



\ 





\ \ 





\ \ 



3 



" \ \ \ 





•\ \ x 



1 





=L 0.5 



\ X \ 



6 

 Z 



\\ \ 



1 0.3 



c 



\\ \ 



c 

 c 



N \ \ 



o 



2 



N \ x 



0.1 







Vl 





Loq lo N=a + b(8-M) \ 



-0.05 







a=-l.02 \ 



-0.03 

 -001 



b= 0.98 N 



i ,. i .... j 



Magnitude, M 



Figure 4. Mean annual occurrence of shallow-focus 



earthquake shocks for the Aleutian and south- 

 eastern Alaska region (from Wilson, 1969; 

 adapted from Berg, 1964) . Trends of World 

 and Japanese data are inserted for comparison. 



Using equation (7), the probability of a tsunami with a magnitude of 

 3.5 or greater is 



n(3.5) = n(3.75) + n(4.25) + n(4.75) 



(8) 



which gives a value of 0.0174 for the Aleutian Trench. This value is 

 based on a relatively short period of data for large tsunamis only. 

 Dividing the trench into 12 segments gives the probability of 0.00145 

 for a tsunami of the given magnitude of 3.5 or greater to be generated 

 at any particular segment of the trench in any given year, assuming an 

 equal probability for each segment. The general equation for a particu- 

 lar segment of the trench becomes 



n(i) = 0.0094 e" - 71 i 



(9) 



26 



