Section VII. SUMMARY 



1. Introduction 



The Alaskan earthquake of Good Friday, March 27, 196U, occurred 

 at 5:36 p.m. (local time) with an estimated magnitude of 8.5 on the 

 Richter Scale. Its epicenter was probably located west of Unakwik Inlet 

 in the Chugach Mountains of Prince William Sound at latitude 6l.l° K. , 

 longitude 1^7.7° W. Its focal depth has been given as between 20 and 

 50 kilometers. 



This earthquake caused violent ground shaking for about 5 minutes 

 during which time massive earth movements took place, horizontally and 

 vertically, over a large extent of south central Alaska and its Conti- 

 nental Shelf. The total movement generated a train of tsunami waves 

 which surged throughout the intricate Alaskan coastline and crossed the 

 Pacific Ocean to the farthest reaches. Numerous seismic sea waves were 

 triggered by submarine slumping of unstable glacial deltas or by land- 

 slides in the fjords along the Alaskan coastline. 



More remotely, the earthquake was responsible for a great number 

 of seismic seiches developing in lakes and reservoirs throughout North 

 America. Seismic sea waves were generated even in the Gulf of Mexico 

 as a result of ground vibration. 



The earthquake occurred at a period of low tide along the open 

 coast of Alaska and in Prince William Sound, but at high tide at the 

 head of Cook Inlet. The full damage potential of the tsunami in Alaska 

 was greatly mitigated by the coincidence of the tremors and low tide. 



2. The Mature of Earth Dislocation and Movement 



The Alaskan Pacific seaboard is one of the most active seismic 

 regions in the world, and forms part of the circimi-Pacific belt of seis- 

 micity which has been responsible for a major portion of the earthquake 

 and volcanic activity throughout the world. Statistics covering some 60 

 years provide the empirical relationship for Equation (l), page 5, that 

 allows a reasonable estimate for the frequency of occurrence of an earth- 

 quake of given magnitude in the Alaskan-Aleutian arc region. According 

 to this, an earthquake of magnitude M - 8.5, such as that of March I96U , 

 has about a l-in-30-year chance of occurrence within this region. This 

 is a somewhat higher frequency rate than applies to Japan, which is 

 also one of the most seismically active zones in the Pacific. 



The region in which the earthquake occurred is typified by a serra- 

 tion of geanticlines and geosynclines which tend to parallel the coast of 

 the Gulf of Alaska. The substructure of the coastal area mainly affected 

 comprises cretaceous sediments of graywacke, folded and warped into this 

 system, within which lie also a series of arcuate faults, roughly parallel 

 with the coast. 



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