Figure 6. Wave record from Wake Island, showing arrival 

 of tsunami (initial motion is positive and 

 remains above normal tide curve for more than 

 an hour) (from Van Dorn, 1964) . 



based on tsunamigenic earthquakes in Japan, where Dy is the focal depth 

 in kilometers and M the magnitude on the Richter scale. Tsunamis usu- 

 ally do not occur for earthquake magnitudes less than that given by 

 equation (11), although a small number of tsunamis of lesser magnitude 

 have been associated with lesser magnitude earthquakes. It should be 

 noted that equation (11) does not consider the location of the earthquake 

 with respect to the coastline, the configuration of the coastline, and 

 possible local resonance effects. The Richter scale is given by 



M = 



(log E - 11.8) 

 1.5 



where E is the earthquake energy in ergs. 



Geller and Kanamori (1977) note that care must be taken when defining 

 earthquake magnitude. Richter (1958) gives higher values for earthquake 

 magnitudes than those listed by Gutenberg and Richter (1954) . The differ- 

 ence results from the relationships used to determine earthquake magni- 

 tudes from surface wave magnitudes and body wave magnitudes. 



Attempts have been made to define lower limits for earthquake magni- 

 tudes associated with disastrous tsunamis. However, the definition of 

 "disastrous tsunamis" may be more a function of the location of the origin 

 and the population in the adjacent coastal zone, rather than an analysis 

 of the actual waves generated. Also, the equations developed to define 

 these limits are based on limited data and do not fully consider coastal 

 configurations and resonant effects . 



A tsunami generated from a dip-slip fault source will have the 

 characteristics of being generated from a line source; i.e., the length 



29 



