III. MECHANICS OF GENERATION 



The generation of large, transoceanic tsunamis results from the 

 displacement of water above the area of uplifted sea bottom associated 

 with a dip-slip fault movement. Crustal displacement progresses along 

 a faultline from some initial source. Ben-Menahem (1961) developed a 

 method for determining the direction, speed, and length of rupture 

 propagating from the epicenter of a given earthquake by using recorded 

 seismic surface waves. Various analyses using this method, as reported 

 in Berg, et al. (1970) for the 1964 Alaskan tsunami give speeds from 

 3.0 to 3.5 kilometers (1.9 to 2.2 miles) per second for rupture propaga- 

 tion and a rupture length of from 600 to 800 kilometers (370 to 500 miles) . 

 Because of the high speed of rupturing, it is generally assumed in analyz- 

 ing wave generation that the total uplifting occurs instantaneously. 



1 . Area and Height of Uplifting . 



Very little data are available on the size of the generating areas 

 and the height of uplifts for various tsunamis which have been recorded 

 at coastal points. After the 1964 tsunami generated in Alaska, extensive 

 surveys were undertaken in the area of origin (Plafker, 1965; Berg, et al . , 

 1970) . These surveys included comparisons of tide levels at surviving 

 tide gages, establishment of previous tide levels by visual observation 

 and interviews with area residents, new hydrographic surveys in areas 

 previously surveyed, establishment of new elevations at bench marks, and 

 measurement of the displacement of sessile marine organisms. The uplifted 

 water area on the Continental Shelf was estimated as 1.1 x 10 11 square 

 meters (1.184 x 10 12 square feet). The potential energy of an incremental 

 area of uplifting is proportional to h 2 , where h is the height of up- 

 lifting. The average value of h 2 was estimated as 4.1 square meters 

 (44.1 square feet). The uplifted area in Prince William Sound was con- 

 sidered to have a limited effect on the tsunami generation because of the 

 restricted connections between the sound and the shelf area. 



An uplifting of the sea bottom will produce a vertical uplifting of 

 the overlying water. As a first approximation, it may be assumed that 

 the uplifting of the water surface equals the uplifting of the sea bottom. 

 The potential energy of the uplifted water is then given as 



n h • 



E = E pg A.h. — (13) 



U\ i * 2 



where 



E = the energy in ergs (foot-pounds) 



p = the density of the seawater and is assumed to equal 1.0252 

 grams per cubic centimeter (1.989 slugs per cubic foot) 



g = gravitational acceleration and is equal to 980.7 centimeters 

 (32.174 feet) per second squared 



A- = an incremental area of uplifting 



h^ = the height of uplifting over the incremental area A. 



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