote past. The deeper- focus earthquakes of the Alaskan region 

 have a tendency to occur beneath or behind the island arc or the coast; 

 the shallower-focus earthquakes towards the Aleutian Trench (Figure 3) 

 (see also Tobin and Sykes, I966). This accords with similar trends else- 

 where in the circum-Pacific seismic belt. Milne and Lee (l939) held that 

 this was manifestation of a vast inclined fault plane extending downward 

 from the ocean trenches" into the mantle below the island or continental 

 land masses. An example of this zone of weakness in the Kurile-Kamchatka 

 segment of the circum-Pacif ic' arc is illustrated in Figiire 6. Benioff 

 (196U), however, notes that deep earthquakes axe essentially missing from 

 the Aleutian segment in this type of formation. 



Besides compressional thrusting in a vertical sense along this fault 

 zone, there is rotational movement counter-clockwise of the entire area 

 within the circum-Pacific belt (Marlette, et al, I965). Worth noting is 

 a long-term secular trend of land emergence from sea level in evidence 

 for much of the Alaskan coastline and probably attributable, at least in 

 part, to deglaciation (Twenhofel, 1952; Plafker, 1965; Hicks and Shofnos, 

 1965). This emergence would also signify some of the forces at work in 

 building up strain within the earth crust in that region. 



3. Vertical Earth Movement during the Alaskan Earthquake 



Perhaps the most notable aspect of the Alaskan earthquake was the 

 great extent and amount of the changes in land level that accompanied it. 

 From the epicenter in northern Prince William Sovmd, the zone of surface 

 deformation (Figiire 7) extends for 500 miles roughly parallel to the 

 trends of the Aleutian volcanic arc and trench and the coast of the Gulf 

 of Alaska. As shown in Figure 7, and in greater detail in Figure 8, an 

 uplift of the land and sea bed has occurred on the seaward side of a 

 hinge line paralleling the Aleutian volcanic arc and passing through the 

 epicenter of the esirthquake. On the northwest or landward side of this 

 hingefline the land level has dropped. Thus most of Prince William Sound 

 has been raised above its former level while much of the Kenai Peninsula 

 and the Kodiak Island group has sunk below the former level. 



This picture of continental change has now been widely reported by 

 Grantz, et al (l96i+), Plafker, et al (l96i+), Bruder (l96i+), Plafker (1965). 

 U. S. Coast and Geodetic Survey (196^1, I965), and others. The vertical 

 tectonic movement in Prince William Sound was determined by the U. S. 

 Geological Survey, mainly by making more than 8OO measurements of dis- 

 placement of intertidal sessile marine organisms along the long intricate 

 embayed coast. These measurements were supplemented at the tidal bench 

 marks by coupled pre- and post-earthquake tide-gage readings made by 

 U. S. Coast and Geodetic Survey and by numerous estimates of relative 

 changes in tide levels by local residents. The amount and distribution 

 of the vertical tectonic movement inland from the coast were defined along 

 the highways connecting the cities of Seward, Anchorage, Valdez and 



Text res\mes on page 13 



